Karl-Heinz Jansen and Dirk Thomas Quak
First published in AHZ 4 2018; 263: 1–15, translated by Chris Kurz, with permission of the Georg Thieme Verlag KG Stuttgart. New York
The homeopathic materia medica of Causticum described by Hahnemann is, due to the mixing of the symptoms of “Tinctura acris sine kali” with those of the Causticum distillate later described in chronic diseases, still uncertain today. However, the substances produced by Hahnemann’s Causticum syntheses are also not clearly deﬁned chemically. We therefore repeated Hahnemann’s production procedure of Causticum several times in a modern research laboratory under various conditions and analyzed the distillations with elaborate chemical investigations, the results of which bring essential and new aspects into discussion.
Keywords Causticum, hydras caustici, silicates, ammonium silicates, trace analysis.
Method of Preparation of “Causticum” by Samuel Hahnemann
(bold emphasis by authors):
“Take a piece of freshly burned lime of about two pounds, dip this piece into a vessel of distilled water for one minute, then lay it in a dry dish, in which it will soon turn into powder with the development of much heat and its peculiar odor, called lime – vapor. Of this fine powder take two ounces and mix with it in a (warmed) porcelain triturating bowl a solution of two ounces of bisulphate of potash, which has been heated to red heat and melted, cooled again and then pulverized and dissolved in two ounces of boiling hot water. This thickish mixture is put into a small glass retort, to which the helm is attached with wet bladder; into the tube of the helm is inserted the receiver, half submerged in water; the retort is warmed by the gradual approach of a charcoal fire below and all the fluid is then distilled over by applying the suitable heat. The distilled fluid will be about an ounce and a half of watery clearness, containing in concentrated from the substance mentioned above, i. e., Causticum; it smells like the lye of caustic potash. On the back part of the tongue the caustic tastes very astringent and in the throat burning; it freezes only in a lower degree of cold than water, and it hastens the putrefaction of animal substances immersed in it. When Muriate of Baryta is added, the Causticum shows no sign of sulphuric acid, and on adding oxalate of ammonia it shows no traces of lime.”
Pierre Schmidt and Jost Künzli von Fimmelsberg recommended purportedly a local application of Causticumin liquid form in simple cases of burns. Our idea to develop a lotion, gel or spray with Causticum for external application required us to manufacture our own Causticum to satisfy the requirements of a sterile manufacturing chain. After an exhaustive study of the literature we were puzzled by the question: what actually is Causticum. A question previous experimental researchers have been puzzled by as well.
Excursion into the Chemistry of Hahnemann’s period
The “caustic principle” as Hahnemann understood it.
In manufacturing Causticum, Hahnemann, like many other chemists and alchemists of his time, was motivated by the search for the “caustic principle” of alkaline and basic substances .
“… the caustic lye salts …, which now forms their composition, gives them also the corrosive property and deserves the name aceture or causticum.” (Hahnemann S: Aetzstoff and Hydras Caustici, Journal for chemistry and physics, in connection with several scholars, LVI volume, hall at Eduard Anton 1829). 
There existed the idea that the effect of alkaline substances on living organisms (corrosive, burning, irritating, biting, tanning, dissolving, etc.) is caused by a specific caustic substance, waiting to be discovered.
“What is the caustic principle found in living lime and corrosive alkalis is not yet clear; but that it does exist, and that it does not depend on an alkaline base, becomes clear on account of the strong medicinal effects of the (tincture-saturated) tincture of caustics “(from: Hahnemann:” Fragmenta “, note to Acris Tinctura, 1824). 
Hahnemann held the opinion that one never encountered absolutely pure substances in chemistry. He considered all matter “composed” and therefore of complex composition. Understandably, he thought that a yet unknown substance had to be responsible for the “caustic action”:
“All material perceptible by our senses, as simple as it appears, is always composed, just as every decomposition is conditioned by a new, different composition. Thus the caustic lye salts are not uncomposed substances, just as the freshly calcined (and extinguished) lime is simply lime earth. “(From Hahnemann: Journal for Chemistry and Physics 1829: Aetzstoff and Hydras Caustici). 
Accordingly, composed matter was considered in Hahnemann’s times to consist of individual particles. “Binding agents and active principle” were also held to be material particles. There was, of course, no notion of charged atomic nuclei and electrons to explain the principle of chemical bonds. The molecular structure of water was also still unknown.
Principles of Modern Acid-Base Chemistry
According to Brønsted and Lowry, we today define a base as a substance capable of accepting H+– ions, i.e., a proton acceptor. In aqueous solutions, the proton acceptor is the hydroxyl ion, OH-. The corresponding cation is called the H+-ion.
At the time of Hahnemann, the principles of acid-base chemistry were still unknown. The hydroxyl ion OH- as proton acceptor (base) and the hydronium ion H3O+ as proton donor (acid) were only postulated in 1887 (44 years after Hahnemann’s death) by Svanthe Arrhenius and incorporated into a complete model in 1923 by Johannes Nikolaus Brønsted.
Modern chemistry explains atomic bonds and electrons involved therein through their difference electronegativity, i.e., the varying capability of atoms to attract electrons in a chemical bond. The dipole moment of water, with its negative charge distribution around the oxygen atom and the resulting positive net-charge around both hydrogen atoms is the mediator of the “caustic property”when a base is added. A OH– -ion is formed by protolysis (the “caustic principle” of alkaline substances).
Acids and basis in the “old school” chemistry
At Hahnemann’s time, the term “alkaline basis”, denoted the alkaline reaction of substances, which formed bases by dissolving in water. Alchemy, the predecessor of chemistry, knew several forms of lye as bases:
- Limestone: calcium carbonate(CaCO3),
- burnt lime, quicklime: calcium oxide (CaO)and
- slaked lime: calcium hydroxide(Ca(OH)2),
but also natron (NaHCO3), sodium carbonate (Na2CO3), potash (K2CO3) and ammoniac (NH3).
The chemical composition of these compounds lay still in the dark and chemical formulas like the ones quoted above were also unknown. The periodic table of elements would only be discovered in 1869.
“… burnt lime (has) … added another substance to its composition, which, unknown to chemistry, gives it its corrosive nature, and its solubility in water to lime water.
This substance, although not acid itself, gives it the caustic power … “(from Hahnemann:” The chronic diseases: Causticum”, 1828).
The term “basic” (in the meaning of alkaline) was hardly used at the beginning of the 19thcentury. Rather, one talked of substances with caustic properties. In English, potassium hydroxide is still today called “caustic potash”. This property as ascribed to the “fire element” (derived from ancient Greek καυστόςmeaning “burnt”), because this corresponded to the haptic and sensory experience associated with these substances. Chemistry back then was largely experienced, felt, smelled and tasted than understood by abstract formulas like today.
The considerable amount of heat released (exothermic reaction) when calcium oxide is slaked with water, forming calcium hydroxide, was interpreted as part of the caustic principle, similar to the corrosive and burning properties on the skin and mucus membranes. These were imagined to be some kind of “fiery agent”, because fire has the quintessential property of “burning”.
Manufacturing of Bases in the 18th century
Through a reaction of burnt lime (calcium oxide) with dissolved soda (sodium bicarbonate) or potash (potassium carbonate) one knew already how to produce the “caustics”, caustic of sodium (NaOH) and caustic of potassium (KOH). This “caustification” (to render caustic, corrosive) of soda and potash was essential to the manufacture of soap. These procedures were common knowledge and already used semi-industrially. From his writings, e.g., Apotherlexikon of 1793 , one can infer that Hahnemann was an experienced chemist, experimenter and physician, who was up-to-date on the knowledge of his time.
Transference of the “caustic principle” onto water
“Causticum sine Kali” meant to Hahnemann “caustic principle separated from potassium”, which he imagined as a separable, material substance he called Causticum:
“I would like to know how such a strange substance, promising as it is for the Arztey art, which gives it the corrosive property as constituent element of the etching bases, and in this composition has such a great affinity for the oxygen, that it quickly becomes with it transformed into chalkegas, which makes the bases (to a certain extent neutralized and) mild, while separating the atmospheric air from the etching bases by adding a completely moist acid to the bases and then by distillation, in combination with water, as hydras caustici I am interested in, I say, how one can still refuse this essential substance of citizenship in the realm of chemistry “(from Hahnemann: Journal of Chemistry and Physics 1829: Aetzstoff and Hydras Caustici). )
We therefore interpret Hahnemann’s manufacturing procedure of Causticum as the attempt to chemically separate the postulated (and as he writes himself) “hitherto unknown by chemistry” caustic (basic) principle (by today’s standards the properties of the hydroxyl ion OH–) from burnt and slaked lime (i.e., calcium hydroxide, Ca(OH)2) and transfer it onto water by distillation.
Saturation of Bases to liberate Causticum
The article published in Journal für Chemie und Physik, 1829, and cited above shows clearly Hahnemann’s intention to transfer the caustic principle to water by distillation. He defends is hypothesis about Causticum:
“When the caustic bases are saturated by a liquid acid, the caustic substance is transferred onto the water in the mixture and yields a Hydras caustici. Distilling this compound of caustic base with the acid – provided the acid is not in excess – over a sand bed until all water has evaporated, drives this new composite (Hydras caustici), by all appearances as pure water, over to the other side. Was the amount of water initially small, and hence the aggregate of the caustic principle with water concentrated, its taste on the tongue will at first be cool, then astringent and finally burning on the palate, similar to Mezereum…”
It was Hahnemann’s intention to isolate the caustic principle as material substance from a “liquid acid” (a solution of potassium sulfate) saturated by a “base” (slaked lime) by distillation. This is the reason which led him to the circuitous process of synthesizing a solution of caustic potash (KOH) as a step to his distillation of Causticum, even though he was familiar with the usual preparation of caustic potash from burnt lime (CaO) and potash (K2CO3).
The Idea of “Hydras Causticum”
Hahnemann’s idea of a “Hydras Causticum” (caustically reacting water), in which water is the carrier medium of the caustic property, is not far removed from the mechanism of protolysis in water.
Particularly considering how clearly Hahnemann already spoke to the reaction of carbon dioxide with water (published in Journal fürChemie und Physik, 1829). He was able to explain the reaction of carbonic acid with calcium hydroxide (slaked lime) only via “Hydras Causticum”, because gaseous carbon dioxide (i.e., CO2, which, however, in water forms H3O+ + HCO3 ) does not react with calcium hydroxide, as he was able to demonstrate in meticulous experiments:
“Perhaps only, or at least most frequently, the caustic principle is contained in three compounds,
- with bases (alkaline salts, Fuller’s earth etc.);
- with carbon (in glowing coals, extinguished under mercury) (Note by the authors: Hahnemann perhaps refers to the writing of Joseph Priestly (1733- 1804), the first to discover oxygen (“air freed of phlogiston”) and
- with water.
Only in the first two cases can Causticum by contact with atmospheric air (the oxygen contained therein) be converted to acids, (Note of the authors: It is not oxygen but CO2which reacts in connection with water. Maybe Hahnemann thought that carbon, in connection with Causticum and oxygen, yields carbonic acid, in the sense of: carbon (C) + oxygen (O2) will not react to H2CO3 but 2OH– + CO2 = H2CO3)) which we will, by tradition, call chalk-acid, while recently they have been termed carbon-acid. Thus the bases become bland and the coal turns to chalk-acidic gas (carbon dioxide).”
At this point Hahnemann goes on to describe the chemical properties of CO2 and not, as he assumes, of Causticum and oxygen. If one mixes “air” (which contains CO2) with water, CO2 is dissolved under dissociation (HCO3 ). Is the solution saturated with calcium hydroxide (slaked lime), chalk precipitates (chalk milk; it is converted to calcium carbonate and hence “bland”).
He calls carbonic acid “chalk-acidic gas”, because of its emergence during burning of lime and because pure carbonic acid can only be produced under very specific circumstances outside of an aqueous solution. It does, under normal conditions, not exist in liquid form.
In his explanations Hahnemann follows the thoughts of the Frenchman Antonine de Lavoisier (1743- 1794) who discovered that solutions of certain oxides (e.g., sulfur dioxide) react acidic. This proved to Lavoisier that all acids had to contain oxygen. A belief that was only disproved by Justus von Liebig (1803-1874) who showed in his Elementar analyse (1831) that there, indeed, exist acids which do not contain oxygen. But Liebig, just as Hahnemann, failed to develop a general model for bases.
The “caustic principle” (Causticum) is Hahnemann’s model for the transformation of slaked lime to chalk-water by introduction of air into the aqueous solution. For him, the decisive factor is oxygen, who reacts with the “chalk-acid gas” (carbonic acid). He considers Causticum to be the catalyst of this reaction.
Isolation of the caustic principle by distillation
Furthermore, he is convinced that the caustic principle can be isolated by distillation, if he saturates the “bases” (slaked lime) with a “wet acid” (solution of potassium sulfate), so that the caustic principle is released.
This is the point at which Hahnemann went wrong, we realize today. Steam distillation does not transfer either acid components (cations) nor alkaline components (anions) to the other phase, since they remain as salts in the distillation flask. Using chemically pure substances, the distillate contains only water (H2O) with its own temperature dependent auto-protolysis. Such a distillate is, ideally,pH-neutral and contains no contaminations. Hahnemann writes:
“Every (acid), even chalk-acid (carbonic acid), separates Causticum from the corrosive bases, in the presence of water with which Causticum combines to Hydras caustici.” 
In his understanding, the principle of base-acid chemistry lies in the transference of the alkaline properties of a substance by way a “substance”, which exists only in connection with water. This is, in principle, correct, except one cannot separate the OH–ion from KOH as the alkaline properties (i.e., the pH) is determined by the modified behavior of valence electrons of molecules in solution. In other words, the alkaline properties are determined by the extent of protolysis (relative abundance of H3O+-ions to OH–-ions) in an aqueous solution.
According to Hahnemann, the “material” property being transferred is corrosiveness. The more of the caustic principle is contained in a substance, the more corrosive it is. This is analogous to the idea of a “heat principle”, which he mentions sometimes. The “heat principle” is, in his interpretation, a substance added to a compound by heating, and which is released again by burning. At his times this theory (Phlogiston theory) was wide spread and capable of explaining oxidation and reduction processes. It yielded, for the first time, a framework to classify certain groups of substances which form acids and bases. It was also the starting point for the investigation of the physics of gases.
Until the beginning of the 20thcentury, it was a commonly held belief that qualitative properties of matter are conveyed by specific carrier substances. The concept of “ether” as a carrier of electromagnetic waves was only finally disproved by quantum physics and Einstein’s theory of relativity. Modern physics has expanded this idea and now speaks of “gauge particles” as mediators of forces, e.g. photons, gravitons and gluons as the quanta for the electromagnetic, gravitational and strong nuclear force, respectively.
Hahnemann’s Causticum hence describes the „relationship“ between acids and bases. It is the substance of common inter-est (inter-est: from Latin inter-esse, that which is in between): they both are “corrosive”
“Entirely and absolutely simple substance are not detected by our senses: no man has ever seen such…This caustic principle in isolation and by itself is likewise undetectable, just as undetectable as are the simple substrates of gases (oxygen, nitrogen, and so forth) to our senses…That, which is part of their composition, also lends them their corrosive property and deserves to be called caustic principle or Causticum.” 
The substance carrying the corrosive property is, to Hahnemann, Causticum.
Experimental Setup and Procedure
Prior to the determination of the experimental setup it was expected that, according to current know-how, the procedure described by Hahnemann can only result in pure water in the condensed distillate. Therefore, considerable attention was put on historic conditions (impurities, apparatus, handling), which determined the experimental setup at Hahnemann’s time.
According to Hahnemann’s original instructions, 50 g (2 oz.) each of slaked lime Ca(OH)2 and potassium sulfate, K2SO4, were homogenized with 50 ml of boiling water in a porcelain mortar. This suspension (“magma”) was heated in a distilling flask to dryness. The escaping vapor was condensed in a cooler and collected in fractions. The entire experiment lasted about 90 minutes, resulting in individual distilled fractions of ca. 10 ml.
Three different apparatus were used for the experiment:
- 500 ml Duran flask with ground glass seal (Schott); Claisen distillation bridge with 40 cm Liebig cooler (Lenz Laborglas); 20 ml Duran culture vials (Schott) with screw top for collection of individual fractions.
- 500 ml Duran flask with ground glass seal (Schott); 1000 ml alchemic still (alembic) of Duran glass with bent run-off (Neubert glass); 20 ml Duran culture vials (Schott) with screw top for collection of individual fractions.
- 1000 ml historic flask of green lime-natron-glass with matching historic alembic; historic glass vials (ca. 30 ml) for collection of individual fractions.
Controlled heating, in order to test the effect of temperature, was accomplished on one hand by an oil bath and a laboratory-grade heating stirrer (Bibby Sterlin) up to 200 °C.
And on the other hand(comparable to a classic sand bed over an open flame) over an electronically controlled heating mantle (Witeg Heating Mantle) up to 420 °C.
Temperature was recorded over the entire duration of the distillation by Pt100 temperature sensors and, additionally, by contactless infrared thermometer.According to setup and research goal, different temperature profiles and different final temperatures between 200 and 400 °C were employed.
The following parameters were measured in the individual distilled fractions:
- Cation-chromatography (detection threshold [DT] ca. 10 μg/l) for Li, Na, K, NH4, Mg, Ca and
- Anion-chromatography (DT ca. 10 μg/l) for F–, Cl–, NO2–, Br–, NO3-, PO43-, SO42- and organic
- Amino-acid analysis (DT ca. 10 pM) for the 40 most commonamino-acids.
- pH-value via apH-sonde.
- Photometry (DT ca. 20 μg/l) for silicates.
- Determination of silicates was only carried out once in some trials (due to the volume required for the test) using 1 ml of each of the final three fractions. In later trials, with already higher concentrations of silicates detected in the mixed fractions, 1 ml was taken from each individual fraction, diluted 1:5 with water, and tested individually for silicates, without mixing.
For the individual trials, chemicals were used from different sources and preparations: For calcium hydroxide we used the following preparations:
- Calcium hydroxide, chemically pure (GPR RECTAPUR®VWR)
- Calcium oxide (VWR), chemically pure, slaked with demineralized water (Sartorius, IP Arium Comfort)
- Certified marble, chemically pure calcium carbonate (Merck), burnt 4 h at 1100 °C in tantalum oxide crucibles, slaked with demineralized water.
- Isar-chalk, calcium carbonate, burnt 4 h at 800 °C in corundum crucibles, slaked with demineralized water
- Isar-chalk, calcium carbonate, burnt 4 h at 1100 °C in corundum crucible, slaked with demineralized water
For potassium sulfate we used the following preparations:
- Potassium sulfate, chemically pure, (GPR RECTAPUR®VWR)
- Potassium hydrogen-sulfate (Alpha Aesar), melted and roasted over open flame (ca. 1100 °C) in a porcelain crucible.
For water we used the following sources.
- Tap water from Fürstenfeldbruck
- Demineralized water from a mixed-bed preparation with activated charcoal filter and UV irradiation. Conductance less than 0,1 μS (Sartorius, IP AriumComfort)
During particular trials, 0.5 g of crushed chalk-natron glass was added to simulate the quality of glass used at the time of Hahnemann.
In reproductions of historic setups used in Hahnemann’s period, original (ca. 1830) glass apparatus was used, which was sealed with dried pig bladders, cut in stripes and re-moistened (as described in the literature of the period). This kind of sealant was also used for several trials in conjunction with modern equipment for simulation purposes.
Each setup was tested by distilling 50 ml of pure water in order to determine the base values for the particular materials used.
Since the particular usage history of the historic apparatus was unknown they were carefully cleaned mechanically and rinsed for several days with demineralized water. The affluent water was examined and found to be unremarkable in all analyzed parameters.
The Chemistry of the Reaction
In his manufacturing instructions for Causticum, Hahnemann uses slaked lime (calcium hydroxide) in conjunction with molten potassium hydrogen-sulfate (potassium sulfate) with the addition of water in a classical exothermic reaction to yield potassium hydroxide (in solution solution) and calcium sulfate (gypsum). This aqueous suspension (termed “Magma” by Hahnemann) was then distilled.
By melting of potassium hydrogen sulfate Hahnemann synthesizes potassium sulfate under evaporation of SO3:
2KHSO4 T —-> K2S2O7+H2O T —-> K2SO4 + SO3
He then goes on to mix pulverized potassium sulfate with slaked lime and hot water:
K2SO4 + Ca(OH)2+Aqua —-> 2 KOH (potassium hydroxide) + CaSO4(gypsum)+ Aqua
Afterwards, he distills the reaction mixture to dryness.
Ratio of Molar Masses
Two ounces (ca 50 g) of each of the reactants are mixed:
50 g potassium sulfate (K2SO4: M =174,3g/mol) =>287mmol 50 g calcium hydroxide (Ca(OH)2: M =74,1g/mol) =>676mmol 50 g Aqua (H2O: M =18g/mol) => 2780mmol
Assuming complete stoichiometric reaction of potassium sulfate in KOH, we arrive at the following stoichiometric yields:
287 mmol K2SO4 + 670 mmol Ca(OH)2 + 2780 mmol H2O
574 mmol KOH + 287 mmol CaSO4 * 2 H2O + 383 mmol Ca(OH)2 + 2206 mmol H2O —->
Checking the input quantities against the yield of the reaction products:
|574 mmol KOH:||(KOH: M = 56,1 g/mol)||=>32,3 g|
|287 mmol CaSO4 *2 H2O||(CaSO4 *2 H2O: M = 172,1 g/mol)||=>49,4 g|
|383 mmol Ca(OH)2||(Ca(OH)2 : M = 74,1 g/mol)||=>28,4 g|
|2206 mmol H2O||(H2O: M = 18 g/mol)||=>39,8 g|
As expected, this yields a mol-mass of 3450 mM (1244 mM without water) and
a total weight of149.9 g (discrepancy due to rounding errors). Stoichiometric
ratios of ions in the reaction product:
KOH (574 / 1244) = 46%
Ca(OH)2 (383 / 1244) = 31%
CaSO4*2H2O (287 / 1244) = 23%
If bumping during boiling (boiling delay) should happen, then the “splashes” ought to contain the individual ions in the calculated stoichiometric ratio.
Calcium hydroxide as “Causticum”: the bumping hypothesis of Grimm
In the article “Causticum: Caustic Principle or Phantasy?”, published 1989, Grimm presumes delayed boiling with ensuing bumping during the distillation process. This has become a widely accepted hypothesis today, even though it is known that Hahnemann was familiar with the problem of bumping during distillation. He describes in detail how to avoid it in his Apothekerlexikon. He recommends using a thermometer and knows about controlled heating using a sand bed.
Furthermore, Hahnemann checks his distillate using specific precipitating reactions to exclude impurities due to bumping or other causes. He wants to ensure that his Causticum is not contaminated by sulfuric acid or calcium hydroxide. He manages to do this, employing state-of-the- art methods of his time.
Exclusion of KOH-Concentrations above 1% by Hahnemann’s Oral Test
Hahnemann starts by describing a test based on taste:
“…tastes astringent at the back of the tongue and extremely burning in the throat…”
Should, as Grimm describes , a bumping during delayed boiling have led to concentrations above 1% of KOH, this would have resulted in cauterized and necrotized tissue in the oral cavity. The oral test, as described by Hahnemann, excludes the possibility of this having happened.
If Transference by Bumping then Transference of All Minerals
Then Hahnemann checks his distillate for impurities using two precipitating reactions:
“…upon addition of salt-acidic barite (barium chloride), no trace of sulfuric acid; and upon addition of oxalic-ammonium(ammonium oxalate), no trace of chalk detectable.”
Grimm’s hypothesis of bumping during boiling (transference of KOH to the distillate) explains the important “corrosive” properties of Causticum, but not the absence of sulfate and calcium.
If one supposes bumping, like Grimm does, as an inadvertent mechanism of transferring minerals or salts, one should be able to detect all other minerals, e.g., calcium sulfate, in their respective abundances, next to KOH.
This, however, Hahnemann disproves using the precipitating reaction with ammonium oxalate:
Ca2+ (aq) + (NH4)2C2O4 → CaC2O4 (white precipitation) + 2 NH4+ (aq)
Hahnemann’s demonstration of absence of sulfur also excludes transference of gaseous SO2 or SO3 dissolved in the vapor to the distillate.
Hahnemann uses the precipitation of insoluble barium sulfate with barium chloride to demonstrate the absence of sulfate ions.
SO42- (aq) + Ba2+ → BaSO4
Possibility of Transference by Bumping Only within the Solubility Product of Barium Sulfate and Calcium Oxalate. Since both Hahnemann’s tests, for sulfate as well as for calcium, were negative, one can safely assume that no bumping during delayed onset of boiling happened.
From our point of view, only transference in minute amounts within the solubility product of barium sulfate and calcium oxalate are conceivable, because those would not have been detectable by Hahnemann. What does this mean for possible concentrations of KOH in the distillate?
SolubilityBaSO4: 2,2 mg·l-1 (18°C)
SolubilityCaC2O4: 6,1 mg·l-1 (20°C)
Of BaSO4 (M=233.4 g/mol) there are, hence, only 9.4 µmol/l soluble. Rounding up to 10 µmol/l this results in a maximum possible ion concentration in the distillate:
10 µmol/l CaSO4*2H2O: → 400 µg/l Ca + 960 µg/lSO4
13,5µmol/lCa(OH)2 → 539 µg/l Ca + 459 µg/lOH
20µmol/lKOH → 780 µg/l K + 340 µg/lOH
If there had been bumping during the distillation process (in the form of minute splashes), then Hahnemann’s detection method excludes concentrations in excess of 1mg/l for all involved ions (anions as well as cations).
This means that Hahnemann’s detection reaction by precipitation of barium sulfate (solubility product 2.2 mg/l) with ammonium oxalate is capable to exclude involuntary transference of the order of the 150-th part of a single droplet.
Concentration of at most 799 µg/l of free OH–-ions can arise. This modifies the calculated pH of water to that of a very weak base (pH 9.67). In vivo this theoretical value is never reached, because the small number of OH–ions are buffered immediately by the reaction equilibrium between liquid and external air. Furthermore, not all OH–ions are completely dissociated.
Skin or mucus membranes neutralize bases having a pH up to 11.5 quickly by CO2-diffusion. Only at a pH above 11.5 does necrosis and hence corrosive damage of the skin arise . Bases typically taste bitter and soapy in higher concentrations. Purportedly the detection level of KOH by taste lies between 1 and 50 mg/l .
Potassium hydroxide in solution, which would be present due to bumping during boiling within its solubility product, would have a maximum concentration of 0.799 mg/l. This cannot be detected by taste, would not lead to corrosive damage of the skin and would not explain the properties as regards taste (“…tastes astringent at the back of the tongue and extremely burning in the throat..”) of his Causticum.
Sublimation of KOH at High Temperatures
Another common explanation for the emergence of a dilute solution of potassium hydroxide is the dry evaporation of KOH in the flask at very high temperatures. The melting point of pure potassium hydroxide, which heralds the beginning of sublimation (evaporation), is 360 °C (the boiling point of KOH is 1327 °C). Temperatures of 300-400 °C are reached, if at all, only toward the end of distillation, after all water has evaporated from the flask. This, however, defines the end of the distillation process, according to Hahnemann. He would have had to expose the flask to high temperatures over a longer period (without apparent result to him) at the end of the process (whenthe “last” drop has already been transferred), in order for the temperature to reach 360 °C in the interior of the flask.
Alembics and glass flasks were very difficult to get and very expensive at the time of Hahnemann. They were made of temperature sensitive chalk-natron glass. It seems unlikely that Hahnemann, who was demonstrably carful, meticulous and following the highest standards, would have exposed them unnecessarily to high temperature variations over a longer period and risk shattering them in the process.
Even if one assumes that temperatures in excess of 360 °C were reached in the distillation flask, a transference of KOH by sublimation to the liquid fraction is still impossible. Without a phase of water vapor or liquid condensate of water (which both are absent at this point in time) gaseous KOH would have to reach the other side of the cooler. However, as soon as the temperature drops below the sublimation temperature of 360 °C, KOH solidifies and is not carried further. In the alembic’s beak the required 360 °C are never reached and decline, at the end of the process, to far below 100 °C. In our experiments we extended the heating period several minutes at a temperature of 400 °C measured at the flask. No rise in potassium concentration was discernible.
We deem it impossible that relevant amounts of KOH could be transferred to the distillate using Hahnemann’s procedure, without transference of other reactants for the following reasons:
- Fractioned analysis of the distillate for cations and anions by ion-chromatography.
- Measurements of conductance.
- Accurately documented temperature profile during distillation.
- Measurement of pH.
- Careful and repeated observation of theprocess.
- Above mentioned reasoning.
Under the given conditions it is implausible that KOH should have reached the distillate by sublimation.
Formation of Ammonia during distillation
Numerous replications and interpretations of the historic experimental setup of Hahnemann confirmed the presence of ammonia in the distillate . Since ammonia reacts alkaline in water, this could be a possible candidate for Hahnemann’s Causticum.
We can identify two possible sources of ammoniac on our experiments :
- Ammonia from ammonia salts contained in feldspar and biotite-minerals present in chalk in variousamounts.
- Ammonia from alkaline hydrolysis of the pig bladders used as seals between flask and alembic.
In our analyses, Ammonia was, in fact, detectable even without using pig-bladder seals. The explanation is the presence of feldspar and biotite-minerals, present in chalk as impurities, which contain ammonia. The melting point of these minerals is far beyond 1100 °C and could, therefore, survive the burning process of chalk. However, ammonia in our analysis is only detectable in minute quantities and only by high-pressure-liquid-chromatography (HPLC). This ammonia is formed only during the last fraction and at high temperatures in the alembic, when the last drop of water evaporates (“distilled up to dryness”). With respect to the entire amount of distillate of ca. 50 ml, Hahnemann would not have been able to detect these trace amounts of ammonia. During distillation, ammonia cannot neither be detected by smell, nor made visible by acetate proof, nor by litmus (pH). Using pig bladder seals on the alembic, significant amounts of ammonia are formed across all fractions (with the highest concentration in the last fraction). They are clearly detectable by olfaction as well as by a litmus test, as they move the pH far into the alkaline region.
Hahnemann Was Familiar with the Chemistry of Ammonia
Ammonia (NH3) is a colorless gas, readily soluble in water. It has a particularly pungent smell and is very poisonous. However, because of its very strong, typical and unpleasant odor the danger of accidental poisoning is very small. The threshold for olfactory detection of ammonia gas in humans falls between 0.018 and 70.5 ppm. The deadly dose by inhalation is LCLo=10000 ppm within 3 hours, and hence is a factor 103 to 105 higher than the olfactory threshold.
Ammonia was discovered 1776 by Johann Kunckel. Carl Wilhelm Scheele demonstrated 1773 the existence of oxygen and nitrogen and recognized nitrogen and hydrogen as the composing agents of ammonia. This was also known to Hahnemann. Scheele published his results 1777 in his only book “Chemische Abhandlungen von der Luft und dem Feuer”, which Hahnemann probably cites in his Apothekerlexikon under Ammonia. In addition, Hahnemann herein describes his experience with Ammonia during the production of Causticum:
“Alkaline Salt of Ammonia: (sal alkali volatile). The so-called volatile alkaline salt, the composition of which has been plausibly explained by recent authors as being of phlogistic (Remark of authors: nitrogen) and flammable (Remark of authors: hydrogen) air by way of the heat principle. This origin demonstrates also, why it can be found in all three kingdoms of nature, even though we draw the most of it from animal substances. Its particularly volatile, pungent odor, especially in conjunction with caustics and Causticum, distinguishes it easily from other substances.” 
Hahnemann was familiar with the smell and chemistry of ammonia:
“In combination with other acids to form neutral salts, it does not have this odor. However, upon grinding the neutral salt (e.g., salt of salmiac) with burnt lime it is released promptly and escapes, the alkaline salt combined with the caustic principle of lime, perceptible by its well-known odor which bites in the nose.” 
He also was well versed in the usual methods of detection of his time:
“Should there be too little of it present, or is its odor masked by other smells for it to be identified, one must only hold an open vial of acetic acid next to it. A white fog (Remark of authors: ammonium acetate) will form above it, if free ammonia was indeed present
… The nature of other compounds with alkaline salt of ammonia teaches us the art of separation…” 
It is, therefore, safe to assume that Hahnemann recognized the ammonium odor during his distillation, which forms naturally in distillations of all bases in conjunction with the use of sealants composed of animal matter. We can also assume that he will have attempted to minimize this effect, particularly since he was a decidedly practical, experienced and well versed chemist.
Ammonia from the sealant material of the alembic
Using the distillation procedure of Hahnemann, an alembic is fixed to the flask. There exist various methods of sealing the alembic to the flask. With today’s distilling apparatus featuring ground glass connectors, one would wrap pig bladder seals around the plug and thereby place the sealant material between plug and glass.
With a historic alembic, however, the sealant (strips of pig bladder) is wrapped around the shaft of the flask and finally the alembic fitted onto it. Thereby the sealant is placed on the outside of the flask without direct contact to the substrate within. It will, however, make contact with the hot condensate of the distillate at the edge of the gap.
Hahnemann’s distillation apparatus
The sealant in Hahnemann’s distillation apparatus comprised of pig bladder, which was dried and cut in strips. These dried strips were moistened before use and wrapped around the flask. Then the alembic was put on top and “glued” to the flask. The “moist bladder” formed a gluey strip around the shaft which exuded gelatin and fat, thereby rendering it impermeable to air and vapor.
The pig bladder is heated within the sealant gap by heat conduction of the glass. Exuded fat starts to boil and splash along with the condensate in the form of micro-drops into the alembic. Additionally, the mixture of fat and protein creeps along the glass surface across the inner surface of the apparatus. Proteins (amino acids) contained in the pig bladder thereby make their way into the vessel. The “steamed” exudate drips in small quantities from the retort back into the flask. By alkaline hydrolysis of the amino acids, small quantities of ammonium (and also low amines) are formed, which are readily detectable by their smell.
Fig 3: Alembic and positioning of pig bladder seal. Drawing of distill taken from Apotherkerlexikon by Hahnemann.
Some more modern alembics, which were available already at the time of Hahnemann, were made of glass and copper. They featured a cylindrical extension at the bottom of the retort, which could be inserted into the flask. Using this rather rare apparatus, the sealant covers the inside of the flask’s sealing surface. The exudate of the pig bladder can then drip directly back into the flask and hence leads to the formation of considerable quantities of ammonia through alkaline hydrolysis.
Consequently, the amount of ammonia produced varies with the localization of the sealant material. It is, however, at sufficient temperature, unavoidable. Hahnemann has detailed knowledge of the properties of ammonia and understands also the way it is formed at the contact of a solution of potassium hydroxide with animal matter. Still, he insists that his Causticum is a unique substance. Why?
Formation of Hydrated Silicates during Distillation
Hahnemann expects to isolate the substance responsible for the alkaline reaction, Causticum (today one would say the hydroxyl-ion), by distillation. In the distillate he finds a substance which he cannot detect directly chemically (as opposed to ammonia), whose taste and odor he can identify, and which is clearly different from ammonia. This substance (his “caustic principle”) constitutes not potassium hydroxide, as chemists later claim, but a by-product of his apparatus, which was largely unknown to the chemistry of his period.
Fig 4: Formation of ammonia from the sealant material and of hydrated silicates from the glass flask in conjunction with hot potassium hydroxide solution. Ammonium silicates are hence formed in the gaseous phase. During heating of a potassium hydroxide solution in a flask made of chalk-natron glass, silicates are formed, which are released from the glass surface during distillation! This is clearly visible as a milky opacity and a roughening of the interior surface in flasks which have been used repeatedly to “cook” alkaline solutions. Because crystalline and liquid potassium hydroxide corrode glass under formation of water soluble silicates, these substances are today only stored in plastic bottles.
Modern distills (like we are using) consist of inert Duran glass, from which nearly no SiO4 molecules (silicic acid) are released. In order to simulate the chalk-natron glass in Hahnemann’s flask, we added a small amount of crunched chalk-natron glass to Hahnemann’s Magma. The effect is surprisingly clearly detectable in the distillate. The surface nanostructure of silica gel (about 500-1000 m2/g) is perfectly suited for absorbing moisture from air or for drying solid substances. This explains the astringent sensation on the tongue and in the throat upon tasting by Hahnemann.
Hydrated silicates are volatile in water vapor. Investigations of the thermodynamics of mono- and di- silicates  show that they are present in water vapor (which explains their presence in our setup) and that they have the property to condense to silicate compounds (e.g., in the presence of aluminum and zeolites).
There existed no procedure to detect silicates directly at the time of Hahnemann. The specific chemistry of hydrated silicates appears to be very complex and was entirely unknown back then. So these silicates remained undiscovered and were understandably interpreted incorrectly by Hahnemann. In fact, he had synthesized a new substance, wholly unknown to chemistry of his time, which is until today used successfully as a homeopathic remedy.
Potassium silicates exhibit an alkaline reaction in water, under formation of OH-:
K4SiO4+H2O —-> 4 K+ + 3 OH– +H3SiO4–
On one hand, the positively charged cation, potassium, is formed but next to it also another cation, H3SIO4 , which is relative to OH positively charged (less electronegative). In this manner a “caustic substance” (dissociated silicic acid) emerges, according to Hahnemann’s imagination.
Hydrated silicates can, under certain circumstances, condense to larger, cyclic or cage-like structures (also known as “soluble glass”) which lend it a viscous consistency. Sodium silicate (or potassium silicate) was then and now produced by melting a mixture of quartz and soda (or potash) at temperatures of 1200-1500 °C. Or, still unknown at Hahnemann’s time, by chemical pulpation of quartz with sodium hydroxide (or potassium hydroxide) in the autoclave at 200 °C. Soluble glasses harden to so-called silica gels upon exposure to air, mostly due to a reaction of alkali meta-silicate with carbon dioxide or carbonic acid, according to this reaction formula:
K2SiO3 + H2CO3 —–> K2CO3 + H2SiO3 (Gel SiO2 • nH2O)
Direct acidification of soluble glass yields first silicic acid which proceeds to polymerize spontaneously to silica-gel. Silica-gel is a gelatinous mass and comprises of spherically shaped poly silicic acids connected by oxygen bridges, and whose interstitial spaces are filled with water. When this watery gel is dried at higher temperatures, a hard silica gel is formed (“Xerogel”). The surface nanostructure of silica gel (about 500-1000 m2/g) is perfectly suited for absorbing moisture from air or for drying solid substances. This explains the astringent sensation on the tongue and in the throat upon tasting by Hahnemann.
Hydrated Silicas and Ammonia: pH-dependent Formation of Ammonia-Silicates
Ammonia forms in water an equally remarkable alkaline solution. It dissociates in water under release of energy to hydroxyl and ammonium ions.
NH3+H2O —–> NH4+ + OH– +Heat
Upon heating and evaporation of the water, ammonia will again completely transition into the gas phase. Silicates are water soluble as silicic acids, which, however, depends strongly on pH and temperature. At a pH of 7 and 25 °C, solubility is at about 100 mg/l. At pH>12 and 100 °C it rises to ca. 1000 mg/l. Silicic acid transitions into the vapor phase, which, by the way, poses a permanent problem for the industrial generation of steam.
We assume the following mechanisms, happening in parallel, during Hahnemann’s distillation:
- Formation of silicic acid at high pH and transition into the vapor phase. —-> SiO2 + 2 H2O Si(OH)4
- Formation of gaseous ammonia through alkaline hydrolysis of the pig bladder eluent and dissociation in the distillate.
NH3 + H2O <===> NH4OH
- Formation of soluble ammonia silicate
2NH4OH + Si(OH)4 ——-> (NH4)2SiO3 + 3 H2O
Silicic acid would condense by deprotonation into chains of poly-silicic acid of various length, which would be detectable by a cloudiness of the solution. An excess of ammonia, however, leads to a disproportionation of the silicic acid and forms soluble ammonia silicate. In water, ammonia-ions and hydrated silicates form monomeric semi-stable ammonia silicates of the formula (NH4)2SiO3 (ammonia-meta-silicate). Ammonia hampers polymerization of silicates and, in this way, stabilizes the silicate solution. After some time, through acidification, cooling or heating, the ammonia silicates can again decompose. Ammonia escapes as a gas and silicic acid is formed, which, in turn, goes on to polymerize to gelatinous meta-silicates.
- Using salts of different origins yielded nothing significantly different from ordinary water in the Duran glass apparatus. Taste test according to Hahnemann was negative.
- Using pig bladder as sealant or adding it to the mixture in the flask resulted in formation of ammonia.
- The historic apparatus made of chalk-natron glass and using pig bladder as sealant yielded ammonia and silicate in the distillate. The taste was as described byHahnemann.
- The particular temperature profile used during distillation had a significant effect on the yield of ammonia.
- The abundance of ammonia increases steadily over the fractions. Concentrations of silica reach a maximum beyond which they do notincrease.
- A trial using classical conditions and the historic apparatus led to crystallization in the first fraction 24 hours after cooling. Following fractions remained clear. Testing for silicate after dissolving the crystals in alkali waspositive.
- Gentle heating (oil bath) resulted in significantly smaller concentrations of ammonia and silicate. Only trials using self-burnt lime, high temperatures, the addition of chalk-natron glass (or using historic glass vessels) and the use of pig bladder as sealant material showed robust analysis results of individual constituents in the distillate (Fig.s 5, 6, 7,8)
Our Causticum distillation experiment with crushed chalk-natron glass resulted in clearly detectable concentrations of 5-6 ml/l in the distilling receiver. A trial with historic chalk-natron glass flask and alembic (both dating from the period of Hahnemann) gave concentrations between 3-4 mg/l even without addition of crushed chalk-natron glass. Distillates are clearly identifiable by taste: the taste like silica gel and leave a strong, lasting feeling of desiccation in mouth and throat. The solution is clear and has a pH of 9.7.
Fig. 7: Individual fractions of distillation F1-F5, evolution of silicate, ammonia and pH.
Fig 8: Cation profile of individual fractions (F1-F5 and pure water distillation) of the distillation experiment using original vessels and pig bladder seals (V16) by HPLC-analysis.
Ammonia silicates exhibit properties which match the chemical and physical properties described by Hahnemann for his Causticumto a surprising degree:
- Completely soluble inwater.
- Can be distilled at ca. 100 °C or are carried over by water vapor.
- Clear and colorless insolution.
- Smell similar to potassium hydroxide.
- Oral sensation of binding water (“tastes astringent at the back of the tongue and extremely burning in the throat”).
- Lowers the freezing point (ammonia meta-silicates are used as anti-freezeagents).
- Hastens putrefaction (through denaturation ofproteins)
- No precipitation with barium (“upon addition of salt-acidic barite, no trace ofsulfuric acid…can be discerned”)
- No reaction with ammonia oxalate (“and upon addition of oxalate of ammonia, no traceof chalk”): absence of calcium = no white precipitate of calcium oxalate.
According to our hypothesis, Hahnemann synthesized alkaline, water vapor voluble, ammonia silicates in aqueous solution instead of the universal caustic principle (Causticum), a certain “caustic” substance from the mixture of potassium hydroxide, calcium sulfate, pig bladder seal and the chalk- natron glass of the distillation flask. He was the first physician and chemist to make them available as homeopathic remedy. Further trials will provide us the opportunity to optimize the composition of Causticum quantitatively and qualitatively further.
Our research on Causticum was funded by the Sanddorf-Foundation Regensburg.
We thank Dr. Kathrin Bretthauser for the exhaustive editing of our text, the glass collectors Birgit and Dieter Schaich for the distills of the period of Hahnemann, Mrs. Claudia Rühle of the Museum of Medical History Ingolstadt, Mrs. Anne Roestel of the German Museum of Apothecary Heidelberg, Mrs. Marianne Hasenmayer of the Glass Museum Spiegelberg, Dr. Susanne Rehn-Taube of the German Museum in Munich and Mrs. BeateSchleh of the Library of the Institute of Medical History of the Robert Bosch Foundation for their helpful informations, Dr. Julie Christoffel of Brahms Pharmacy in Regensburg for the production of Causticum spray and Causticum potencies according to our manufacturing protocol, Mr. Wolfgang Courth of the Leonardo Pharmacy in Hamburg for providing low potencies of Causticum for analysis, Mrs. Christina Grundler for the possibility to burn lime, the butcher shop Boneberger in Fürstenfeldbruckfür numerous pig bladders, Prof. Sylvia Schnell of the University Gießen for numerous laboratory glasses.
Editors Note: The authors would like to discuss their results about the correct way of producing Causticum and its influence on the homoeopathic pharmacopoeia, with a producer of homeopathic remedies, a pharmacy. If you would like to get in touch with the authors regarding the same, please post a message in the comment box below.