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.