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.