Potassium hydroxide, KOH

The Potassium hydroxide, KOH can be prepared by methods similar to those employed for the corresponding sodium derivative. The chief processes are the electrolysis of the chloride, and the interaction of the carbonate or sulphate in aqueous solution with slaked lime. In the sulphate process, evaporation of the mother-liquid yields the anhydrous hydroxide, the monohydrate, or the di-hydrate, the formation of each product depending on the concentration. For laboratory use, the substance can be prepared free from carbonate by a method described by Jorissen and Filippo.

The solubility of potassium hydroxide has been studied by Pickering, some of his results being given in the appended table:

Temperature, °C	-40	-30	-20	-10	0	10	20	30	40	50	60	70	80	90	100	110	120	140	140
Grams of KOH per 100 grams of water	61.2	82.4	87.9	92.3	99.2	106.1	112.7	122.2	136.9	143.9	150	157.9	163.4	177.7	187.3	201.2	214.4	235.5	281.6

The density of the solution saturated at 15° C. is 1.536.

Potassium hydroxide or caustic potash is the type of a strong base. In aqueous solution it is very extensively dissociated into its ions, and the properties of hydroxidion are, therefore, very strongly developed. Even in very dilute solution it colours litmus blue and phenolphthalein red. Somewhat stronger solutions have a soapy feeling, because they dissolve the skin of the fingers and convert it into a slimy mass; they exhibit a similar solvent action on fats, horn, hair, and like animal substances. Acids of all kinds are neutralised, i.e. converted into potassium salts, and neutral salts containing other metals are mostly decomposed in such a way that potassium salts are formed and the metals are deposited as hydroxides.

Since the last reaction is largely made use of in analysis and for of caustic potash is added to the solution of a salt the metal of which forms a sparingly soluble hydroxide, this hydroxide will be precipitated, because so much hydroxidion is introduced into the solution by means of the potash that the solubility product of the hydroxide in question is greatly exceeded. Since, now, the hydroxides of almost all the metals except the alkali metals are less soluble than potassium hydroxide, their salts are all decomposed in the above manner by potash solution.

Thus, solutions of zinc salts give a white precipitate of zinc hydroxide with caustic potash; solutions of nickel salts, a green; and copper salts a blue precipitate of hydroxide. Ammonium salts on being heated after the addition of caustic potash, evolve ammonia gas, which can be detected by its smell and by the fumes which it gives with hydrochloric acid, because the ammonion undergoes transformation with the hydroxyl to form water and ammonia.

All these reactions are clue to hydroxidion and not to kalion, for the same reactions are given when the latter is replaced by natrion or the ion of any other alkali metal. What has just been said is, therefore, not a description of caustic potash in particular, but of the strongly dissociated hydroxides in general.

In chemical properties the potassium derivative resembles sodium hydroxide, the aqueous solution being a strong base, a solution containing 6.7 gram-molecules of the hydroxide per litre having the maximum OH'-concentration. The hydroxide readily absorbs ozone, the product being possibly the heptoxide, K₂O₇.

Three hydrates have been isolated, the monohydrate, dihydrate, and tetrahydrate, melting respectively at 143° C., 35.5° C., and -32.7° C., the transition-point of the first and second being 32.5° C., and of the second and third -33° C.

The action of aqueous solutions of potassium hydroxide on sulphur is similar to that of sodium hydroxide or of concentrated ammonium hydroxide.