General Sources of Potassium Salts. - The chief source of potassium salts is the ash left by the combustion of plants or trees; but there are two subsidiary sources, viz. nitrate of potassium, which is found native, and bitartrate of potassium, which is deposited from wine in the process of fermentation.

General reactions of Potassium Salts. - In analysis, potassium is distinguished from all other bases, excepting magnesium, sodium, and ammonium, by not being precipitated by ammonium sulphide nor carbonate of ammonium. The positive reactions by which its presence is ascertained are - (1) its precipitation when converted into an acid tartrate; (2) its precipitation by perchloride of platinum; (3) the violet colour it imparts to flame.

The sparing solubility of the acid tartrate is the test which is used in the U.S.P. to distinguish all salts of potassium. The reagent employed is tartaric acid in the case of potassium hydrate, carbonate, and bicarbonate; in the case of the tartrate of potassium and sodium, acetic acid is used. In the case of most other salts a saturated solution of bitartrate of sodium is added to their aqueous solution. Potassium chlorate is calcined and the reagent added to a solution of the residue. Potassa sulphurata is decomposed by boiling with hydrochloric acid, the sulphur removed by filtration, and the nitrate neutralised by soda before the reagent is applied. No test for potassium is given in the case of potassium bitartrate or permanganate.

This test is only employed in the British Pharmacopoeia in four instances - viz. neutral tartrate, acetate, bromide, and iodide. In the case of the neutral tartrate the test is applied by adding a small quantity of acetic acid, and thus producing acid tartrate. In the case of the acetate, bromide, and iodide, it is applied by adding tartaric acid. On the addition of perchloride of platinum to chloride of potassium a double chloride of potassium and platinum is formed, and falls as a sparingly soluble pale-yellow precipitate. If the potassium salt be other than a chloride, part of the chlorine in the platinum salt is used up to convert the potassium into a chloride, and thus loss of the expensive reagent takes place. To avoid this loss hydrochloric acid is always to be added before the addition of the platinum salt. This reaction is not used for the bromides and iodides, because bromide and iodide of platinum would be formed and a loss of the reagent would occur. In testing some potassium salts, modifications are observed in the mode of applying the test. Before applying it to the chlorate the salt is calcined, oxygen is thus driven off, and the residue, consisting of chloride of potassium, does not require the addition of hydrochloric acid. The permanganate is also calcined, but the potash contained in the residue, after being dissolved out by water from its admixture with manganese dioxide requires to be treated with acid as usual. In the case of the sulphide the hydrochloric acid causes the evolution of hydrogen sulphide, which must be removed by boiling, and causes also the precipitation of sulphur, which must be removed by filtration before the addition of platinum chloride.

Preparation Of Potassium Salts

Prepared from

By

Potassium carbonate, B. and U.S.P.

Wood ashes

Lixiviating, evaporating, and crystallising.

Liquor potassae, B.P. " potassii, U.S.P.

Potassium carbonate

Treating solution with slaked lime and partially evaporating.

Caustic potash, B. and U.S.P.

Do.

Ditto, and evaporating to dryness.

Potassium bicarbonate, B. and U.S.P.

Do.

Passing carbonic acid gas into solution.

Potassium sulphite, U.S.P.

Do.

Passing sulphurous acid gas into strong solution until acid, adding equal weight of potassium carbonate and crystallising.

Prepared from

By

Potassium acetate, B. and U.S.P.

Potassium carbonate

Dissolving in acetic acid.

Potassium citrate, B. and U.S.P.

Do.

Neutralising with citric acid.

Potassium hypophos-phite, U.S.P.

Do.

Decomposing by hypophosphite of calcium.

Potassium chlorate, B. and U.S.P.

Do.

Treating with lime and chlorine.

Potassa sulphurata, B. and U.S.P.

Potassium carbonate and sulphur

Heating together.

Potassium ferrocyan-ide, B. and U.S.P.

Potassium carbonate

Fusing with animal matter and iron; lixiviating and crystallising.

Potassium cyanide, B. and U.S.P.

Potassium ferrocy-anide

Igniting either alone, or with carbonate of potassium. The former process is official B.P. and gives a purer, the latter a more abundant product.

Potassium acid tartrate, B. and U.S.P.

Crude tartar or argol

Treating with charcoal or clay.

Potassium tartrate, B. and U.S.P.

Acid tartrate of potassium

Neutralising with potassium carbonate.

Potassium nitrate, B. and U.S.P.

Native

-

Potassium sulphate, B. and U.S.P.

Acid sulphate left from admixture of sulphuric acid and potassium nitrate in the preparation of nitric acid

Neutralising with carbonate of potassium or calcium.

Potassium permanganate, B. and U.S.P.

Chlorate of potassium, caustic potash, and oxide of manganese

Ignition together, boiling and neutralising.

Potassium bichromate, B. and U.S.P.

Chromate of potassium

Treating with sulphuric acid.

Potassium iodide, B. and U.S.P.

Potash and iodine .

Mixing and heating with charcoal.

Potassium bromide, B. and U.S.P.

Potash and bromine.

As in the iodide.

General Action of Potassium Salts. - According to Ringer, potassium is a protoplasmic poison destroying muscle, nerves, and nerve-centres when applied to them sufficiently long and in a sufficiently concentrated form. But this action is not peculiar to potassium, for sodium, ammonium, hydrocyanic acid, and probably many other substances possess it. Potassium salts differ from sodium salts in diffusing more readily through membranes. They are more easily absorbed and more easily excreted than sodium salts. In the living organism they occur chiefly in the solid structures, such as blood-corpuscles and muscles, while sodium salts occur chiefly in the fluids of the body.

When applied to muscle, potassium salts in minute doses may increase its contractile power (p. 135); but in larger doses, or when continued for a longer time, they diminish its power and finally paralyse it altogether. They remove the excessive prolongation of muscular contraction produced by veratrine, barium, calcium, strontium, and by large doses of sodium or lithium (p. 135).

They have a somewhat paralysing action on motor nerves. They paralyse also the nerve-centres, generally after a primary, transitory, excitement.

A peculiar difference in the action of sodium and potassium salts locally applied to the intestine has been already noticed (p. 383). Large doses paralyse the muscular fibre of the intestines, and it is possible that this paralysing action is the cause of the digestive disturbances which the prolonged use of potassium salts causes (Rossbach).

When administered by the mouth they may produce, like other salts in large doses, irritation of the gastro-intestinal canal. They are, however, so quickly excreted that they can hardly produce poisoning by their action on the heart while circulating in the blood; they probably modify the nutrition of the tissues and act as alteratives. It is probable that potassium salts may accumulate to a certain extent in the body in the same way as sodium chloride (p. 601). By feeding animals with potassium salts the poisonous action of barium may be lessened. Cash and I have now found that when injected simultaneously with salts of barium (cf. p. 137), they will antagonise the action of the latter, and prevent death from an otherwise lethal dose of barium. Similar experiments with potassium and veratrine have given negative results. The prolonged use of potassium salts is apt to cause some depression, and larger doses continued for some time may diminish the force of the circulation. They do not paralyse the heart when given by the mouth, but when injected directly into the veins they produce transitory excitement, clonic spasms, paralysis, and death.

Death is preceded by convulsions, and is caused by stoppage of the heart while respiration still continues. Even after both heart and respiration have ceased and the animal is apparently dead, life may be restored by the patient use of artificial respiration, and mechanical irritation of the heart by compressing the cardiac region. After the heart has thus been induced to beat spontaneously, respiration still remains in abeyance for some time. The nerve-centres are also paralysed, and neither voluntary movement nor reflex action occur for some time. When reflex excitability returns it is often much exaggerated, so that a slight shake or gentle touch on the surface may cause spasms. In this respect potassium somewhat resembles atropine, and the possible explanation of this action has already been discussed (p. 171 etseq.).

The effect of potassium salts on the circulation somewhat resembles that of digitalis. In large doses they cause a rapid fall of the blood-pressure and pulse-rate. Smaller doses cause a slight fall of both pulse-rate and pressure, followed by a rise of both. During the rise of pressure, however, the pulse becomes again slow, and continues so even when the pressure again begins to fall to the normal. The rise of pressure occurs even when the spinal cord is divided, and probably depends on contraction of the arterioles (p. 281).