The “bismuth” frequently referred to in the report of Smethurst’s trial is the Bismuthi Subnitras, B. P., also known by the various names of “Bismuthum Album,” “White Bismuth,” “Trisnitrate of Bismuth,” “Subnitrate of Bismuth,” “Magistery of Bismuth,” “Pearl White,” &c. This compound is a basic nitrate of bismuth, Bi N 04, H2 0: it is insoluble in water, and is a heavy, white, minutely-crystalline powder, much used in medicine, and also as a cosmetic. The name “bismuth” is misleading as applied to this drug, which is not bismuth, but a salt of that metal.
Ordinary subnitrate of bismuth frequently contains various adulterations and impurities. The most usual adulterants are carbonate of lead, carbonate of lime and phosphate of lime (Royle’s Materia Medica, 1876): among the impurities which have been found are ammonia (Piper, Pharm. Journ., Ap. 21, 1877), arsenic, lead, iron, chlorine, and sodium salts. Some specimens of bismuth subnitrate analysed by Herapath contained 1 grain of arsenic in 1000: others contained as much as 1 grain in 433. Taylor, also, found arsenic in three samples out of five examined by him. Riche (J. Pharm. et Chim., 5, 384) states that the majority of commercial samples of bismuth subnitrate contain lead and arsenic, the former to the extent of ·03-·04 per cent. (as sulphate), and the latter (as arsenious acid) to ·002-·01 per cent., while Chas. Ekin (Pharm Journ., 3, III., 381) remarks that this preparation of bismuth is often very impure, containing much subchloride, copper, and occasionally lead. On the other hand, three specimens of subnitrate of bismuth, analysed by Bernays, contained no arsenic, lead, or carbonic acid, while the percentage of bismuth oxide present closely approximated to the theoretical amount. Moreover, in the Practitioner, Mar. 1871, p. 191, the results of the examination of six samples of bismuth subnitrate are given, the only impurities found being traces of chlorine and sulphuric acid: there was no arsenic. Hence it is evident that subnitrate of bismuth does not always contain arsenic: and the quantity of this impurity, when present, is so minute as (having regard to the small doses in which the drug is usually prescribed in medicine) to be insufficient to produce the graver symptoms of arsenical poisoning.[196]
The presence of arsenic in bismuth subnitrate may easily be detected by Marsh’s test.
Subnitrate of bismuth nearly always contains arsenic and other impurities, when it has been prepared from commercial bismuth. The British Pharmacopœia, therefore; very properly directs that purified bismuth (Bismuthum Purificatum, B. P.) should be used in the preparation of this drug: the B. P. method of purifying the metal is as follows. 10 ounces of bismuth and 1 ounce of nitrate of potash are fused together in a crucible, heated and stirred, until the salt has solidified into a slag over the metal: the salt is now removed, another ounce of nitrate of potash is added, and the remainder of the process is repeated. The fused bismuth is now poured into a mould, and allowed to cool.
Herapath states that the arsenic is not all removed by this process, and he proposes to boil the nitrate in solution of a caustic alkali, which removes the arsenic and converts the bismuth into oxide, from which the salts can be prepared (Royle).
From the purified metal subnitrate of bismuth can be prepared by the following process, which is that given in the British Pharmacopœia. 2 ounces of purified bismuth are gradually added to a mixture of 4 fluid ounces nitric acid with three ounces distilled water: when effervescence has ceased, heat is applied for a few minutes, and the solution is decanted from any insoluble residue. The liquid is concentrated by evaporation to 2 fluid ounces, and poured into half a gallon of distilled water. The precipitate formed (Bi N 04, H2 0) is well washed by decantation, filtered, and finally dried at a temperature not exceeding 150° F.
In the event of the bismuth used not having been thoroughly purified, and being therefore still likely to contain a trace of arsenic, a modification of the above process, recommended by R. Schneider (Journ. Prakt. Chem., 1879, 418), may be employed. It consists in heating the acid before the bismuth is added, and continuing the heating until the metal is dissolved. If arsenic be present, the solution will contain in suspension a white precipitate of bismuth arsenate, which is nearly insoluble in nitric acid, and can be removed by filtration through asbestos, before the solution is diluted. Cold nitric acid would convert any arsenic present into bismuth arsenite, which is readily soluble in nitric acid, and could not, therefore, be separated by filtration.
“Grey powder” is Hydrargyrum cum Creta, B. P., and consists of one part of metallic mercury in a very finely divided state, mixed with two parts of chalk. It is made by rubbing mercury and prepared chalk together until metallic globules are no longer visible. The mercury in this preparation always becomes in course of time more or less oxidised, the amount of oxide formed varying according to the length of time the mixture has been kept, and the extent of its exposure to the air.
Iron, silica, and phosphoric acid, might be present in very small quantities as impurities, in many samples of grey powder: caustic lime could not possibly occur, unless the specimen had been subjected to a red heat, which would drive off the mercury and so spoil the preparation. Antimony and arsenic would rarely be met with as impurities in grey powder, and if present, would only be in very minute quantities.