Method of Fresenius and Von Babo[5]
—Moist Method.—The substance which is suspected to contain the poison, if solid, is reduced to a pulp, and is mixed with sufficient water until of the consistency of thin gruel. The urine should be evaporated to one-fourth or one-sixth of its volume. Fæces should be well stirred up with distilled water. The substance is then placed in a large flask together with crystals of potassium chlorate; each 100 grammes of the substance require 3 to 4 grammes of potassium chlorate. Pure HCl of the same weight as the original substance is then added, the flask is placed on a water-bath and gently heated. Care must be taken that the heating is not too brisk, as otherwise the evolution of the chlorine peroxide takes place too rapidly. If necessary, additional crystals of potassium chlorate are added from time to time until the fluid becomes limpid and of a slight yellow colour, or, if there is much organic matter, until it assumes the appearance and colour of thin oatmeal gruel. On account of more gradual evolution of chlorine, the chlorate that is present before the flask is heated acts much more energetically, weight for weight, than fragments added after the liquid is heated, as a great deal of the gas then escapes without rendering any service. If the substance contains sugar, starch, or alcohol, extra care must be taken to avoid frothing over. When the fluid contents are clear or reduced to a thin consistency, the liquid is transferred to an evaporating basin, and allowed to remain on a water-bath until the smell of chlorine has disappeared; it is then filtered while hot. The whole of the organic matter is not destroyed by this process, fatty substances especially being resistant; but if the organic matter is reduced to small fragments, any mineral poison present will be liberated.
The objections raised against this process are that some important poisons—such, for instance, as arsenic and antimony, specially the former—are liable to escape partially in the form of vapour, and that others, such as lead and silver, may remain as insoluble precipitates on the filter. As regards the first objection, it is to be observed that, when the hydrochloric acid is diluted with water (as in a moist method of destroying organic matter), any arsenic which may be present in the hot solution is not given off with its acid aqueous vapour, arsenious chloride dissolved in hydrochloric acid being volatile only when the solvent is concentrated. Any possibility of loss may be avoided, however, by furnishing the flask, in which the organic matter is being destroyed, with a condenser and receiver. The second objection, as far as lead is concerned, is met by taking care to filter the solution whilst hot; if only a limited amount of lead is present, it then remains in solution as chloride so long as the liquid is hot, and will consequently pass through the filter. A considerable quantity is kept in solution in the cold, as it forms a combination with potassium chloride, which is more soluble than lead chloride alone. If a large amount is present, it will not all be found in the filtrate; the substance left on the filter, therefore, must always be tested for lead. In toxicological work, however, the amount of lead present is not as a rule more than will remain dissolved in the cold. Silver chloride, being insoluble either in hot or cold water, will not pass through the filter; consequently the salts of silver require dealing with in a special manner.
Dry Method.—This is effected by heating the finely divided substance to redness, so that it is either carbonized or completely incinerated. When cold, the residue is drenched with nitric acid, and sufficient heat is afterwards applied to drive off the free acid. The nitrate of the metal is then dissolved in water, filtered, and dealt with according to the kind of metal present.
The dry method is unsuitable in the case of the more volatile metals, as arsenic, antimony, and, in a lesser degree, lead, tin, and zinc. Further, it is extremely difficult and troublesome to carry out with large masses of organic matter. It is convenient with small amounts, and in the absence of the more volatile metals yields good results.
The following two methods are given by Glaister[6] on the one hand, and Dixon Mann[7] on the other. Both methods are good. It will be seen that Glaister recommends the estimation of the lead as sulphide.
When minute quantities of lead are present in combination with large amounts of organic matter, the dry process is tedious, difficult to carry out, and uncertain in its results. The plan adopted in the elimination of lead by Dixon Mann is as follows:
The urine is evaporated down to the consistency of gruel, and the fæces mixed in distilled water to a like consistency. They are then treated by the wet method, as given above. The filtrate after cooling is placed in a glass cell, the bottom of which consists of a sheet of vegetable parchment; the cell is immersed to such a depth in a deeper cell, containing distilled water acidulated with a few drops of sulphuric acid, that the liquids in the inner and outer cells stand at the same level. A piece of platinum-foil enclosing a surface of about 50 square centimetres, constituting the kathode, was submerged in the liquid contained in the inner cell, a similar piece of platinum-foil, constituting the anode, being immersed in the outer cell. The pieces of foil are so placed as to be opposite each other, separated by the parchment diaphragm. A current, 3 or 4 volts, is then passed through for from six to eight hours, after which the foil is removed from the inner cell and gently washed and dried. The metallic lead is dissolved off the foil with dilute nitric acid aided by heat, and after driving off most of the free acid the solution is decomposed with dilute sulphuric acid with an equal volume of alcohol added. It is then set aside for twenty-four hours. The precipitate of lead sulphate is washed with water containing 12 per cent. of alcohol, until all the free acid is removed; it is then separated by decantation, ignited, and weighed. The amount of lead is calculated from the weight of the sulphate; 100 parts of sulphate are equal to 68·319 parts of metallic lead.
Whether the moist or the dry process is used, the residue after the primary filtration should be tested for lead, which may be present as sulphate and remain undissolved. If the original substance contains lead as sulphate, the salt should be dissolved with heat in an aqueous solution of ammonium tartrate to which a little free ammonia has been added, then precipitated with sulphuretted hydrogen (100 parts of lead sulphide equal 86·61 parts of metallic lead). It is better, however, to convert the sulphide into sulphate by treating it with nitric and subsequently sulphuric acid, after which it is ignited, weighed, and the amount of the metal calculated by the lead sulphate factor.
Having obtained the substance by decomposing the organic matter, two methods of estimation may be made use of: