3. Sulphuretted hydrogen, or ammonium sulphide, gives a black precipitate.
4. Potassium bichromate a yellow precipitate.
Detection of Lead in Organic Mixtures
The contents of the stomach or vomited matters must be diluted with water and filtered. The residue left on the filter, washed with distilled water, should be set aside for further examination; the filtrate and washings acidified with nitric acid. A current of sulphuretted hydrogen passed through the solution will then throw down the whole of the lead, should any of that metal be present, in the form of a brownish-black sulphide, which may be collected on a small filter and dried. The sulphide, boiled with dilute nitric acid, is partly converted into insoluble sulphate, and in part dissolved as nitrate. The carefully neutralised solution may be either tested at once or carefully concentrated. In either case, the production of a bright yellow precipitate, with a solution of bichromate of potash, and a similar one with a solution of iodide of potassium, may be taken as conclusive of the presence of lead. The portion of lead deposited as sulphate will be found to be soluble in a solution of pure potash, the resulting liquid giving a brown-black precipitate on the addition of sulphide of ammonium.
The insoluble residue left on the filter should be incinerated in a porcelain crucible, either with or without nitric acid, care being taken not to raise the temperature more than is necessary to produce the desired effect: the carbonised mass boiled with dilute nitric acid evaporated to dryness, extracted with distilled water, and then filtered, the filtrate tested as before mentioned. It is often useful, as a preliminary test for the presence of lead in a soluble form, to dip a piece of bibulous paper into the clear liquid obtained by submitting the contents of the stomach or vomited matters to filtration, and then exposing the paper to the action of a current of sulphuretted hydrogen. If lead be present, blackening of the paper will take place.
To detect the lead in the urine and fæces, Dixon Mann advised the urine to be evaporated to the consistency of gruel, and the fæces to be mixed with distilled water to a similar consistence; the organic matters are then to be destroyed with hydrochloric acid and chlorate of potash with the aid of heat, and the solution filtered. The filtrate is then placed in a cell with a parchment bottom, and this into another cell containing distilled water acidulated with sulphuric acid. Two pieces of platinum foil are now placed in the inner and outer cells, separated by the parchment; that in the inner cell is connected with the cathode, that in the outer with the anode of four Grove cells, and the current closed for several hours. The lead, if present, is deposited on the platinum connected with the cathode.
The magnesium method (Marsden & Abram) is useful. A strip of pure magnesium is placed in the urine, to which has previously been added ammonium oxalate, about 1 gramme to 150 c.c. Lead, if present, is deposited on the magnesium in about half an hour, but it may take some hours. The strip is washed in distilled water and dried—warmed with a crystal of iodine the yellow iodide will form; or the deposit may be dissolved in HNO₃ and tested in the usual way.
Goadby recommends inoculating the urine with bacillus coli, which during its growth produces H₂S; this precipitates any lead as sulphide. After filtering, the filtrate is dissolved in a small quantity of 10 per cent. nitric acid and tested by the usual methods.
Recapitulation of the Leading Facts with
regard to Poisoning by Lead