—The group reagent for lead is sulphuretted hydrogen in acid or alkaline solution. Lead is precipitated by this reagent as a black precipitate. Where no other metals are present, and where no organic matter is present at the same time, there is very little difficulty, and the usual method of the determination of lead in water by means of sulphuretted hydrogen is exceedingly easy.
Potassium iodide gives a yellow precipitate soluble on warming, and forms large crystals on the tube. Hydrochloric acid and chlorides give needle-shaped crystals, soluble in heat, and crystallizing out when cold. But the double chloride of potassium and lead is more soluble in heat, and still more soluble in cold, than is the pure chloride. This fact is made use of in the process to be described presently in separating lead from organic mixtures.
The direct examination is rarely possible or satisfactory, but the potassium cupric acetate method is one that may be applied to the qualitative estimation of lead in the tissues.
Method for detecting Small Quantities of Lead qualitatively.
—Dry and incinerate the material to be tested. Extract with hot dilute nitric acid, after repeated incinerations, finally extracting with ammonium acetate. Filter, incinerate, and take up the residue in dilute nitric acid. Evaporate to dryness and add a few drops of dilute acetic acid, and transfer drop to microscope slide.
To the drop on the microscope slide add one drop of dilute copper acetate solution, and two to three drops of saturated solution of potassium nitrate. Stir up the drops and well mix with a platinum wire, allow to stand for a few minutes, and then examine with a two-thirds objective. If lead is present, violet-black cubes of potassium copper lead nitrate appear (K2CuPb[NO3]).
This test is said to give a reaction in the presence of 0·00003 gramme.
Determination of Lead in the Urine.
—The quantity of lead passed by the kidneys into the urine is always small, even in acute lead poisoning. Further, the lead is excreted in organic combination, and is therefore difficult to detect. For absolute quantitative examination it is essential to evaporate the whole bulk, using at least ¹⁄₂ gallon of the fluid, and proceeding with the concentrate in the manner indicated in the estimation of lead.
For the qualitative examination many methods have been suggested, but all of them are more or less fallacious.