(1) Colorimetric; (2) gravimetric.
Where the estimation is to be made gravimetrically, the substance is always obtained as a sulphate, and the lead estimated by weighing as a sulphate. This process is an exceedingly tedious one when a large number of small samples have to be estimated, as in the determination of the amount of lead dust present in the air. On the other hand, it is probable that the use of the balance in the estimation of lead as sulphate is more accurate where larger quantities up to 10 milligrammes are present; but where only 2 or 3 milligrammes are present in the amount of substance examined, the experimental error in washing is too large to warrant the expenditure of time required in this form of estimation, and the colorimetric method is used.
Detection of Lead in Organic Mixtures.
—Acidulate the organic substances, reduced to fine proportions, with nitric acid, heat for some time, then permit to cool; filter, wash residue, and mix washings with filtrate; concentrate filtrate; pass H2S; place mixture in warm place to allow precipitate to settle; after which decant supernatant fluid, collect precipitate on tared filter, thoroughly wash, dry on water-bath, and weigh. One part of sulphide is equivalent to 0·9331 part of lead oxide and 1·5837 parts of acetate of lead.
The electrolytic method is better adapted for the detection of minute quantities of lead, as, for example, in the urine or fæces or in vomited matter. The urine may be evaporated to a viscous state; the others, finely broken up, are treated in the same way, after HCl is added, as recommended above, the mixture heated, and pinches of powdered chlorate of potash added, as necessary, to break down organic matter. The heating is continued until the odour of chlorine disappears, after which it is filtered and the filtrate allowed to cool. The filtrate is then placed in the outer cell of a two-celled arrangement, not unlike a dialyser, the bottom of which is formed of vegetable parchment, the outer cell containing distilled water acidulated with H2SO4. Into the inner cell is placed a piece of platinum-foil measuring about 50 square centimetres of exposed surface, which is connected with the kathode or negative pole of four Grove cells, and into the outer cell is placed a like-sized piece of platinum-foil connected with the anode or positive pole. These pieces of foil are so placed in relation to one another that they are only separated by the parchment. The galvanic circuit being now closed for some hours, any lead in the filtrate will be deposited on the platinum-foil connected with the kathode in the inner cell. The foil is then removed, carefully washed, and the metallic lead dissolved by dilute nitric acid aided by heat, after which the solution is concentrated until most of the free acid is driven off; dilute sulphuric acid is added to throw down the sulphate, alcohol being also added to expedite precipitation. The precipitate is allowed to settle for twenty-four to thirty-six hours, filtered on a tared filter, washed with water containing 12 per cent. of alcohol, dried, ignited, and weighed. One part of sulphate is equivalent to 0·68319 part of metallic lead and to 1·25 parts of acetate of lead.
The estimation of the lead, especially if the amount be small, may be more accurately made by the volumetric colorimetric method. The metallic lead deposited on the platinum-foil is dissolved in nitric acid, and distilled water added, and an aliquot portion placed in a Nessler glass. A few drops of freshly-prepared H2S water, or H2S gas itself, may be added to or bubbled through the contents of the glass so as to form lead sulphide. The colour formed is now matched in a similar glass, using a standard solution of lead nitrate, and forming the lead sulphide as before.
Some have advocated the magnesium wire deposition test, originally devised by von Jaksch, and modified by Hill Abram, for the detection of lead in the urine of persons who are suspected to be suffering from chronic lead poisoning (see [ante]).
The colorimetric method of estimating lead has been made use of with very great success by Duckering in the estimation of lead in the air of potteries. The method is that devised by Mr. Vernon Harcourt[8], with a few modifications. The whole of the method is given, as observation on the quantity of lead present in the air of lead factories helps greatly in suggesting rational methods of precaution.
“After drying and weighing the filters, the following is the method. Solutions required:
“Nitric Acid.—One part of pure concentrated nitric acid to three parts of water.