“A strip of pure magnesium is placed in the fluid to be examined. Ammonium oxalate in the proportion of about 1 gramme to 150 c.c. is added. If lead is present, it is deposited on the magnesium. A deposit is seen within half an hour, but we have usually left it twenty-four hours. The slip is then washed with distilled water and dried. Confirmatory tests: (1) Warm the slip with a crystal of iodine (yellow iodide proves lead, cadmium may be ignored); (2) dissolve deposit in HNO3, and apply usual tests. The magnesium can be used again after careful washing with acid and distilled water. The surface of the magnesium, when used, must be bright and free from oxide. The delicacy of the method has been tested with aqueous solutions containing known quantities of lead, also with normal urine to which known quantities of lead have been added. In all cases a control experiment was performed to insure the freedom of the materials from lead. Lead has been detected when present in the proportion of 1 part to 50,000, whether in simple aqueous solution or in urine.”
Shufflebotham and Mellor[3] describe the following method as one by which lead may be detected in organic tissue, and in each case this necessitated a large amount of evaporation. The method has value, but the difficulty of dealing with large quantities of fuming nitric acid, and the addition of this acid from time to time during the operations, render it difficult unless a good fume chamber is at hand. Shufflebotham and Mellor state that they obtained no reaction with the potassium chloride-hydrochloric acid method suggested by Dixon Mann.
“On the Detection of Lead in Urine and Post-Mortem Specimens.—A piece of kidney of 20 c.c. capacity was cut up into about a dozen pieces. These were placed in an evaporating basin, and about 50 c.c. of fuming nitric acid were poured into the dish. Dense brown fumes of nitrogen oxides were evolved. When the action had subsided (in from two to three minutes), the dish was placed upon a sheet of asbestos, and allowed to simmer over the Bunsen flame for about an hour. If the frothing appears in danger of running over the sides of the dish, stirring with a glass rod or removal of the flame for a short time may be necessary. Twenty-five c.c. of the fuming acid were added at intervals of a quarter of an hour, and this process was repeated three times. The destruction of the organic matter was so complete that the whole of the piece of kidney passed into complete solution. The solution was then evaporated down to a few c.c., neutralized with caustic soda, filtered, and treated with hydrogen sulphide. A dark-brown precipitate of lead sulphide was obtained. With potassium chromate a yellow precipitate of lead chromate was obtained with the same specimen of kidney which gave a negative result with the KClO3-HCl method of destroying the organic matter. Our reagents, dishes, etc., were then examined with a blank test, but we found no lead.
“Urine.—We then sought the presence of lead in the urine of Cases 2, 3, and 4. Half a gallon of urine was evaporated down to dryness in each of two basins. In one basin the residue was heated until it was charred. Both residues were then treated separately with fuming nitric acid, as just described. The uncharred residue passed into solution, and on cooling deposited a white sediment. The mother-liquor was neutralized and tested in the usual way. A brown precipitate of lead sulphide was obtained in Case 2, while in Case 3 a well-marked black precipitate was obtained. The urine of Case 4 gave a negative result. The charred residue did not pass completely into solution, and the tests for lead were not so well defined as when the residue was uncharred. This shows that care must be taken to prevent charring of the residue during evaporation.”
A method has recently been described by Hebert[4]—a modification of Trillet’s. This method is based upon the fact that peroxide of lead, when mixed with tetramethyl of diphenyl-methylen, gives in acetic acid solution a fine blue coloration. Unfortunately, a number of other peroxides give the same blue coloration, amongst them manganese, potash, copper, magnesium. In addition, the sodium peroxide used to convert the lead present into the peroxide also gives a bright blue coloration with the reagent, even if present in minute quantities.
The test is made in the following manner: The substance is incinerated, sulphuric acid added in the usual way, and the substance evaporated to dryness. It is treated with a cold solution of sodium hypochlorite. The hypochlorite is then removed partly by washing and partly by heating, and the reagent is then added directly to the substance in the capsule, and if a peroxide is present the blue colour results.
Unfortunately, this dissociation-point of the hypochlorite and the temperature at which peroxide of lead is changed back again to the oxide are very close together, being only about 25° C. In addition, it is very difficult to remove the last traces of the substances which give a blue coloration in addition to lead. One of us (K. W. G.) has made extensive trials with this method, as, if it had been a reliable process, it would have been one which would have considerably facilitated the estimation of lead in small quantities. The method has been adopted by certain French observers, who by drawing 2 c.c. of blood from the median basilic vein, and estimating the lead present in this small amount by means of the blue coloration, have sought to show that at least 25 milligrammes of lead were circulating in the blood of the body. In addition to other grave considerations, the fact that the reagent itself is colourable by certain other peroxides which exist in the blood-ash renders these figures entirely untrustworthy.
2. Quantitative Methods of Estimation.
—Two methods may be used in the detection of lead in organic substances, either in organic fluids or in solids, and are generally termed the “wet” and “dry” methods, from the original treatment of the substance.
In the dry method the substance is incinerated with or without the addition of sulphuric and nitric acid; in the wet the material is treated with hydrochloric acid and potassium chlorate. Subsequent treatment in both cases is on the same lines.