Another test for readily detecting lead in water, is sulphuretted chyazate of potash, first pointed out as such by Mr. Porret. A few drops of this re-agent, added to water containing lead, occasion a white precipitate, consisting of small brilliant scales of a considerable lustre.
Sulphate of potash, or sulphate of soda, is likewise a very delicate test for detecting minute portions of lead. Dr. Thomson[25] discovered, by means of it, one part of lead in 100000 parts of water; and this acute Philosopher considers it as the most unequivocal test of lead that we possess. Dr. Thomson remarks that "no other precipitate can well be confounded with it, except sulphate of barytes; and there is no probability of the presence of barytes existing in common water."
Carbonate of potash, or carbonate of soda, may also be used as agents to detect the presence of lead. By means of these salts Dr. Thomson was enabled to detect the presence of a smaller quantity of lead in distilled water, than by the action of sulphuretted hydrogen. But the reader must here be told, that the use of these tests cannot be entrusted to an unskilful hand; because the alkaline carbonates throw down also lime and magnesia, two substances which are frequently found in common water; the former tests, namely, water impregnated with sulphuretted hydrogen gas, and nascent sulphuretted hydrogen, are therefore preferable.
It is absolutely essential that the water impregnated with sulphuretted hydrogen, when employed as a test for detecting very minute quantities of lead, be fresh prepared; and if sulphate of potash, or sulphate of soda, be used as tests, they should be perfectly pure. Sulphate of potash is preferable to sulphate of soda. It is likewise advisable to act with these tests upon water concentrated by boiling. The water to which the test has been added does sometimes appear not to undergo any change, at first; it is therefore necessary to suffer the mixture to stand for a few hours; after which time the action of the test will be more evident. Mr. Silvester[26] has proposed gallic acid as a delicate test for detecting lead.
FOOTNOTES:
[11] Dalton, Manchester Memoirs, vol. iv. p. 55.
[12] Marsden's History of Sumatra.
[13] Manchester Memoirs vol. x. 1819.
[14] Observations on the Water with which Tunbridge Wells is chiefly supplied for Domestic Purposes, by Dr. Thomson; forming an Appendix to an Analysis of the Mineral Waters of Tunbridge Wells, by Dr. Scudamore.