Method of Detecting Lead, When Contained in Water
One of the most delicate tests for detecting lead, is water impregnated with sulphuretted hydrogen gas, which instantly imparts to the fluid containing the minutest quantity of lead, a brown or blackish tinge.
This test is so delicate that distilled water, when condensed by a leaden pipe in a still tub, is affected by it. To shew the action of this test, the following experiments will serve.
EXPERIMENT.
Pour into a wine-glass containing distilled water, an equal quantity of water impregnated with sulphuretted hydrogen gas: no change will take place; but if a 1/4 of a grain of acetate of lead (sugar of lead of commerce), or any other preparation of lead, be added, the mixture will instantly turn brown and dark-coloured.
To apply this test, one part of the suspected water need merely to be mingled with a like quantity of water impregnated with sulphuretted hydrogen. Or better, a larger quantity, a gallon for example, of the water may be concentrated by evaporation to about half a pint, and then submitted to the action of the test.
Another and more efficient mode of applying this test, is, to pass a current of sulphuretted hydrogen gas through the suspected water in the following manner.
EXPERIMENT.
Take a bottle (a) or Florence flask, adapt to the mouth of it a cork furnished with a glass tube (b), bent at right angles; let one leg of the tube be immersed in the vial (c) containing the water to be examined; as shewn in the following sketch. Then take one part of sulphuret of antimony of commerce, break it into pieces of half the size of split pease, put it into the flask, and pour upon it four parts of common concentrated muriatic acid (spirit of salt of commerce). Sulphuretted hydrogen gas will become disengaged from the materials in abundance, and pass through the water in the vial (c). Let the extrication of the gas be continued for about five minutes; and if the minutest quantity of lead be present, the water will acquire a dark-brown or blackish tinge. The extrication of the gas is facilitated by the application of a gentle heat.
The action of the sulphuretted hydrogen test, when applied in this manner, is astonishingly great; for one part of acetate of lead may be detected by means of it, in 20000 parts of water.24
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. Thomson25 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. Silvester26 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.
15 It is absolutely essential that the tests should be pure.
16 Philosophical Magazine, vol. xv. p. 252.
17 Transactions of the Royal Society of Edinburgh, vol. viii. p. 259.
18 Sir G. Baker, Med. Trans. vol. i. p. 280.
19 Lamb on Spring Water.
20 Medical Trans. vol. i. p. 420.
21 Van Swieten ad Boerhaave, Aphorisms, 1060. Comment.
22 Medical Comment. Dec. 2, 1794.
23 Lambe on Spring Water.
24 See An Analysis of the Mineral Waters of Tunbridge Wells, by Dr. Scudamore, p. 55.
The application of the sulphuretted hydrogen test requires some precautions in those cases where other metals besides lead may be expected; because silver, quicksilver, tin, copper, and several other metals, are affected by it, as well as lead; but there is no chance of these metals being met with in common water.—See Chemical Tests, third edition, p. 207.
25 Analysis of Tunbridge Wells Water, by Dr. Scudamore, p. 55.
26 Nicholson’s Journal, p. 33, 310.