7. Potassium sulfide. An alkaline solution of potassium sulfide made by mixing equal volumes of 10 per cent potassium hydroxide and a saturated aqueous solution of hydrogen sulfide. 8. Potassium oxalate. Crystals. 9. Potassium sulfate. Crystals. 10. Alcohol. Ninety-five per cent. 11. Alcohol. Fifty per cent. 12. Acetic acid. Fifty per cent. 13. Nitric acid. Concentrated acid (Sp. gr. 1.42). 14. Nitric acid. Dilute 1 part of the concentrated acid to 10 parts with distilled water. 15. Hydrochloric acid. (Sp. gr. 1.20.) 16. Sulfuric acid. Concentrated acid (Sp. gr. 1.84). 17. Sulfuric acid. Dilute the concentrated acid with an equal volume of distilled water. 18. Urea. Crystals. LEAD. Concentrate (1)[D] rapidly by boiling in a 7–inch porcelain dish over a free flame 3 or 4 liters of the sample to be tested, or more if very small amounts of the metals are present, to a volume of about 30 cc. Add 10 or 15 cc. of ammonium chloride solution to assist in the separation of the sulfides, then add a few drops of concentrated ammonium hydroxide, and saturate with hydrogen sulfide. Allow to stand some time, preferably over night, add a little more ammonium hydroxide and hydrogen sulfide, boil the contents of the dish a few minutes, and filter. The precipitate (2) may consist of lead, zinc, copper, and iron sulfides and the suspended organic matter. The soluble coloring matter is in the filtrate (3). Wash the precipitate a few times with hot water, place the precipitate and the filter paper in the original dish and boil with dilute nitric acid, rubbing down the sides of the dish, if necessary, to detach any adhering sulfide precipitate. After again filtering and washing several times with hot water, evaporate the filtrate and washings in the original dish to a bulk of 10 to 15 cc., cool, add 5 cc. of concentrated sulfuric acid, and heat until copious fumes of sulfuric acid are evolved. Footnote D: The numbers in parentheses refer to tables 10–12, pages 55–56. If lead is present dilute the contents of the dish slightly with water, and treat them with 150 cc. of 50 per cent alcohol, in which the lead sulfate is insoluble. Allow to stand some time, preferably over night, filter off the lead sulfate, and wash it with 50 per cent alcohol. Save the filtrate for the determination of zinc.