─────────────────────────────────────────────────────────────────────── 11. Concentrate sample to 75 cc. Add 2 cc. conc. H_{2}SO_{4} for clear, soft waters and 5 cc. for alkaline or turbid waters. Electrolyze following procedure in 7 and 8. ─────────────────────────────────────────────────────────────────────── Table 12.—SCHEME FOR DETERMINATION OF ZINC ONLY. ─────────────────────────────────────────────────────────────────────── 13. Follow scheme for all three metals as given in Table 10 through section 5. Nearly neutralize the filtrate with H_{2}SO_{4}, concentrate to less than the capacity of the dish and electrolyze as directed in section 9. ─────────────────────────────────────────────────────────────────────── MINERAL ANALYSIS. RESIDUE ON EVAPORATION. See description of method (p. 29). The residue should be dried one hour at 180° C. Turbid waters should be filtered, and the composition of the suspended matter should be determined separately or the amount of it reported as suspended matter. ALKALINITY AND ACIDITY. See description of method (pp. 35–41). CHLORIDE. See description of method (pp. 41–43). NITRATE NITROGEN. See description of method (pp. 23–25). SEPARATION OF SILICA, IRON, ALUMINIUM, CALCIUM, AND MAGNESIUM.[10][48] SILICA. Evaporate in platinum 100 to 1,000 cc. of the sample or sufficient if possible to form a residue weighing 0.4 to 0.6 gram, and preferably containing 0.1 to 0.2 gram of calcium. When the residue is nearly dry add 1 cc. of hydrochloric acid (1 to 1) and, after moistening the sides of the dish, evaporate to dryness. Dry at 180° C. and if much organic matter is present char it in a radiator. Moisten the residue with dilute hydrochloric acid and expel the excess of acid by heating on the water bath. Add a few drops of hydrochloric acid, dissolve in hot water, and filter. Wash the residue with hot water. Evaporate the filtrate to dryness, repeat the filtration, and combine the two residues. If great accuracy is not required the second evaporation with hydrochloric acid may be omitted. Ignite and weigh the insoluble residue. Add 2 drops of concentrated sulfuric acid and a little hydrofluoric acid, volatilize the acids, ignite, and weigh again. Report the loss in weight as silica (SiO_{2}). A weight of non-volatile matter exceeding 0.5 mg. should be analyzed. IRON AND ALUMINIUM.