18.0│ 120 19.6│ 110 21.5│ 100 ───────────────────────────────────┴─────────────────────────────────── The results of turbidity observations shall be expressed in whole numbers which correspond to parts per million of silica and recorded as follows: Turbidity between │ 1│and│ 50│recorded to nearest│unit " " │ 51│ " │ 100│ " " " │ 5 " " │ 101│ " │ 500│ " " " │ 10 " " │ 501│ " │ 1000│ " " " │ 50 " " │1001│ " │greater│ " " " │ 100 COEFFICIENT OF FINENESS[80] The quotient obtained by dividing the weight of suspended matter in the sample by the turbidity, both expressed in the same unit, shall be called the coefficient of fineness. If the quotient is greater than unity the matter in suspension is coarser and if it is less than unity it is finer than the standard. COLOR. The “color,” or the “true color,” of water shall be considered the color that is due only to substances in solution; that is, it is the color of the water after the suspended matter has been removed. In stating results the word “color” shall mean the “true color” unless otherwise designated. The “apparent color” shall be considered as including not only the true color but also any color produced by substances in suspension. It is the color of the original unfiltered sample. The platinum-cobalt method of measuring color shall be considered as the standard, and the unit of color shall be that produced by 1 part per million of platinum. COMPARISON WITH PLATINUM-COBALT STANDARDS.[43] _Reagents._—Dissolve 1.246 grams of potassium platinic chloride (PtCl_{4}2KCl), containing 0.5 gram platinum, and 1.00 gram crystallized cobalt chloride (CoCl_{2}.6H_{2}O), containing 0.25 gram of cobalt, in water with 100 cc. concentrated hydrochloric acid, and dilute to 1 liter with distilled water. This solution has a color of 500. Dilute this solution with distilled water in 50 cc. Nessler tubes to prepare standards having colors of 0, 5, 10, 15, 20, 25, 30, 35, 40, 50, 60, and 70. Keep these standards in Nessler tubes of such diameter that the graduation mark is between 20 and 25 cm. above the bottom and of such uniformity that they match within such limit that the distance from the bottom to the graduation mark of the longest tube shall not exceed that of the shortest tube by more than 6 mm. Protect the tubes from dust and light when not in use.