_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.
_Procedure._—The color of a sample shall be observed by filling a
standard Nessler tube to the height equal to that in the standard tubes
with the sample and by comparing it with the standards. The observation
shall be made by looking vertically downward through the tubes upon a
white or mirrored surface placed at such angle that light is reflected
upward through the column of liquid.
Water that has a color greater than 70 shall be diluted before making
the comparison, in order that no difficulties may be encountered in
matching the hues.
Water containing matter in suspension shall be filtered, before the
color observation is made, until no visible turbidity remains. If the
suspended matter is coarse, filter paper may be used for this purpose;
if the suspended matter is fine, the use of a Berkefeld filter is
recommended. The Pasteur filter shall not be used as it exerts a marked
decolorizing action.
The apparent color, if determined, shall be determined on the original
sample without filtration. The true and the apparent color of clear
waters or waters with low turbidities are substantially the same.
The results of color determinations shall be expressed in whole numbers
and recorded as follows:
Color between 1 and 50 recorded to nearest unit
" " 51 " 100 " " " 5
" " 101 " 250 " " " 10
" " 251 " 500 " " " 20.