Color-producing matters and turbidity-producing matters are different in their natures, and the methods which must be used to remove them are different.
THE MEASUREMENT OF COLOR.
The colors of waters are measured and recorded by comparing them with colors of solutions or substances which are permanent, or which can be reproduced at will. One of the earliest methods of measuring colors of waters was to compare them with the colors of the Nessler standards used for the estimation of ammonia in water analysis. The Nessler standards were similar in appearance to yellow waters, and their colors depended upon the amounts of ammonia which had been used in preparing them, and a record was made of the standard which most closely resembled the water under examination.
The method was open to the serious objections that the hues of the standards did not match closely the hues of the waters; that the colors produced with different lots of Nessler reagent differed considerably, and therefore the exact values of results were more or less uncertain; and further, that the numbers obtained for color were not even approximately proportional to the amounts of coloring matter present. Because of this peculiarity, in filtration the percentage of color removal, as determined by the use of these standards, is not even approximately correct, but is much above the truth.
In the Lovibond tintometer, which has been extensively used in England, the standards of color are based upon the colors of certain glass slips, which are in turn compared with standard originals kept for that purpose. This process answers quite well, but is open to some objections because of possible uncertainties in the standardization of the units.
Another method of measuring colors is to compare them with dilute solutions of platinum and cobalt. The ratio of cobalt to platinum can be varied to make the hue correspond very closely with the hues of natural waters, and the amount of platinum required to match a water affords a measure of its color, one part of metallic platinum in 10,000 parts of water forming the unit of color.
This standard has the advantages that it can be readily prepared with absolute accuracy in any laboratory, and that by varying the ratio of platinum to cobalt the hues of various waters can be most perfectly matched. It is important that the observations should not be made in too great a depth, as the discrepancy in hues increases much more rapidly than the depth of color.
For further information regarding colors the reader is referred to articles in the American Chemical Journal, 1892, vol. xiv, page 300; Journal of the American Chemical Society, vol. ii, page 8; vol. xviii, 1896, pp. 68, 264, and 484; Journal of the Franklin Institute, Dec. 1894, p. 402; Journal of the New England Water Works Association, vol. xiii, 1898, p. 94.
AMOUNT OF COLOR IN AMERICAN WATERS.
New England surface-waters have colors ranging from almost nothing up to 2.00. The colors of the public water-supplies of Massachusetts cities have been recorded in the reports of the State Board of Health for some ten years. The figures given were recorded first upon the Nessler standard, and afterwards upon a modification of the same, known as the natural water standard. The figures given are approximately equal to those for the platinum color standard, the relations between the two having been frequently determined by various observers and published in the above-mentioned papers. The accompanying diagram shows the colors in several Massachusetts supplies, as plotted from the figures given in the published reports.