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 (PtCl42KCl), containing 0.5 gram platinum, and 1.00 gram crystallized cobalt chloride (CoCl2.6H2O), 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. |
COMPARISON WITH GLASS DISKS.[[105]]
As the platinum-cobalt standard method is not well adapted for field work, the color of the water to be tested may be compared with that of glass disks held at the end of metallic tubes through which they are viewed by looking toward a white surface. The glass disks are individually calibrated to correspond with colors on the platinum scale. Experience has shown that the glass disks used by the U. S. Geological Survey give results in substantial agreement with those obtained by the platinum determinations, and their use is recognized as a standard procedure.
COMPARISON WITH NESSLER STANDARDS.
Inasmuch as the Nessler scale[[62]] and the natural water scale[[22]][[49]] which agrees with it except for colors less than 20, have been largely used in the past, the old results may be converted[[117]] into terms of the platinum standard by means of the ratios in Table 3, but they must not be considered as universally applicable as the variable sensitiveness of the Nessler solution introduces an uncertain factor.
| Table 3.—Values for converting colors by the natural water scale into colors by the platinum standard in parts per million.[[B]] | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Modified Nessler or natural water standard. | 0.00. | 0.01. | 0.02. | 0.03. | 0.04. | 0.05. | 0.06. | 0.07. | 0.08. | 0.09. |
| Platinum-cobalt standard color. | ||||||||||
| 0.00 | 0 | 2 | 4 | 6 | 8 | 9 | 11 | 13 | 15 | 17 |
| .10 | 18 | 19 | 20 | 20 | 21 | 22 | 23 | 24 | 24 | 26 |
| .20 | 26 | 27 | 27 | 28 | 29 | 29 | 30 | 31 | 32 | 32 |
| .30 | 33 | 34 | 34 | 35 | 35 | 36 | 37 | 37 | 38 | 38 |
| .40 | 39 | 40 | 40 | 41 | 42 | 42 | 43 | 44 | 45 | 45 |
| .50 | 46 | 47 | 47 | 48 | 48 | 49 | 50 | 50 | 51 | 51 |
| .60 | 52 | 53 | 53 | 54 | 54 | 55 | 56 | 56 | 57 | 57 |
| .70 | 58 | 58 | 59 | 59 | 60 | 60 | 61 | 61 | 62 | 62 |
| .80 | 63 | 64 | 64 | 65 | 66 | 66 | 67 | 68 | 69 | 69 |
| .90 | 70 | 71 | 72 | 73 | 74 | 75 | 77 | 78 | 79 | 80 |
| 1.00 | 81 | 82 | 82 | 83 | 84 | 84 | 85 | 86 | 87 | 87 |
| 1.10 | 88 | 89 | 89 | 90 | 91 | 91 | 92 | 93 | 94 | 94 |
| 1.20 | 95 | 96 | 96 | 97 | 98 | 98 | 99 | 100 | 101 | 101 |
| 1.30 | 102 | 103 | 103 | 104 | 105 | 105 | 106 | 107 | 108 | 108 |
| 1.40 | 109 | 110 | 110 | 111 | 112 | 112 | 113 | 114 | 115 | 115 |
| 1.50 | 116 | 117 | 117 | 118 | 118 | 119 | 120 | 120 | 121 | 121 |
| 1.60 | 122 | 123 | 123 | 124 | 125 | 125 | 126 | 127 | 128 | 128 |
| 1.70 | 129 | 130 | 130 | 131 | 132 | 132 | 133 | 134 | 135 | 136 |
| 1.80 | 136 | 137 | 137 | 138 | 139 | 139 | 140 | 141 | 142 | 142 |
| 1.90 | 143 | 144 | 144 | 145 | 146 | 146 | 147 | 148 | 149 | 149 |
| 2.00 | 150 | |||||||||
[B]. Zero on the true Nessler scale is about 15 on the platinum scale.
LOVIBOND TINTOMETER.
The value of the readings of tint and shade by the Lovibond tintometer[[66]][[82]][[83]] has not been commensurate with the labor involved, but it is necessary to make a record of the reflected tint and shade[[50]] of some waters. The standard color disks used in teaching optics may be used for the purpose.
Procedure.—The white disk supports three movable standard color sectors, red, yellow, and blue, and one movable black sector. All are mounted on a device which can be revolved rapidly, blending the colors into a uniform tint or shade. A scale around the circumference of the disk is used to indicate the percentage of each color or white or black in the blend.
Place the sample in a battery jar on a white ground; adjust the sectors so that when blended the tint or shade will match the reflected tint or shade of the sample. Report the percentages of red, yellow blue, white, and black in the blended tint or shade.
ODOR.[[4]][[14]][[53]][[72]][[92]][[114]][[115]][[121c]]
The observation of the odor, cold and hot, of samples of surface water is important as the odors are usually indicative of organic growths or sewage contamination or both. The odor of some ground waters is caused by the earthy constituents of the water-bearing strata. The odor of a contaminated well water is often contributory evidence of its pollution. A study of the organisms as directed under Microscopical Examination (p. [90]) is a valuable adjunct to physical and chemical examination of water. Certain odors distinguish or identify certain organisms, as, for example, the “fishy” odor of Uroglena, the “aromatic” or “rose geranium” odor of Asterionella and the “pig pen” odor of Anabaena. Observe and record the odor, both at room temperature and at just below the boiling point, as follows:
COLD ODOR.
Shake the sample violently in one of the collecting bottles, when it is half to two-thirds full and when the sample is at room temperature (about 20° C.). Remove the stopper and smell the odor at the mouth of the bottle.
HOT ODOR.
Pour about 150 cc. of the sample into a 500 cc. Erlenmeyer flask. Cover the flask with a well-fitting watch glass. Heat the water almost to boiling on a hot plate. Remove the flask from the plate and allow it to cool not more than five minutes. Then agitate it with a rotary movement, slip the watch glass to one side, and smell the odor.