TURBIDITY.
The turbidity of water is due to suspended matter, such as clay, silt, finely divided organic matter, microscopic organisms, and similar material.
TURBIDITY STANDARD.[[110]]
The standard of turbidity shall be that adopted by the United States Geological Survey, namely, a water which contains 100 parts per million of silica in such a state of fineness that a bright platinum wire 1 millimeter in diameter can just be seen when the center of the wire is 100 millimeters below the surface of the water and the eye of the observer is 1.2 meters above the wire, the observation being made in the middle of the day, in the open air, but not in sunlight, and in a vessel so large that the sides do not shut out the light so as to influence the results. The turbidity of such water is arbitrarily fixed at 100 parts per million.
For preparation of the silica standard dry Pear’s “precipitated fuller’s earth” and sift it through a 200–mesh sieve. One gram of this preparation in 1 liter of distilled water makes a stock suspension which contains 1,000 parts per million of silica and which should have a turbidity of 1,000. Test this suspension, after diluting a portion of it with nine times its volume of distilled water, by the platinum wire method to ascertain if the silica has the necessary degree of fineness and if the suspension has the necessary degree of turbidity. If not, correct by adding more silica or more water as the case demands.[[A]]
[A]. This method of correction very slightly alters the coefficient of fineness of the standard, but does not noticeably affect its use.
Standards for comparison shall be prepared from this stock suspension by dilution with distilled water. For turbidity readings below 20, standards of 0, 5, 10, 15, and 20 shall be kept in clear glass bottles of the same size as that containing the sample; for readings above 20, standards of 20, 30, 40, 50, 60, 70, 80, 90, and 100 shall be kept in 100 cc. Nessler tubes approximately 20 millimeters in diameter.
Comparison with the standards shall be made by viewing both standard and sample sidewise toward the light by looking at some object and noting the distinctness with which the margins of the object can be seen.
The standards shall be kept stoppered, and both sample and standards shall be thoroughly shaken before making the comparison.
In order to prevent any bacterial or algal growths from developing in the standards a small amount of mercury bichloride may be added to them.
PLATINUM WIRE METHOD.[[42]]
This method requires a rod with a platinum wire 1 mm. in diameter inserted in it about 1 inch from one end of the rod and projecting from it at a right angle at least 25 mm. Near the other end of the rod, at a distance of 1.2 meters from the platinum wire, a small ring shall be placed directly above the wire through which, with his eye directly above the ring, the observer shall look when making the examination.
The rod shall be graduated as follows: The graduation mark of 100 shall be placed on the rod at a distance of 100 mm. from the center of the wire. Other graduations shall be made according to Table 1, which is based on the best obtainable data. The distances recorded in Table 1 are intended to be such that when the water is diluted the turbidity readings will decrease in the same proportion as the percentage of the original water in the mixture. These graduations are those on what is known as the U. S. Geological Survey Turbidity Rod of 1902.[[105]]
| Table 1.—Graduation of turbidity rod. | |
|---|---|
| Turbidity (parts per million). | Vanishing depth of wire (mm.). |
| 7 | 1095 |
| 8 | 971 |
| 9 | 873 |
| 10 | 794 |
| 11 | 729 |
| 12 | 674 |
| 13 | 627 |
| 14 | 587 |
| 15 | 551 |
| 16 | 520 |
| 17 | 493 |
| 18 | 468 |
| 19 | 446 |
| 20 | 426 |
| 22 | 391 |
| 24 | 361 |
| 26 | 336 |
| 28 | 314 |
| 30 | 296 |
| 35 | 257 |
| 40 | 228 |
| 45 | 205 |
| 50 | 187 |
| 55 | 171 |
| 60 | 158 |
| 65 | 147 |
| 70 | 138 |
| 75 | 130 |
| 80 | 122 |
| 85 | 116 |
| 90 | 110 |
| 95 | 105 |
| 100 | 100 |
| 110 | 93 |
| 120 | 86 |
| 130 | 81 |
| 140 | 76 |
| 150 | 72 |
| 160 | 68.7 |
| 180 | 62.4 |
| 200 | 57.4 |
| 250 | 49.1 |
| 300 | 43.2 |
| 350 | 38.8 |
| 400 | 35.4 |
| 500 | 30.9 |
| 600 | 27.7 |
| 800 | 23.4 |
| 1000 | 20.9 |
| 1500 | 17.1 |
| 2000 | 14.8 |
| 3000 | 12.1 |
Procedure.—Lower the rod vertically into the water as far as the wire can be seen and read the level of the surface of the water on the graduated scale. This will indicate the turbidity.
The following precautions shall be taken to insure correct results:
Observations shall be made in the open air, preferably in the middle of the day and not in direct sunlight. The wire shall be kept bright and clean. If for any reason observations cannot be made directly under natural conditions a pail or tank may be filled with water and the observation taken in that, but if this is done care shall be taken that the water is thoroughly stirred before the observation is made, and no vessel shall be used for this purpose unless its diameter is at least twice as great as the depth to which the wire is immersed. Waters which have a turbidity greater than 500 shall be diluted with clear water before the observations are made, but if this is done the degree of dilution shall be reported.
TURBIDIMETRIC METHOD.
Several forms of turbidimeter or diaphanometer[[73]] have been suggested for use. The simplest and most satisfactory form is the candle turbidimeter.[[116]] This consists of a graduated glass tube with a flat polished bottom, enclosed in a metal case. This is supported over an English standard candle and so arranged that one may look vertically down through the tube at the flame of the candle. The observation is made by pouring the sample of water into the tube until the image of the flame of the candle just disappears from view. Care shall be taken not to allow soot or moisture to accumulate on the lower side of the glass bottom of the tube so as to interfere with the accuracy of the observations. The graduations on the tube correspond to turbidities produced in distilled water by certain numbers of parts per million of silica standard. In order to insure uniform results it is necessary to have the distance between the top rim of the candle and the bottom of the tube constant, and this distance shall be 7.6 cm. or 3 inches. The observations shall be made in a darkened room or with a black cloth over the head.
It is allowable to substitute for the candle an electric light. Calibrate the apparatus to correspond with the United States Geological Survey scale. The figures in Table 2 on page [8] are believed to be approximately correct for the candle turbidimeter but should be checked by the experimenter. It is allowable to calibrate the tube of the instrument with waters of known turbidity prepared by making a series of dilutions of the silica standard with distilled water. From the figures obtained in calibrating plot a curve from which the turbidity of a sample may be read when the depth of water in the tube has been obtained.
| Table 2.—Graduation of candle turbidimeter. | |
|---|---|
| Depth of liquid (cm.). | Turbidity (parts per million of silica). |
| 2.3 | 1000 |
| 2.6 | 900 |
| 2.9 | 800 |
| 3.2 | 700 |
| 3.5 | 650 |
| 3.8 | 600 |
| 4.1 | 550 |
| 4.5 | 500 |
| 4.9 | 450 |
| 5.5 | 400 |
| 5.6 | 390 |
| 5.8 | 380 |
| 5.9 | 370 |
| 6.1 | 360 |
| 6.3 | 350 |
| 6.4 | 340 |
| 6.6 | 330 |
| 6.8 | 320 |
| 7.0 | 310 |
| 7.3 | 300 |
| 7.5 | 290 |
| 7.8 | 280 |
| 8.1 | 270 |
| 8.4 | 260 |
| 8.7 | 250 |
| 9.1 | 240 |
| 9.5 | 230 |
| 9.9 | 220 |
| 10.3 | 210 |
| 10.9 | 200 |
| 11.4 | 190 |
| 12.0 | 180 |
| 12.7 | 170 |
| 13.5 | 160 |
| 14.4 | 150 |
| 15.4 | 140 |
| 16.6 | 130 |
| 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.