TEMPERATURE.

The temperature of the sample, if taken, shall be taken at the time of collection, and shall be expressed preferably in degrees Centigrade, to the nearest degree, or closer if more precise data are required. The thermophone^[109] is recommended for obtaining the temperature of water at various depths below the surface.

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]

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.

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

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.

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.

EXPRESSION OF RESULTS.

Express the quality of the odor by a descriptive epithet like the following, which may be abbreviated in the record:

Express the intensity of the odor by a numeral prefixed to the term expressing quality, which may be defined as follows:

Numerical value.	Term.	Definition.
0	None.	No odor perceptible.
1	Very faint.	An odor that would not be detected ordinarily by the average consumer, but that could be detected in the laboratory by an experienced observer.
2	Faint.	An odor that the consumer might detect if his attention were called to it, but that would not attract attention otherwise.
3	Distinct.	An odor that would be detected readily and that might cause the water to be regarded with disfavor.
4	Decided.	An odor that would force itself upon the attention and that might make the water unpalatable.
5	Very strong.	An odor of such intensity that the water would be absolutely unfit to drink. (A term to be used only in extreme cases.)