I N D C S T R I A L A-VD ENGINEERING C H E M I S T R Y
832
Vol. l i , S o . 8
Permanent Standards as a Possible Source of Error in Iron Determinations’ By Edward S. Hopkins BALTIMORE C I T Y R’ATER D E P A R T M E N T , BALTIMORE, M D .
ERMAXENT color standards as the measure for small amounts of iron have been recommended in Standard Methods of Water Analysis (American Public Health Association) for years. Since the procedure of analysis as outlined in this text is the guide for practically every water and sewage laboratory in this country, any erratic condition therein is of interest. The 1917 and subsequent editions recommend the use of 50-cc. Nessler tubes for comparison of the colors produced by potassium thiocyanate and iron solutions when using the potassium platinic-cobalt chloride permanent standards. Previous to that issue the 100-cc. tube was recommended. As a matter of record, although the 50-cc. tube was standard, the 1920 edition gave a platinic-cobalt table equal to making the total volume 100 cc. The 100-cc. tube is preferable to the 50-cc. tube. A sharper differentiation in color is obtained, making possible more accurate comparisons and increased number of tubes, and the numerical value of the iron solution to the standard color solution is in ratio of 1: 1rather than 1: 2. It has been realized in this laboratory for a number of years that the tint and intensity of color produced by the cobalt chloride solution was in excess of that produced by a corresponding iron solution, and that comparison was extremely difficult. Because of this condition, instead of using the arbitrary amounts of cobalt chloride for the preparation of the standards, as given in the text, only the amount necessary to produce a tint comparable with the given iron color is added. Recently Whipple2 showed that the cobalt chloride usually purchased varied in composition and stated that if the correct formula (CoC12.6Hz0) was used the tints would match when the standards were made in accordance with standard methods. Assuming that Jackson3 and Whipple2 used the so-called “standard 50-cc. tube,” in which the mark is 210 mm. high and the tube 17 mm. in diameter, it is of interest to note the variations in tint of color when using Nessler tubes of other measurements. There are four different Xessler tubes being sold today. Their measurements are:
P
Tube 1 2 3 4 5
Standard 50-cc. tube Low form 50-cc. 50 cc. using 100-cc. tube Standard 100-cc. tube Low form 100-cc. tube
Height of mark Mm. 204 141 76 282 152
Diameter Mm. 17 21 2s
17 28
When the standard amounts of cobalt chloride solution, as given in the text, are placed in the various tubes and diluted to the mark, the colors produced are in excess of those given by iron solutions of the same value, as shown in the accompanying table and curve. Tube 1 2 3 ’ 4 5
IRONVALUEIN COLORSOLUTIONS-----0.03 0.05 0.07 0.10 Mg., F e necessary to match color tints 0.095 0.073 0.055 0.012 0.035 0.115 0.093 0.065 0,040 0.013 0.155 0.105 0.075 0.045 0.014 0.160 0.120 0.085 0.055 0.020 0.210 0.155 0,100 0.055 0.020 0.01
0.15
0.130 0.155 0.215 0.205 0.300
1 Presented before the Division of Water Sewage and Sanitation at the 69th Meeting of the American Chemical Society, Baltimore, hld., April 6 to 10, 1925. 2 Exg. 5 Contr., 62, 80 (1924). 8 Tech. Quart., 1%320 (1900).
It will be noticed in the table that an excess of iron varying from 0.002 mg. to 0.15 mg., depending upon the size of tube and concentration of standards, is necessary to match the tints. Comparison of these colors was made by looking vertically downward through the tubes upon a white or mirrored surface placed a t such an angle that the light may reflect upward through the column of liquid. It is recognized that the difference in tints between the standards and iron solutions, together with the question of eye sensitiveness to color, may in the hands of another analyst slightly vary the figures presented here.
A
s
Iron value in color solutions Base line represents correct amount of Fe solution to match color solution A m o u n t s of Iron S o l u t i o n Necessary to M a t c h T i n t of P e r m a n e n t Color Standards
Six different iron salts, three lots of ammonium thiocyanate, and five lots of potassium thiocyanate were checked against each other. Color intensities produced by the different lots of potassium thiocyanate gave similar results, but the ammonium thiocyanate produced excessive color corresponding to 0.1 mg. iron in most instances. No discrepancy existed between the various iron salts. In conclusion, it is suggested that the colorimetric value of the cobalt-platinic standards as given in “Standard Methods’’ be considered as approximate and that these solutions be compared with actual iron colors to establish their proper intensity and tint in the Kessler tube habitually used.
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