Permanent Color Standards for Determination of Phosphate by Molybdenum Blue Method E. P. PARRY1 and A. L. McCLELLAND* D e p a r t m e n t o f Chemistry, University o f Connecticut, Storrs, Conn.
This note describes the characteristics of sohtions which have been found to be very suitable as permanent color standards for the routine estimation of phosphate in the concentration range from 10 to 100 parts per hillion of phosphorus. Application of the standards to the field determination of phosphate in natural waters is described elsewhere (7).
I
S T H E course of the development of a field method for the determination of phosphate, it became evident that there is a
definite need for stable, well-matching, permanent color standards for use with the molybdenum blue method of phosphate determination. The most widely used method for the determination of traceb of phosphorus involves the reduction of a heteropoly molybdophosphate to give a blue color, commonly referred to as molybdenum blue (8). The need for standards is emphasized by the fact that the molybdenum blue solutions fade after about 15 minutes. This method can be adapted for field use in the concentration range from 10 to 100 parts per billion phosphorus bjusing tall-form Nessler tubes of 50-ml. capacity as the viewing tubes and permanent solution color standards for comparison of the unknowns. Several permanent solution color standards have been proposed for visual color comparison of the molybdenum blue solutions in the phosphate determination (1, a, 6). However, Woods and Mellon (8) have found that most of the color standards suggested do not give a good visual match with the reduced molybdophosphate; one mixture (6) is no more stable than the reduced molybdophosphate itself. Permanent color standards are needed which are not only stable upon exposure to air and sunlight but also closely match the color of the reduced molybdate. Color standards composed of mixtures of copper sulfate and bromophenol blue in an acetate buffer of p H 4.53 make very satisfactory and useful color standards for phosphate determination in the range from 10 to 100 parts per billion of phosphorus. The color developed by this system is stable, and matches the color well enough so that unknown phosphate solutions can be determined to l t 5 parts per billion. Table I gives the amounts
of the components needed to prepare the standards. -4lthough these values will prepare solutions which show a reasonably close color match, in every instance the final adjustment should be made empirically against a known phosphate solution undet conditions identical with those to be used in the actual unknown determination. Changes in hue of the standard can easili be made by changing the pH of the solution, as this changes the hue of the bromophenol blue, Figure 1 compares the spectral transmittancy curves of a molybdenum blue solution containing 500 parts per billion of phosphate phosphorus and its visually matched color standard. A Model B Beckman spectrophotometer with 1-cm. cells was used to obtain the curves. The fact that the spectrophotometrir curves do not exactly match does not necessarily mean that the colors do not match by visual comparison. To obtain a measure
Table 11. Variation of Spectral Transmittance with Time for Copper Sulfate-Bromophenol Blue Color Standard Solution A mixture of 4.88 mg. of CuSOa and 0.920 mg. of bromophenol blue in 10 I d . of a sodium acetate-acetic acid buffer solution of p H 4.53, in solutions continually exposed in east window for time indicated Wvti\ r Length, hlr
500 525 550 575 no0 625 650 675 700 725 750 800 850
- -~
0 days
89 88 85 81 79 78 78 81
5 2 0
6 2 0 8 6
Transmittance, R 3 5 days 9 days 18 days ... 97.2 98.5 ... 94.7 ... 91.6 ... 88.8 ... 90.3 89 5 89.3 88.0 88 4 87.5 84.7 84.3 84 0 81.3 81.1 81 6 79.0 79 3 78.7 77.6 77.5 78 0 78.4 78 3 78 7 81.1 81 2 81 3
24 days 96.5 96.2 94.4 91.3 88.9 91.0 88.6 85.0 81.6 79.3 78.3 79.0 81.5
1 Present address, Department of Chemistry, State College of Washington, Pullman, Wash. f Present address, Chemical Department, E. I du Pont de Nemours dr Co , Wilmington, Del.
Table I. Volumes of Copper Sulfate (0.00883 Gram' of Copper per MI.) and Bromophenol Blue (0.010~0)Needed to Prepare Color Standards (Diluted to 30 ml. with buffer of pH 4.53) Concn. Bromophenol Phosphorus, Blue, cuso,, P.P.B. M1. MI. 10 0.40 0.085 20 0.70 0.150 30 0.96 0.210 1.17 0.330 40 1.40 0.400 50 60 1.60 0,500 1.79 0.620 70 1.95 0.750 80 2.17 0,900 90 2.40 100 1.oon 2.82 120 1.145 150 3.40 1.540
550
1% YLVC
LIYaTW
650
IYILLIUI*IONIl
Figure 1. Spectral Transmittancy Curves for Molybdenum Blue Test on 500 Parts per Billion of Phosphorus and Its Visibly Matched Color Standard I-em. cells used in Model B Beckman spectrophotometer Color standard
0 Standard phosphntr
140
141
V O L U M E 27, NO. 1, J A N U A R Y 1 9 5 5 of visual matching the chromaticity coordinates, x and y, based on the Interns,tional Commission on Illumination standard observer and coordinate system ($), were calculated. The method of ten selected coordinates (6)and Standard Illuminant C were used. For the reduced molybdophosphate, values of z and y were 0.153 and 0,200, respectively; for the visually matched color standard, 0.149 and 0.190. These figures show that the solutions match well in color hue. With the aid of the Maxwell triangle ( 4 ) to indicate hue deficiency, an even closer match probably could be obtained by s slight empirical adjustment of solution pH. The stability of the color standards is demonstrated by the data of Table 11. Spectral transmittancy curve^ were determined periodically on a mixture of 4.98 mg. of copper sulfate and 0.920 mg. of bromophenol blue in 10 ml. of acetate buffer of pH 4.53 during the time i t remained in an east window exposed to considerable direct morning sunlight. There was no change in the absorption spectrum of the mixture even after 24 day& indicating that the color standards have good stability characteristics.
The calor standards should prove valuable for routine u8e in the visual estimation of traces of phosphate by the molybdenum blue method. LITERATURE CITED
Deniphs. BuU. SOC. p h m . Bordeazlz. 65, 107 (1027). (2) F l o r e n t i n , Ann. china. anal. et china. appl., 3,205 (1921). (3) Mellon, M. G.. “Andlytioal Absorption Spectroscopy,” p. 522, New York. John Wiley & Sons. 1950. (4)Ibid., p. 525. (5) Ibid.. p. 535. (6) Meyer, Science. 7’2, 174 (1030). (7) Parry. E. P., and MoClellmd, A. L.. Piogressive Fish CdLrrist. 16, No. 4, 158 (1054). (8) Woods, J. T., and Mellon. M. G., IND.END.CHEM.,ANA,,.Eo.. 13, 760 (1941). (1)
R ~ . O E I Y Bfor D review July 6, 1954. Accepted September 27. 1954. This investigation was supported b y Federal Aid to Fish Restoration Funds under Dingell-lohnson Project No. F-3-R.
Method for Sparking Thin Sheet Samples for Spectrographic Analysis Application to Manganese and Niobium Determination in Stainless Steel F. P. LANDIS and L.
P.
PEPKOWITZ
Knolls Atomic Power Laboratory, General Electric Ce., Schenectady, N. Y.
.4 technique has heen developed for sparking thin sheets of steel a n d applied to the spectrographic determination of manganese and niobium. The sample is a n l e d by helium d u r i n g sparking.
A
TECHNIQUE has been developed for sparking thin sheets of steel for the spectrographic determination of manganese and niobium. Normally, reproducibility and accuracy are achieved by using massive samples which do not become appreciably warmer than room temperature when sparked. However, when thin sheets of stainless steel are sparked, the heat produced by the spark is sufficient to cause oxidation of the sheet on the unsparked side. Whenever this evidence of overheating occurs,
manganese and niobium values are very erratic and unusually high, manganese being affected much more than niobium. It is believed that, because of the high local temperatures of the steel, excessive volatilization of manganese and niobium occurs (with respect to the amount of iron volatilized). An attempt to cool the sample while sparking by attaching it to a solid block of steel or copper was unsuccessful, probably because of the poor thermal contact of the thin sample with the cooling block. More successful was the technique of using a flow of cooled helium on the upper or unsparked side of the sample during the analyBis. Helium was chosen because of its high thermal conductivity. To accomplish helium cooling, the clamp on the Petrey stand sample holder wa8 replaced with a hollow fitting into which the cooled gas could flaw and impinge on the upper surface of the thin sample (Figure 1). The gas was passed through a flom-meter and then through a copper coil immersed in an ice bath and from there into thc Petrey stand clamp. Ice was used a8 a cooling
Table 1. Effect of Variation in Coolant Flow He Flow LiterdMih.
0
4
Anparent
% Mn 0.76 1.09 1.98
15
28
F i g u r e 1. A p p a r a t u s
0.61 0.95 1.73
2.80
2.75
1.38
0.95 0.92 0.93 0.99
i.oo
1.15 1.17 9
Apparent 7% N b
0.70
0.71 0.70 0.71 0.61
0.68
0.67
0.79 0.74 0.74
0.61 0.63 0.53
0.60
0.60 0.55
0.57 0.62 0.58
0.54 0.53
0.60
0.49
0.51
