505
1 U L Y 1947 The solvents niust be of reagent grade and must not contain any preservatives-chloroform of reagent grade usually contains ethyl alcohol as a preservative, so that a constant reading cannot be obtained on the water differential thermometer. Hanson and Bowman ( 4 ) have shown that the zero point of the differential thermometer is obtained more quickly by dispersing a small amount of gum rubber (about 0.05%) in the solvent. Thiis recommended in the present microprocedure. Solid samples are weighed and pressed into platinum or glasb boats, thereby eliminating the need of a pellet press. The platinum boat does not short the platinum heater because only relatively small surfaces come into contact. Table I gives the values obtained with varying weights of different organic samples with a number of solvents.
The modified apparatus was made by Distillation Products, Inc., Rochester, N.Y, LITERATURE CITED (1) Colson, A. F., Analyst, 57, 757 (1932). (2) Cottrell, F. G., J . Am. Chem. SOC.,41, 721-9 (1919). (3) Hallett, L. T., in “Scott’s Standard Methods of Chemical Analy-
sis,” 5th ed., Vol. 11, pp. 2530-3, New York, D. Van Nostrand Co., 1939. (4) Hanson, W. E., and Bou-Inan, ,J. R., IND. ENG. CHEM.,-1N.4L. , ED.,11,440-2 (1939). (5) Menzies, A. W. C., and \Tripht. P. L.. J r . , .I. A m . Ch,em. ,Sot.. 43, 2314-23 (1921). C ~ M M C S I C A T I1114 O X f r o m the Kodak Kesearrh Laboratories.
Colorimetric Determination of Cobalt Using Nitroso-R Salt HOBART H. WILLARD AND SA3IUEL KAUFMAN, Department of Chemistry, University of Michigan, A n n Arbor, Mich.
HE colorinietric determination of small amounts of cobalt,
100
Tusing the nitroso-R salt, has received increasing attention recently ’with the publication of several papers on the subject.
However, there has been an apparent inconsistency in the choice of a filter to be used in the colorimeter. Hixson and McNabb (1) suggested a green filter ( 5 0 0 t o 570 mg) and Young, Pinkney, and Dick (5)suggested a blue filter (Hilger blue S o . 1). Sandell ( 2 ) presents a curve based on data of R . -4.Johnson using a green filter (Cenco KO. 2), while on the next page he suggested that the transmittancy be measured at 510 mfi. I t seemed likely that the difference between these filters rva5 enough to affect the accuracy and sensitivity of the method. The purpose of this paper was to determine the best filter to use for thc dcxtermination of cobalt with nitroso-R salt.
40
eo
3 c
70
t U
e 5 “ O C
e
+ s o t
C
u L
‘2
40
--
APPARATUS AND SOLUTIONS
The absorption and‘transmittancy curves were made in 1.000cm. cells using a Beckmann quartz spectrophotometer, Model UV. The slit width was varied but the maximum band width used was 5 mp. Since the nitroso-R salt itself gives a yellow color in aqueous solution, the reagent vias used as the blank in both the absorption and transmittancy curves. Standard solutions of cobalt were made from cobaltous chloride hexahydrate to contain 2 X 10-4, 4 X 8 X 10-4, 1 x 10-3, and 5 X 10-3 mg. of cobalt per ml. of solution. Standard cobaltous chloride solutions were used instead of standard ore samples because there was no doubt Y S to the validity of the method, only a question as to which was the best filter. Solutions needed for the determinations were: Sodium acetate, 100 grams dissolved in 200 ml. of hot water and filtered. Con-
2
4
6
a
IO
I,?
Cobalt concn. x IO4 mg./ml.
Figure 2
centrated sulfuric acid. Sitroso-R salt (1-nitroso-2-hydroxynaphthalene-3,6-disodium sulfonate), 0.5 gram in 100 nil. of water. To 10 ml. of standard solution in a 50-ml. volumetric flask were added 2 drops of concentrated sulfuric acid, 2.0 ml. of sodium acetate solution, and 1.0 ml. of nitroso-R salt solution. The resulting solution was shaken vigorously and then made up to 50.0 ml. This solution was used directly in the spectophotometer cell$ and undiluted nitroso-R salt solution was used as the blank. DISCUSSlON
Figurt, 1 is a plot of wave length versus per cent transmittancy made at several concentrations of cobalt. It is apparent from the curves that the maximum absorption occurs a t a wave length of 420 mp. This indicates clearly that a blue filter is the best one to use. In order to shoiv the effect of the choice of a filter on sensitivity, a number of curves have been drawn plotting per cent transmittancy against concentration, using selected wave lengths (Figure 2 ) ; it is apparent from t h r slope of the 420 my curve that this viave length gives the greatest sensit,ivity. At concentramg. per nil. of cobalt, Beer’s law does not tions below 2 X hold. LITERATURE ClTED
350
I
I
I
I
1
400
450
500
550
1
600
650
WUVQ
LQngth mU ,
Figure 1
(1) Hixaon, A. (1946).
K.,and XIcKabb, TT. M.,Meletul Pinidling, 44, 208-9
(2) Sandell, E. B., “Colorimetric Determination of Traces of Metals,” pp. 201-2, New York, Interscience Puhlishers, 1944. (3) Young, R. S., Pinkney, E. T., and Dirk. R., IND. EXG.CHEM., ANAL.ED.,18, 474-6 (1946).
