V O L U M E 22, NO. 3, M A R C H 1 9 5 0
499
mination of mercury by cyanide, or vice versa, 0.1, 0.01, and 0.001 molar solutions of mercuric nitrate and of potassium cyanide were buffered with sodium acetate and a few drops of a 0.1yo solution of 4,4’-bis(dimethy1amino) thiobenzophenone in acetone were added to them. When mercury solutions were titrated with cyanide the color change was from blue-green to pale yellow, the end point being very sharp with 0.1 and 0.01 molar solutions and satisfactorily sharp with 0.001 molar solutions. When cyanide solutions were titrated n-ith mercury solutions, the color change was from pale orange to blue, with the same degree of sharpness as the reverse titration. In all cases the titrations were reproducible and the end points reversible. With accurately standardized solutions, a combining ratio of mercury to cyanide of I tn 2 was found in either direction. The presence of chlorides did not interfere. It was found that aqueous solutions of chlorides cannot he
titrated with silver using 4,4’-bis(dimethylamino)thiobenzophenone as an indicator because its sensitivity is within the solubility range of silver chloride. However, chlorides in alcohol, acetone, or other solvents in which silver chloride is insoluble can be titrated with standard aqueous silver nitrate solution to a sharp end point. LITERATURE CITED
(1) (2) (3) (4) (5)
Baither, O., Ber., 20, 1731, 3289 (1887). Fehrman, W., Zbid., 20, 2857 (1887). Madelung, W., J . prakt. Chem., 114, 20 (1926). Schoenberg, A . , and Urban, W., Ber., 67, 1999 (1534). Tarbell, D. S., and Wystrach, V. P., J . Am. Chem. Soc., 68, 2110 (1946).
RECEIVED January 12. 1949. Presented before the Division of Analytical a n d Micro Chemistry at the 113th Meeting of the A & r m I c A x C H E X I ~ A L SOCIETY, Chicago, 111.
Control of Nitrite Interference in Colorimetric Determination of Phosphorus ARNOLD E. GREENBERG, LEON W. WEINBERGER, A N D CLAIR N. SAWYER .Massachusetts Institute of Technology, Cambridge, Mass.
IS T H E colorimetric detrrruiiliition of phosphorus ( 1 1, irivolving reduction of ammonium phosphomolybdate by stannous chloride, nitrites have bren found to interfere. Figure 1 s h o w the effect of various cnorirentrations of nitrite nitrogen on the transmittance of light through a standard phosphorus solution using a wave length of 690 mp. -4s t,he nitrite nitrogen concentration increases, the hlue color rapidly fades and as little HS 1 p.p.m. in the trst harnple introduces a considerable error in the determination. In order to o~er(:onirthis effect, various agents have bee11 twied. An excess of stannous chloride (more than 0.25 ml.) will rontrol limited concentrations of nitrites. Hon-ever, as the intenqity of the blue color formed is affected seriously by varying conwntrations of stannous chloride, it does not seem desirnhle t o use a n iinknon.n exrrw of this rrepmt.
Table I.
Control of Kitrite Interference in Determination of Phosphorus by Sulfamic Acid
N02-N, P.P.M.
n 2
5 10
15 20 25
Light Transmittance, %. after 10 Minutes 0 . 4 P.o.m. nhosnhorus plus moiybdate0 . 4 P.p.m. phosphorus sulfamic reagent
..fi
20 fi
26.7 65.0 87.8 87.0 88.6 89.0
29.9 29.4 29.0 27.6 29.5 30.4
27 -.
I
Sodium azide when added to the sample in various conc.entrations controls nitrite interference, but it cannot be included as a component of the molybdate reagent because of the production and evolution of hydrazoic acid, a toxic and explosive gas. Sulfamic acid was found to give good control of nitrite interference, as can be seen from Table I. I t can be combined with the molybdate reagent. The sulfamic acid should first be dissolved in a minimum quantity of water and then added to a ‘ cooled sulfuric acid-molybdate solution. Ten grams of sulfamic acid per liter of reagent are sufficient to control concentrations of nitrites up to 25 p.p.m. Although the modified reagent does not appreciably affect light transmission by standard phosphorus solutions, for precise work a standard curve should be developed n-ith the reagent. The reagent i. stable for periodq as long as 3.5 days.
I
851 h
E”
0 75
0,
$2
65 U I-
E 55 v,
545 I-
1.5 PPM.
f-‘
LITER.4TURE CITED
(1) Am. Pub. Health Assoc., “Standard Methods for Examinatmn of
Water and Sen-age,” 9th ed., p. 80, 1946.
1.0 PPM.
RECEIVED June 16, 1949
C
z 35
W 0
a
W p.
25 2
0.5 PPM. 0.0PPM. 6 IO 14 18 TIME IN MINUTES
Correction 2
Figure 1. Effect of Nitrite Concentration on Transmittance in Phosphorus Determination
In the article on “Differential Analysis with a Beckman Spectrophotometer” [Bastian, Robert, Webering, Richard, and Palilla, Frank, ANAL.CHEY.,22, 164 (1950)l in Figure 3 all the slit readings should be preceded by decimal points: 0.29, 0.24, 0.215 ’ 0.168, and 0.13 mm.
