Analysis of Binary Mixtures by Second-Order Differential Reaction Rates SIR: This communication clarifies a statement in a previous correspondence by Papa, Mark, and Reilley ( 5 ) . That communication stated that Siggia and Hanna (3, 6) used an expression of Lee and Kolthoff (4). The graphical method of Siggia and Hanna ( I , 3, 6, 7 ) does not require the Lee and Kolthoff equation. There is a relationship between Siggia and Hanna’s graphical approach and the Lee and Kolthoff approach because the equation used by Siggia and Hanna to plot their data is the classic secondorder equation (6). Lee and Kolthoff derived their expression from the same classic equation to describe mathematically the same chemical situation as Siggia and Hanna describe graphically, but the mathematical Lee and Kolthoff approach cannot be used with the Siggia and Hanna approach.
To analyze chemical systems with the Lee and Kolthoff expression, one requires rate constants which necessitates operation a t fixed conditions. The graphical method does not require the rate constant since the plot automatically takes this into account. This provides not only flexibility for wider applications, but also provides flexibility to choose analysis conditions particularly amenable to resolution and/or accuracy and precision. The niathematical “double point” technique ( 5 ) also circumvents the limitations of the “single point” techniques (4, 6). However, it should be pointed out that the graphical approach has an added advantage in that many data points are plotted which tends to eliminate error resulting from random scatter of the data.
LITERATURE CITED
(1) Block, J., llorgan, E. Siggia, S., AXAL.CHEM.35, in press. ( 2 ) Glasstone, S., “Textbook of Physical Chemistry,” 2nd ed., p. 1055, Van
Sostrand, New York, 1946.
(3) Hanna, J. G., Siggia, S.,Ss.41,. CHEK 34, 547 (1962). (4) Lee, T. S.,Kolthoff, I. M.,Ann. N.Y . Acad. Scz. 53, 1093 (1951). ( 5 ) Papa, L. J., Mark, H. B., Jr., Reilley, c . s., A S A L . CHEM. 34, 1513 (1‘362). (6) Siggia, S., Hanna, J. G., Ibid., 33, 896 (1961).
(7) Siggia, S.,Hnnna, J. G., Ibid., 35, in press.
SIDNEY SIGGIA
Olin Research Center NeJT Haven, Conn.
H. B. MARK California Institute of Technology Pasadena, Calif.
Zinc as Masking Agent in the Spectrophotometric Determination of (Ethylene dinitri1o)tetraacetric Acid in Urine SIR: ;in earlier paper published by Cherney et al. ( I ) described a method for the quantitative determination of ( e t h y l e n e d i n i t r i l o ) tetraacetic acid (EDTA) in urine based on the color of the chromium EDTA complex. dttempt.; t o apply this procedure in our laboratory demonstrated rather wide variations vith known amounts of EDT.4. The method requires that one posieq.; not only the urine specimen uhieh is t o be analyzed but also an earlier EDT.1-free urine from the same individual mhich serves as a blank. Furthermore, urines that were EDTAfwe did not always give the same abwrbancc (blank) nith the color reagent en u hen obtained from the same individual. I t n as, therefore, impossible t o determine the EDT.4 content of an isolated urine Ypecimeii l x the Chwney procedure. If a mean.; for masking out EDTA could be devised, it should then be possible t o set up a scheme of analysis in n hich each specimen might serve as its own blank. This would increase the utility of the determination. Investigation of a number of metal ions (31
slioned that zinc formed a sufficiently stable chelate with EDT.4 to mask completely the reaction between chromium and EDTA. The method decribed here is based upon this ma&ing effect. EXPERIMENTAL
Reagents. Potassium Dichromate -1 0.55% solution of Solution. reagent grade potassium dichromate in distilled water is used. This solution is stable indefinitely when stored in borosilicate glass bottles. Arsenious ilcid Reagent. Transfer 4.95 grams of reagent grade arsenious oxide to a 500-ml. volumetric flask. Add 3.5 grams of sodium hydroxide and dissolve in about 150 ml. of distilled water with warming. Neutralize with glacial acetic acid using phenolphthalein as indicator and then slowly add 200 ml. of glacial acetic acid with mixing. Cool, dilute to volume with distilled water, and mix thoroughly. This solution is stable indefinitely when stored in borosilicate glass bottles, Arsenious Acid-Zinc Acetate Reagent. Transfer 4.13 grams of reagent grade zinc acetate dihydrate to a 250-ml.
volumetric flask. Dissolve in arsenious acid reagent solution and dilute to the mark with the same solution. Mix thoroughly. This reagent is stable indefinitely when stored in borosilicate glass bottles. EDTA Standard Solutions. Transfer loo-, 200-, and 300-milligram portions of dried reagent grade (ethylenediamine)tetraacetic acid into 100-ml. volumetric flasks. Add one sodium hydroxide pellet per 100 milligrams of EDTA. Dissolve in distilled water and dilute to volume. This solution is stable indefinitely when stored in plastic bottles. Procedure. Prepare two tubes for each sample. Pipet 2 ml. of the urine specimen into each tube. Add 2 nil. of arsenious acid reagent to one and 2 ml. of arsenious acid-zinc acetate reagent t o the other and mix. This latter tube serves as a blank. Add 2 ml. of potassium dichromate solution to each and mix. Allow to stand for 35 minutes a t room temperature and read each sample against its corresponding blank a t 560 mp. Prepare standards as above but use 2 ml. of distilled water in place of the standard solution to obtain a blank. All measurements were made with a Coleman VOL. 35, NO. 3, MARCH 1963
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