336 Table
INDUSTRIAL AND ENGINEERING CHEMISTRY II. Comparison of Results Obtained with Spectrographic, Polarographic, and Colorimetric Methods
(Recoveries of cadmium added to 100-ml. portions of urine) --admiurn Recovered Spectro ra hic Polaro ra hic Colorimetric Cadium Added rnetfoz metto$ method Microprana
Micrograms
Measured on 0-60 1 range
solution, and within 5y0 when more cadmium is present, can be obtained by spectrographic means. In the case of the polarograph, it is sometimes difficult to measure accurately the diffusion waves obtained when 1 to 3 micrograms of cadmium are present in 3 ml. of solution, although there is no difficulty in identifying the diffusion wave. Another difficulty occurs a t high recorder sensitivity when traces of cadmium are to be determined in the presence of large quantities of lead. At the highest galvanometer sensitivity, it is possible to determine 10 micrograms of cadmium in the presence of a concentration of lead ten times as great. When lead is present in a much greater quantity, the galvanometer sensitivity is too great to permit both waves to appear on the chart. I n such ’cases, the lead may be removed by extraction from the strongly alkaline tartrate solution, as described under “Colorimetric Method”. The cadmium is then shaken into 0.2 N hydrochloric acid and the polarographic estimation is repeated. Large amounts of bismuth oxychloride also interfere polarographically, but this interference may be eliminated by filtration and re-evaporation of the filtrate to dryness. The interference by bismuth oxychloride may also be eliminated by a re-extraction of the cadmium with dithizone as mentioned above for lead. Other metals such m nickel, cobalt, and zinc give diffusion waves above that of cadmium and consequently do not interfere.
Vol. 16. No. 5
It will be noted from Table 11, in which the recoveries by the three methods are compared, that cadmium was detected in the samples to which no cadmium had been added. From some of the data in Table I, it seemed likely that the considerable quantity of c a h i u m present in the blank was due to its presence in the reagents. This was proved by analysis of large volumes of sulfuric and nitric acids, for i t was found that two thirds of the blank was due to the 10 ml. of sulfuric acid, and the rest to the approximately 50 ml. of nitric acid used in each digestion. In the matter of the choice of one of these methods for routine use, the authors have found that the polarographic method is the most convenient, particularly when only occasional samples are to be run. The spectrographic method, in their opinion, offers the most rapid and economical means of analysis, if large numbers of samples are to be handled daily. The colorimetric method, while the most sensitive, is the most laborious and time-consuming, but provides a reliable means for determination of cadmium when polarographic or spectrographic equipment is not available. LITERATURE CITED
Bambach, K., and Burkey. R . E.. ISD. ENG.CHEM.,ANAL.E D . , 14,904 (1942).
Cholak, J., Hubbard, D. M.,and Burkey, R. E . , Ibid. 15, 754 (1943).
Cholak, J . , and Story, R.V., Ibid., 10,619 (1938). Cholak, J., and dtory, R. V., J . Optical SOC.A m . , 31,730 (1941). Ibid., 32, 502 (1942). Cowling, H., and Miller, E. J., IND.ENG.CHEM., ANAL.ED.,13, 145 (1941).
Dwyer, F. P.,Australian Chem. Inst. J . Proc., 4,26-34 (1937). Fairhall, L. T.,and Prodan, L., J . Am. Chem. SOC.,53, 1321 (1931).
Fischer, H., Angew. Chem., 50, 919-38 (1937). Hubbard, D. M., IND.ENG.CHEM.,ANAL.ED., 13,915 (1941). Hubbard, D.M., and Scott, E . W., J . A m . Chem. Soc., 65,2390 (1943).
Lingane, J. J., and Kerlinger, H., IND.ENQ.CHEM., ANAL.ED., 12, 750 (1940).
Mahr, C., Mikrochim. Acta, 111,300 (1938). Pierce, W. C., and Nachtrieb, N. H., ISD. ENG.CHEM.,ANAL. E D . , 13, 774 (1941). Prodan, L., J . I d . Hug., 14, 132 (1932); 14, 174 (1932). Sandell, E. B., IND.ENG.CHEM..ANAL.E D . ,11, 364 (1939).
Spectrophotometric Determination of Leuco Crystal Violet after Oxidation with Benzoyl Peroxide WM. SEAMAN, A. R. NORTON, J. T. WOODS, AND J. J. HUGONET Calco Chemlcal Division, American Cyanamid Company, Bound Brook, N. J. A method i s reported for determining leuco crystal violet b y oxidizing with benzoyl peroxide and measuring, by means of the spectrophotometer, the intensity of the color formed. The method has a precision represented b y a standard deviation for a single value of ~ 0 . 2 5 %of the total leuco crystal violet. The effect of impurities upon the accuracy i s discussed.
0
NE method described in the literature for the production of crystal violet involves the initial preparation of leuco crystal violet, p,p’,p“-methenyltris-(N,Ndimethylaniline),followed by oxidation to the finished product. The proper control of the oxidation necessitates a knowledge of the content of the leuco form. As far as the authors know, no method of determining this compound has been published; previous t o the development of the method reported here leuco crystal violet had been determined merely by ascertaining the amount of material insoluble in a solution which was about neutral after the acid-insoluble constituents had been removed. Obviously, this method is not
specific for the leuco crystal violet, so that it became desirable t o find a more satisfactory method. Since the colorless leuco crystal violet is converted t o the colored form by oxidation, the possibility of utilizing this behavior as the basis of a colorimetric method was considered. After a consideration of oxidants, benzoyl peroxide was chosen. A study was then made of the optimum conditions of temperature, time, and concentration for color formation with this oxidant. The detailed method of analysis is given below, followed by a discussion of the experiments which determined the conditions of analysis. METHOD OF ANALYSIS
APPARATUS.Most of the color measurements were made using a modified automatically recording Hardy spectrophotometer (2, 3, 4 ) . Calibrated cells, approximately 1 cm. in length, were used. This spectrophotometer plots the spectral curve as log log 1/T against a logarithmic function of thc wave length, referred to as the octaval and measured in constant resolution units (c.r.u.).
ANALYTICAL EDITION
May, 1944
337
Thc color intensity of this solution is ineasured within a period of not more than one hour. The transmittancy is measured at 263 c.r.u. (580 mp) and the amount of leuco is calculated from the following
