Document zQ8zZJ0y6mX3xKBNQOKmR55d3

/ COPY CC: DR. R. A. KEHOE August 8, 1944 Mr. J. L. Stecher E. I. du Pont de Hemours and Co. Wilmington, Delaware Dear Mr. Stecher: I am attaching hereto for your information copies of two letters of comments on Chambers Works'Experimental Reports. The first of these, dated July 18, contains the comments of Mr. Gsmbrill of our Chemical Research Laboratory at Detroit on "Experimental Report Ho. TEL 44-8, Chambers Works Method for Lead Analyses by Dithisone". The second set of comments cpmes from Dr. Kehoe and is concerned with Report Ho. TEL 44-10, "Detection of Small -Amounts of Lead in Air Spectra- graphically." If you have any comments to make on these comments, I should be very, glad to have them. Yours very truly J. H. Schaefer 1 -1 KE 0001050 July 18, 1944 MEK0RAEEU1I TO: Dr. Calingaert | SUBJECT* Chambers Works Method for Lead Analyses by Dithizone, Experimental Report No. TEL 44-8 - June 12, 1944 She method described is based on the method developed at the Kettering Laboratory of Applied Physiology for the determination of lead in biological samples. In this method (of. Ind. & Eng. Chem. Anal. Ed 9. 493 - 1937) the solution of the Pb containing*'ammonium citrate and KCN is adjusted to pH 7.6. The Pb, and any Bi which may be present, is then extracted -with ditbizone-CHClg solution. Iron and other.metals which may be present are not extracted. ` The CHCI3 solution of the pb and Bi is then extracted with dilute aoid and the aoid solution of Pb and Bi is adjusted to pH 2. Bi is then removed by dithizone-CHClg extraction, and the aqueous phase after Bi removal is readjusted to pH 7.5 and again the pb is removed by dithisone extraction. Again t h e p b is removed from the CHCI3 by extraction with a standardized amount of dilute acid. To this acid solution is added a standardized amount of NH4OH-KCN solution to obtain a pH of 9.5. The pb is then extracted by dithisone for color measurement, using standardized amount of pur dithizone in CHCI3 . The amounts of dithizone used depend on the amount of Pb present i n the sang>le. In the Chambers Works Method, the preliminary extractions, as mentioned above, to ob- tain the pb free of interfering metals are not made. Rather the solution containing the pb is adjusted to pH 2, then HH4DH-KCN solution added to bring the solution to pH 7.8 and the color density of the Pb-dithizone complex obtained directly on t h e - traction from this solution. According to Sandell (1), Hopkins and Williams (2), and Wlehmann (3) only approximately BOf, of the Pb is extracted at pH 7.8, unless a large excess of dithizone is employed. Generally when extracting Pb at pH 7.5, 5 ml. increments of the dithisone solution are used ajad drained eaoh time from the aqueous phase until a green dithizone solution is obtained. Suoh &n extraction procedure thus permits an estimate of the amount of pb present, and the selection of the proper standard (see below) for the final color measurement of the pb complex. If Fe is present which according to the method is indicated by a brown color in the dried salts obtained by evaporating the I-KI solution used for absorbing the TEL, then a special extraction to remove Pe is suggested. This special extraction is aotually the first extraction generally used by most workers to obtain the lead free from other interfering metals. " ' O . f - 2- n is given in. the Chambers Works Method for adjusting the solution to pH 2* on is made regarding Bi , and if Bi is not to he considered then there is or no reason to adjust to pH 2 in the first place* Further, if Bi were present to he removed, the solution should be adjusted to p H 2.8-3.0, for as reported hers as well as in LTD 42-19, the extraction of Bi at pH 2 does not give 10($ action of the Bi (of, also references 1 and 2). the development of the method, standard Fb solutions were used containing from 0-40 [orograms of Fb, of them only 6 were reported for 0*12 mlorograms of Pb and 5 for from [2-50 mlorograms of pb. The analysis of 8 spot samples (2.1 ou. ft.^however show 7 to ^contain less than 3 mlorograms of Fb and only one to contain above that or 7 mtcrograms of Pb. For this reason and for accuracy standard Curves should be prepared for Fb ranges i. e, 0 to 10, Q to *26, etc. micrograms of Pb. Attempting to estimate the pb - present in a sample containing 0-10 microgrems of pb from a 0-40 miorogram pb curve causes all the readings to be made in the most inaccurate portion of the curve, I Although the Chambers Works Method may be satisfactory, it does not seem that idle best procedure has been used particularly fer low pb concentrations wherein an accuracy of _0.1 to 0.2 microgrems of Fb is desired for Semples containing as little as 1 to 2 mlorograms of pb. '' " Note is also made that all glassware used in the manipulations shall be made lead free by treatment with concentrated HNOg. Fb(NQg.)g is insoluble in concentrated HNOg and for this reason voluminous washings with water must be made to completely remove any Pb contamination from the glassware. Dilute BHOg solutions are preferred for deleading glassware for dithisohe pb determinations* The description of the Kettering Laboratory Method attached to the Chambers Works Method is in error in the paragraph on Preparation of the Sample. The addition of the HH4OH-KCN solution has been omitted. Therefore, according to the description given, only Bi would be extracted, no pb would,be removed, . (1) Colorimetric Determination of Traces of Metals. E. B. Sandell, Interseienoe Publishers, Ino*, New York, 1944. (2) Organic Reagents for Metals, 4th Edition, Research Staff EopkLns and Williams, ^ Hopkins w n d Williams, TLtd.,,,Is^on, 1943* Y. ?'*!? (3) Isolation and Detenni nation of Traces of Metals, H* J. Wichmann, Ind, and Eng, ,Chem. Anal. Ed, 11, 6 6 , 1939. \ \", KJ= 0001052