ANALYTICAL EDITION
188
Vol. 5, No. 3
TABLE111. EFFECTOF INCREASED EXCESSOF PRECIPITANT gravimetrically (as silver chloride). I n all analyses reported UPON PRECISION OF RESULTS herein, the Volhard procedure was employed. EXCESS OF
AgNOs ADDED
(Ow: theoretical)
COMPOUND ANALYZED
cc. 1. p-Chlorobenzoic acid
4
8 12
2. p-Bromoacetanilide
4
3. Bromobenzene 4. 1,2,4,5-Tetrachlorobenzene 5. Iodobenzene
8 12 4 8 12 4 8 12 4
8 12
LITERATURE CITED HALOQEN HALOGEN THEORETICAL FOUND (1) Bacon, J. Am. Chem. SOC.,31, 49 (1909). % % (2) Drogin and Rosanoff, Ibid., 38, 711 (1916). 22.66 22.78 (3) Favrel and Bucher, Ann. chim. anal. chim. appl., 32, 2(e), 22.60 321-324 (1927). 22.68 (4) Jenkins, McCullough, and Booth, IND. ENG. CHEM., 22, 32 37.45 37.34 (1930). 37.31 37.31 (6) Lemp and Broderson, J. Am. Chem. SOC.,39, 2069 (1917). 50.92 51.17 (6) Maryott, Am. J. Sci., 30, 378 (1910); Chem. News, 103, 1 51.15 (1911). 51.11 (7) Proner, Ph.D. theais, Nancy, p. 79 (1925). 65.71 65.58 (8) Stepanow, Be?., 39, 4056 (1906). 65.59 (9) Van Duin, Rec. trav. chim.,45, 363 (1926). 65.69 (10) Walker and McRae, J. Am. Chem. Soc., 33, 598 (2911). 61.79 62.22 61.91 RECEIVED January 10, 1933. 62.01
It is apparent that the larger excess of standard silver nitrate accounts for smaller differences between the theoretical and experimental halogen percentages. But the primary reason for the use of a larger excess of the standard solution is that the end-point transition seemed much sharper than ordinary, because of the increased quantity of silver thiocyanate appearing in the back-titration.
Improved Laboratory Screw Clamp w. A. SPERRY 11th and Washington Sts., Wilmington, Del.
SUMMARY I n the determination of nuclear halogen in organic compounds, by the method described, the effects of a variation of several factors were studied and the following conclusions attained: 1. Drogin and Rosanoff (8limit the sample weight to 0.2 to 0.3 gram, regardless of halogen content of the material analyzed. Consequently a high percentage error occurs in the analyses of substances of low halogen content. The authors have found it advisable to adjust the sample weight in accordance with the halogen content, so that it is equivalgnt to 30 to 35 CC. of 0.1 N AgN03. 2. The blank determination, the volume ratio of silver‘ nitrate to ammonium thiocyanate and the standardization of the silver nitrate (against C.P. sodium chloride) must be carried out under parallel conditions of acidity, volume of solution titrated, and titration in the large porcelain dish. 3. The analysis of halogenated nitro compounds requires an excess over the quantities of sodium and alcohol specified by Drogin and Rosanoff. 4. The method is not suited to the analysis of volatile substances. Preliminary attempts to determine the percentage of chlorine in chloroprene yielded low results due to loss by volatilization, (The sample of chloroprene was supplied by Ira Williams of the Jackson Laboratory] E. I. du Pont de Nemours & Co.) 5. If the substance analyzed is crystalline, difficultly soluble in alcohol, and highly refractory chemically, the fineness of division is extremely important to the success of the analysis, I n the earlier literature divergent results for hexachlorobenzene and benzene hexachloride were reported. Similar difficulties were encountered in the authors’ analysis of tetrachlorobenzene. These discrepancies were probably due to the state of division of the sample and also to insufficient refluxing with the reducing media. In such cases it was found necessary to increase the quantities of sodium and alcohol employed to twenty per cent over those proposed by Drogin and Rosanoff, and to extend the time of heating to 2 hours after addition of the last portion of sodium. 6. I n all analyses, filtration of the precipitated silver halide is necessary. Should the filtrate produce interfering colors on the addition of indicator solution, the analysis must be repeated, with the excess of precipitant determined
A
S THE ordinary laboratory screw clamp is equipped with a small, narrow screw head, the thumb and fingers of the operator often suffer injury when frequent tightening or adjusting is performed. It is often necessary to use pliers to open or close this type of clamp, especially if used on pressure tubing or when the screw has become corroded. This prompted the writer to provide the clamp with an enlarged knurled head, having a broader gripping surface and a greater diameter. This also simplifies fine adjustment when the clamp is used for regulating the flow of gases or liquids through rubber tubing. The device tried first consisted merely of a new screw, with enlarged head, turned from solid rod. This was soon replaced by s i m p l y brazing a k n u r l e d b r a s s cup of the a p p r o p r i a t e depth and diameter to the original screw head. The brazing was accomplished t h r o u g h a s m a l l hole in ,__ 1-1-2__ ’__ --b/ the center of the cup and ,-__ - -- _ , when properly executed I ; gives a strong union. / ,;_____ 1 i___: I 1 A more convenient deI\qj vice is the auxiliary head turned from stainless steel FIGURE1. METHODS FOR IMOF LABOand given a set of triangular PROVING EFFICIENCY HATORY SCREW CLAMPS “lands and grooves” as eleA stainless steel. B , screw head ments of the interior taper. griphed by lands and grooves. C, D , interior taper, 1 on 10 This steel head is d r i v e n knurled. with 24 lands and grooves 0.031 incd onto t h e screw head of deep. the customarv clamm the “lands” bitiig into “te brass and effecting a secure hold. Since screw clamps are of inexpensive construction and are ordinarily short-lived] this head may be removed and used again when the clamps are discarded. I
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RIDCEIVBD February 11, 1933.
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