INDUSTRIAL AND ENGINEERING CHEMISTRY
14
Fym
3 shows a lot of A. P. I. gravity versus molecular weig t for the normafparaffins and for Midcontinent strai htrun and cracked stocks. The line for straight-run stocks digers slightly from that previously presented. This difference is due to a slight difference in the Midcontinent pipe-line crude, and not to a change in procedure, as was shown by checking some of the old samples. Figure 4 is a similar plot of molecular weight against 50 per cent A. S. T. M. boiling point (760 mm.). Since there is no stem correction in the A. S. T. M. method, the 50 per cent point shown by pure substances of various boiling points was actually determined, and a correction plot prepared so that the 50 per cent boiling point which would be shown by each pure hydrocarbon could be computed. These computed values, not the true boiling points, are used in Figure 4. Figure 5 is a plot of the kinematic viscosity versus molecular weight. In this plot the results of various determinations in the literature, as well as some other unpublished work, have been incorporated, in the hope of correlating viscosity, vis-
Vol. 7, No. I
cosity index, and molecular weight for the heavier fractions. The data are somewhat conflicting, and the lines as drawn are, at the upper end, to be regarded as only a first attempt to obtain the desired result. LITERATURE CITED (1) British Engineering Standards Association, No. 188 (1929). ENQ.CHHM.,24, 1369 (1932). (2) Epperson and Dunlap, IND. (3) FitzSimons and Bahlke, Am. Petroleum Inst., Proc. 10th Ann. Meeting, 11, No. 1, Sect. 111, 70-2 (1930); Oil Gas J., 28, 164 (December 5, 1929). (4) Gullick, J . Inst. Petroleum Tech., 17, 541 (1931). (6) Parks and Huffman, IND.ENQ.CHEM.,23, 1138 (1931). (6) Steed, J.Inst. Petroleum Tcch., 16,783 (1930). (7) Wilson and Wylde, IND. ENQ.CHEM.,15, 801 (1923). R ~ C E I V EAugust D 27, 1934. Presented before the Division of Petroleum Chemistry a t the 88th Meeting of the American Chemical Society, Cleveland, Ohio, September 10 to 14, 1934.
t
Colorimetric Determination of Small Quantities of Chlorides in Waters H. B. RIFFENBURG, Virginia Polytechnic Institut,e, Blacksburg, Va.
T
HE method generally adopted for the estimation
of the chlorides in water has consisted of titrating a known quantity of water with a standard solution of silver nitrate, using potassium chromate as the indicator, and calculating the chloride content in parts per million or a similar denomination (1). The chlorides in most rain waters and many ground waters are present in too small a quantity to be determined accurately by this method. A study of over two hundred articles on the composition of rain water in different parts of the world made by Riffenburg (6) in 1923 showed the chloride content to vary from a trace to over 50 parts per million, with the average about 3.0 parts per million. Some authors used a 0.1 N or weaker solution of silver nitrate to titrate the chlorides, and many did not discuss,the method used. Such a variable chloride content indicates that either the method used or the analysis is faulty, and this is confirmed by determination of chloride content in lakes, rivers, and many ground waters. Jackson’s chloride maps (4) show that some natural waters j n the northeastern part of the United States contain as little as 0.2 p. p. m, Lake Superior water showed 1.1 p. p. m. and sixteen rivers in the United States showed less than 2.0 p. p. m. of chlorides (3). Twenty-three samples of rain water collected in Washington, D. C., in 1923-1924 and analyzed by Riffenburg (6) contained a maximum of 3.0 p. p. m. and an average of about 1.3 p. p. m. Samples collected in central and upper Michigan and Blacksburg, Va., averaged about the same. Williams’ (2) analyses of rain waters collected in ten different places in the United States showed averages from 1.4 to 0.25 p. p. m. Williams evaporated a larger quantity down to 25 cc. and titrated it with a silver nitrate solution, 1 cc. of which is equivalent to 0.5 mg. of chloride. These analyses show the importance of developing a method that will more accurately estimate very small quantities of chlorides in waters. An attempt was made to increase the accuracy of the titration by using dilute solutions and greater or smaller volumes of water, but this failed until a modified colorimetric method was used. The colorimetric modification consists of filling one, or preferably two, Nessler tubes to the mark with the sample to be tested, adding 1 ml. of potassium chromate prepared as in ( I ) , and mixing
well. One tube is used as the control and to the other is added the silver nitrate solution until the usual red color can be detected in the sample when compared to the control tube. The silver nitrate should be added drop by drop, shaking the tube well, and comparing it with the control after each drop. The silver nitrate used contained an equivalent of 0.05 mg. of chloride per cubic centimeter; it should not be stronger than this and should not be too weak. Test tubes may be used, but Nessler tubes have been found more satisfactory, since they have olished bottoms and present a longer column of liquid through wkch to view the color change. Sharper color changes can be produced by allowing the sun to shine on a piece of white paper or white porcelain under the tubes and protecting the tube from the light. Distilled water may be used as the control if the quantity of sample is not sufficient to fll two tubes, and the same control may be used for a number of samples if none shows a different shade of the chromate color. If a different shade of color appears, the sample itself will have to be used as the control. The burets and pipets used in these analyses were certified by the U. S. Bureau of Standards and the other volumetric apparatus used was calibrated. Fairly close checks were obtained by evaporating 500 ml. to 50 ml. and titrating by the usual method, using a yellow light to increase the sharpness in the end point. Analyses of twenty samplek of rain water collected at Blacksburg, Va., during the past year and a half show a maximum of 0.4 p. p. m., a minimum of 0.05 p. p. m., and an average of 0.23 p. p. m. LITERATURE CITED (1) Am. Public Health Assoo., “Standard Methods for the Examina-
tion of Water and Sewage,” 7th ed., 1933. (2) Collins, W. D., and Williams, K. T., IND.ENQ.CHEM.,25, 244 (1933). (3) Ddle, R: B.,U. 8. Geol. Survey, Water Supply Paper 236 (1909). (4) Jackson, D. D., Ibid., 144 (1905). (5) Riffenburg, H. B., Ibid., 560,31-53 (1925). RECEIVED October 4, 1934. Preaented before the Division of Water, S e a age, and Sanitation Chemistry a t the 88th Meeting of the American Chemios1 Society, Cleveltlnd, Ohio, September 10 t o 14, 1934.
