INDUSTRIAL AND ENGINEERING CHEMISTRY
January 15, 1931
Although Colin and Lievin (9) in 1918 have used a phosphate buffer in connection with iodometric sugar determinations, they have made no critical study of it. Most of the recent work has been with sodium hydroxide or sodium carbonate. With the latter the brisk effervescence of carbon dioxide upon acidification increases the danger of loss of iodine by volatility. This fact, in addition to other favorable aspects, led to an experiment as to the effect of the time interval upon the extent of oxidation of the pure glucose with the phosphate buffer. The results are given in Table 11. Conclusion
It seems safe to conclude that the presence of amino substances will tend to give slightly higher values for glucose by the iodometric method and that the condensation reaction between the aldol sugar and amino acid will not have a significant effect unless the solution has been allowed to stand for some time in an alkaline condition before the analysis is made.
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Such a situation might follow a defecation procedure as has been pointed out in a previous paper. On the whole the phosphate buffer gives more satisfactory results than a carbonate buffer of the same pH. The results vary but little over a considerable time interval; there is less danger of loss of iodine on acidification of the reaction mixture and less error due to the presence of fructose and amino substances. Literature Cited (1) Cajori, J . B i d . Chem., 64, 617 (1922). (2) Colin and Lievin, Bull. soc. chim., 23, 403 (1918). (3) Englis and Byer, IND. END. CHEM.,Anal. Ed., 2, 121 (1930). (4) Englis and Dykins, I b i d . , 8, 17 (1931). (5) Euler and Brunius, Z physiol. Chem., 165, 259 (1926). (6) Euler and Brunius, Ibid., 161, 265 (1926). (7) Kolthoff, Z. Untersuclz. Nahrungs Cenussm., 46, 131 (1923). (8) Kolthoff, “Volumetric Analysis,” Vol. 11, p. 446, Wiley, 1929. (9) Oliver, University of Minnesota, M.S. Thesis, 1927. (10) Slater and Acree, IND. ENO. CHEM.,Anal. Ed., 2, 274 (1930). (11) Vosburgh, J . A m . Chem. Soc., 42, 1696 (1920).
Determination of Phosphates in Presence of Silica in Boiler? Water’ Elwood W. Scarritt ELGINSOFTENERCORPORATION, ELGIN,ILL.
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H E greatest source of error in the colorimetric determination of phosphate in boiler waters and in other alkaline solutions is the interference of silica or silicate. Relatively minute quantities of silica cause the production of a blue color similar to the color due to phosphate. This interference has recently been noted by many workers in the boiler field. The present paper describes a method Ghat minimizes this difficulty and produces reliable results in the presence of silica. When the colorimetric method for phosphate determination was first used in this laboratory certain samples developed a clear blue color more like the blue of copper sulfate solution than the characteristic violet-blue of the phosphate when treated with slightlv acidified molvbdate and hydroquinone solutions, and-subsequently neutralized with sohium carbonate in the presence of sodium sulfate. It had been suggested that iron interfered, but this was found not to be the cause of the clear blue color in the samples of boiler water, zeolite-softened well water, or certain untreated natural waters which contained no phosphate. Investigation showed the color to be due to silica. The presence of silica also intensified the color due to phosphates in samples of known phosphate content, causing errors in the phosphate estimation as great as 100 per cent. Since the blue color is proportional to the amount of phosphate or silicate ion in solution in the sample, it was obviously necessary to remove the silicate if the phosphate was to be determined accurately. It was not convenient to dehydrate or precipitate the silicate, but by acidifying to the right degree it was found possible to repress the ionization of the weak silicic acid to a point where it developed no noticeable blue color, while the stronger phosphoric acid remained sufficiently ionized to react with the molybdate and produce a blue color proportional in intensity to the concentration of phosphate present. It is important to make up the phosphate standards with the same amount of acid used in testing the samples, as f
Received September 4, 1930.
the quantity of acid used influences the depth of color produced, Solutions Required
Standard sodium phosphate, Na3P04.12H20-0*320 - in 40 ml*H2°* gam per liter* ml* = P * P * mP04 (2) fhlfuric acid--600 ml. H 2 0 PIUS 300 ml. 95% c+p* HzS04*
(3) Ammonium molybdate, (“4)s M07021.4H20-92.3 grams per liter plus 38 ml* coned* H2S04* (4) Hydroquinone, C6Hd0H)2-23 grams per liter plus ml*coned* HzS04. (5) Alkaline sodium sulfite-183 grams NaOH 16 grams NazS03 per liter Hzo*
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Procedure
To a 40-ml. sample to be tested for phosphate, add 5 ml. sulfuric acid and mix thoroughly. Add 1 ml. of molybdate solution and 1ml. of hydroquinone solution. Again shake vigorously and allow to stand 5 minutes. Then add 15 ml. of the alkaline sodium sulfite. If phosphate is present a blue color will form which is then compared with standards of known PO4 content prepared in exactly the same way as the sample being analyzed. For general laboratory analysis it is found convenient to make up standards of 5, 10, 20, 30, 40, and 50 p.p.m. I n the boiler room, however, it is usually sufficient to prepare only two standards representing the upper and lower limits of allowable PO4 concentration. If the color developed in the boiler water is less than 20, for example, more sodium phosphate should be fed. If the color is greater than 30, the addition of phosphate may be curtailed. Large test tubes of uniform size or 100-ml. Nessler tubes will be found convenient for comparing the colors. The comparison should be made a t once since the phosphate color begins to fade after about 10 minutes. There is also a tendency for the silicate color t o appear if the sample is allowed to stand 5 to 10 minutes after neutralizing the acid with the alkaline sulfite solution.
