Determination of Silicon in Sea Water THOMAS G. THOMPSON AND HAROLD G. HOULTON, University of Washington, Seattle, Wash.
T
HE presence of compounds containing silicon dissolved
in natural waters was apparently first noted by Bergman (3) in 1770 and the first quantitative data were reported by Bunsen (4) in 1847. T h e e years later, Forchhammer ( 7 ) noted the presence of silicates in sea water collected near Copenhagen. I n 1891 Murray and Irvine (11) summarized the quantitative data of other investigators on the silicon content of sea water, elaborated considerably on previous work, and attempted to show the role of the compounds containing silicon in the sea. This pioneer work may now be criticized because of the admitted lack of suitable analytical refinement and the failure to prevent contamination of the water samples. I n 1905 Raben (12) made a detailed study of the determination of silicon in sea water using a gravimetric method which has since been improved by Wells (18). This procedure is undoubtedly the most reliable of the gravimetric methods, but the main objections to it are the large supply of water necessary for a determination, the length of time required, and the ease with which a sample may be contaminated. The first application of a colorimetric method for silicates was that by Jolles and Neurath (8). These investigators used as their reagents concentrated nitric acid and a 16 per cent solution of potassium molybdate. The yellow color, developed by the formation of the silicomolybdate complex, was shown to reach maximum intensity when heated for a short interval. Solutions of water glass of known silicon content, accorded the same treatment as the samples, were used as standards. Later workers ( I O , I S , 14) eliminated heating and substituted the use of other mineral acids. In 1914 Winkler (19) introduced the use of potassium chromate solutions as artificial color standards and thus avoided any contamination of water-glass standards by glass vessels. Dienert and Wandenbulke (6) modified Winkler's procedure by the introduction of standards prepared from picric acid solutions. Atkins (2) showed that the colorimetric method was applicable to sea water, while the preparation of the picric acid standards for such determinations has been made the subject of investigation by King and Lucas (9). COLORIMETRIC DETERMINATION OF SILICON The chemistry of the method, as shown by Asch ( I ) , depends upon the formation of a yellow heteropoly acid, having the probable formula of HsSi(Mo2OT)e when an acidified solution of a soluble silicate is treated with ammonium molybdate. The method is accurate and is particularly advantageous because of the small amount of sample required and the ease and rapidity of manipulation. The procedure and preparation of the reagents required are as follows: AMMONIUMMOLYBDATE REAGENT.Ten grams of ammonium molybdate are dissolved in 90 grams of distilled water. The heptamolybdate, (NH&Mo70~a.4Hz0,is the usual compound employed for this reagent and should be examined from time t o time to insure freedom from silicates. PREPARATION OF PURE PICRIC ACID. Forty grams of picric acid are dissolved in 100 ml. of benzene on a water bath. The resulting solution is filtered with suction through a Buchner funnel, leaving behind by decantation as much water as possible. The benzene is then partially evaporated, under reduced pressure, until water again appears in dark globules. These globules are decanted off and the excess benzene evaporated completely at reduced ressure, until the picric acid is dry and free from benzene. T i e drying under reduced pressure is performed on a water bath. gmall quantities of picric acid are placed in clean, dry test tubes, and the tubes are sealed and stored until ready for use.
PREPARATION OF PICRIC ACID STANDARDS. A solution is prepared by dissolving 314.2 mg. of the vacuum-dried, recrystallized picric acid in distilled water and making up to a volume of exactly 1 liter. This solution has a color equivalent of 10 mg.-atoms of silicon per kilogram for sea water of chlorinities from 14.5 t o 19.5 per mille. Fifty milliliters of the solution are diluted to a volume of 1 liter and portions of it are taken and diluted in volumetric flasks to 250 ml. Four milliliters of the picric acid solution, when diluted to 250 ml., are equivalent to 0.008 mg.-atoms of silicon per kg. when a 50-ml. Nessler tube is filled to the graduation. Solutions with different portions of icric acid are thus prepared and transferred to glass-sto gottles. When standards are desired, a series of 50-ml. tubes are filled to the mark with these solutions. Each increase of 2 ml. of the picric acid on dilution to 250 ml. ives an increase of 0.004 mg.-atoms of silicon per kg. A series of such standards is most convenient for field work and is stable over a considerable period of time. If it is desired to report as milligram-atoms of silicon per liter, the original picric acid solution should contain 307.5 mg. of picric acid per liter instead of 314.2 mg. METHOD. To 50 ml. of freshly sampled sea water, secured directly from the sampling a paratus as it arrives aboard ship, 2 ml. of the ammonium mofybdate reagent are added and 4 drops of 18 N sulfuric acid. After 5 minutes, the resulting color is compared either with a picric acid standard in a colorimeter or with a series of standard solutions in Nessler tubes.
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FURTHER EXPERIMENTAL INVESTIGATIONS A study of four possible factors that might influence the colorimetric determination of silicon in sea water was made and the observations noted are summarized below. POSSIBLE DIFFERENCESIN BEHAVIOROF META- AND ORTHOSILICATES. To ascertain whether an inherent error in the determination may be encountered when different ions containing silicon are present, two standard solutions, one of sodium metasilicate and the other of sodium orthosilicate, were prepared. Weighed amounts of pure silica were fused with sodium carbonate and sodium hydroxide, respectively. The fused masses were dissolved in water, neutralized, and each diluted to a volume of 1 liter. Immediate analysis of these solutions colorimetrically gave the following: SILICA
True value Mg.-at./l. Orthosilicate Metasilicate
0.083 0.083
Value obtained Mg.-at./l. 0.082 0.082
These data were explained by the fact that one atom of silicon occurs in the ortho ion as well as in the meta ion and thus the same coordination of the silicon atoms with the molybdate radicals takes place and develops the same color intensity. EFFECT OF TEMPERATURE ON SILICOMOLYBDATE COLOR. A series of samples containing the same amount of silicon was run a t various temperatures from 7" to 25" C., but change in color intensity could not be detected with either the samples or the standard picric acid solutions. EFFECT OF FUNGUS GROWTHS IN PICRIC ACIDSTANDARDS. A fungus growth was noticed in several picric acid standard , solutions and the colonies started were permitted to continue. Three months later, on dilution of the standard and comparison with freshly prepared solutions, no diminution in color intensity was discernible. EFFECTOF LONGSTANDING ON PICRIC ACID STANDARDS. Picric acid solutions to be used for the preparation of standards were made in March, 1930. They were allowed to stand 17 months, part of the time subjected to the action of sunlight. I n August, 1931, these samples were diluted in the 417
418
ANALYTICAL EDITION
usual manner and compared with freshly prepared solutions.
No change whatsoever was observable. One year later (August, 1932), standards were again made from these solutions and compared with those freshly prepared and no change was noted. This fact appears t o contradict the observations reported by Atkins (6),who noted a change in 3 months, but the authors have likewise observed that continual exposure of the diluted solution to the dust of the air, etc., may cause a fading in color within the period mentioned by Atkins. However, these diluted solutions may be kept for a considerable period in stoppered tubes, the stoppers being removed only when comparisons are to be made.
USE OF COLORIMETRIC METHOD As the result of a detailed study of the method used in the laboratory and aboard ship, the following notes summarize certain observations and precautions necessary in the application of the method: 1. A greenish tinge is sometimes obtained with water having more than 0.07 mg.-atoms of silicon per kg., especially if Nessler tubes are employed. This interference may usually be eliminated by the use of the colorimeter or by the reduction of the size of the sample to 25 ml. es, 100-ml. sam les should 2. With waters of low si1 be utilized. A sensitivity is with Nessler tuge series of 2 parts of silicon to 100,000,000 parts of water. 3. An excess of sulfuric acid causes a diminution in the color intensity, but no effect is noted with slight excesses of the molybdate solution. The yellow color of the hetero oly acid reaches its maximum intensity within 5 minutes after t f e addition of the reagents and remains constant for nearly 3 hours. However, the samples should be compared with the standards as soon as possible to avoid any increase in silicates caused by the dissolving action of the sea water on the glass container. 4. A maximum probable error of 5 er cent occurs when Nessler tubes are employed and this may ge reduced by the employment of the colorimeter. 5. Dissolved compounds containing iron or phosphorus in sufficient quantities to affect the determination are not encountered in sea water (1, 15). 6 . Organisms and finely divided inorganic material, if present in sufficient quantities, may be removed by filtration or centrifuging. 7. Contamination from glass bottles containing the sea water samples is ver marked, as shown by Atkins (2) and Thompson and Johnson 6‘7). Thus water that has been ex osed to glass for some length of time should not be analyzed &r silicon. If containers other than glass are employed, they soon corrode and the sediment settling to the bottom may absorb silicates. Thus for reliable results analyses should be performed only on freshly sampled sea water. 8. The c. P. picric acid contains varying quantities of water, and such material should never be used for the preparation of standards without recrystallization and drying.
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9. Picric acid standards should never be prepared with sea water, as the dissolved salts have a very pronounced “salt effect.” 10. Should it be deemed advisable to dilute samples with distilled water, an examination of the latter for silicon should first be made. 11. To insure concordant results on long trips, it is advantageous to prepare two standard stock solutions from two different batches of pure picric acid. The comparison standards are made from one of these solutions and these are checked occasionally against the second standard. 12. Various means of reporting the quantit of dissolved silicates in sea water occur in the literature. T l e authors feel that the most logical form of reporting the results is that recommended by a committee of chemists representing the different marine and oceanographic institutions on the Pacific Coast of Canada and the United States. The committee recommended that the constituents of sea water be reported as milligram-atoms of the element determined per kilogram of water. A milligramatom is defined as the result obtained when the number of milligrams of the determined element per kilogram of sea water is divided by the atomic weight of the element ( 5 ) . 13. The silicon content varies considerably in sea water ranging from less than 0.01 mg.-atom of silicon per kg. in surface waters where there is marked plankton growth to as hi h as 0.3 mg.-atom in the bottom ocean waters. Generally speafing, the silicon content increases with depth, and coastal waters as well as those of estuaries will show a seasonal fluctuation.
LITERATURE CITED Asoh, W., “Silicates in Chemistry and Commerce,” p. 16, Constable, 1913. (2) Atkins, W. R. G., J . Marine Biol. Assoc. United Kingdom, 13, (1)
154 (1932): 14, 89 (1926): 15, 91 (1928): 16, 822 (1929).
(5) Bergman, T., “De aquis Upsaliensibus,” Upsala, 1770. (4) Bunsen, R., Liebigs Ann. Chem., 62, 49 (1847). (5) Carter, N. M., Moberg, E. G., Skogsberg, T., and Thompson, T. G., Proc. Fifth Pacific Sci. Congr., 1933 (in press). (6) Dienert, F., and Wandenbulke, F., Bull. soc. chim., 33, 1131-90 (1923).
Forohhammer, G., Proc. Roy. SOC.Edinburgh, 2, 38, 303 (1850). (8) Jolles, A., and Neurath, F., 2. angew. Chem., 1, 315 (1898). (9) King, E. J., and Lucas, C. C., J. Am. Chem. Soc., 50, 2395 (7)
(1928).
(10) Lincoln, A. T., and Barker, P., Ibid., 26, 975 (1904). (11) Murray, J., and Irene, R., Proc. Roy. SOC.Edinburgh, 18, 229 f1891). (12)
Riben,‘ E., Wiss. meersuntersuchungen (KieE), 8, 100, 287
(13)
Salvadoni, R., and Pellini, G . , Chem. Zentr., 71, Pt. 1, 834
(14) (15) (16) (17)
Sohreiner, O., J. Am. Chem. Soc., 25, 1056 (1903). Sund, O., J. conseil intern. ezploralion mer, 6, 24-245 (1931). Thayer, L. A., IND.ENG.CHEM.,Anal. Ed., 2, 276 (1930). Thompson, T. G., and Johnson, M. W., Pub. Puget Sound
(18) (19)
Wells, R. C., J . Am. Chem. Soc., 44, 2187 (1922). Winkler, L. W., 2. anorg. Chem., 27, 511-12 (1914).
(1905); 11, 319 (1910); 16, 226 (1914).
(1900).
Bid. Sta., Univ. Wash., 7, 345 (1930).
RECEIVED August 16, 1933.
Note on Shaffer and Hartmann Combined Carbonate-Citrate Method for Determination of Glucose J. 0. HALVERSON AND F. W. SHERWOOD, Nutrition Laboratory, Agricultural Experiment Station, Raleigh, N. C.
I
N MANY respects one of the most rapid and convenient
methods for the determination of dextrose after the hydrolysis of starch by a malt solution is that of Shaffer and Hartmannl in which the “combined carbonate-citrate solution” is used. This is a single stable alkaline copper solution to which is added sufficient potassium iodide and potassium iodate to yield a 0.1 N iodine solution when acidified. With this combined reagent it is necessary only to add the dextrose solution, boil, cool, acidify, and titrate the excess iodine liberated. I n checking this method with a washed starch and also 1
Shaffer, P. A., and Hartmann, A. F., J . E d . Chern., 46, 365 (1921).
against dextrose (No. 41, Bureau of Standards), the amount recovered for the latter, calculated from the copper : glucose ratios of Shaffer and Hartmann, averaged 2.3 mg. more than that started with. In other words, in the authors’ hands and under their conditions, with the standardization of uniform heating by an electric hot plate2 in bringing the solution to a boil, there was somewhat more reduction from a given amount of glucose than that reported by Shaffer and Hartmann. A preliminary examination of the data showed that there 2 I. D. Jones has found that the rate of heating by the slightly fluctuating electric current may be readily controlled by the use of an ammeter and rheostat.
