NOVEMBER 15, 1936
ANALYTICAL EDITION
Literature Cited
455
(1) Hurdg
(5) Meulen, H. ter, and Heslinga, J., “Nouvelles MQthodesd’Analyse Chimique Organique,” p. 17, Paris, Dunod, 1932. (6) Russell, W. W., and hlton, J. W., IND.ENQ.CHEM.,Anal. Ed., 5, 384 (1933). (7) Russell, W. W., and Marks, M. E., Ibid., 6,381-2 (1934).
(4) Ibid., 53, 118-25 (1934).
RECEIVED July 16, 1936.
D.9 ‘‘PYr0his of Carbon ComPo~nds~” P. lo59 New York, Chemical Catalog Co., 1929. (2) Marks, M. E., IND.ENQ.CHEM.,Anal. Ed., 7, 102-3 (1935). (3) Meulen, H. ter, Rec. truv. chim., 43, 899-904 (1924). . ,
Photometric Determination of Silicate in Sea Water REX J. ROBINSON AND HERBERT J. SPOOR Chemical and Oceanographic Laboratories, University of Washington, Seattle, Wash.
T(4
HE colorimetric method of Dienert and Wandenbulcke
is now used universally for the determination of silicate in marine waters. In this method the yellow silicomolybdate complex, which develops quickly in a sulfuric acid solution, is compared with permanent standards prepared from picric acid. The quantity of picric acid in the standards as recommended by Dienert and Wandenbulcke has been shown to be erroneous and more recently has been changed (6, 7 ) . Winkler’s method (14)’ which utilizes a hydrochloric acid solution of molybdate for the reagent and potassium chromate solutions as standards, is recommended by the American Public Health Association (1). Steffens (9) did not think that the reddish yellow color of a potassium chromate solution exactly matches t h a t of the silicomolybdate complex, but Swank and Mellon (11),after completing a spectrophotometric investigation of both picric acid and potassium chromate standards, concluded t h a t potassium chromate standards buffered with sodium tetraborate match very well and are preferable to t h e picric acid standard. The photometer has been receiving increasing attention in colorimetric analysis because of its great sensitivity, its removal of colors which interfere with t h e normal visual method, the elimination of color standards, and the ease with which the factors of the method may be studied. Strohecker, Vaubel, and Breitwieser (10) have made a photometric investigation of the Dienert and Wandenbulcke method, using fresh waters. The authors’ paper is concerned primarily with the photornetric determination of silicate in marine waters.
Preparation of Reagents and Standard Solutions
and sodium chloride were weighed directly. Magnesium chloride and calcium chloride were precipitated as carbonates with sodium carbonate and dissolved with a calculated quantity of hydrochloric acid. The previously weighed salts were diluted to a volume of 1 liter. This solution was preserved in a paraffined bottle to prevent silicate contamination from the glass. STANDARD PHOSPHATE SOLUTION.Pure dry potassium dihydrogen phosphate (4.084 grams) was dissolved in distilled water and diluted to 1 liter. One milliliter contains 0.030 mg. at. of phosphorus. Solutions containing 0.0003 and 0.00003 mg. at. per ml. were prepared by dilution. AMMONIUM MOLYBDATE REAGENT.Ten grams of c. P. (NH4)6Mo~Oza.4H20were dissolved in 90 ml. of silicate-free distilled water. SULFURIC ACIDSOLUTION.A 6 N solution was prepared from concentrated c. P. acid. PICRICACID STANDARD.Vacuum-dried picric acid (307.5 mg.), recrystallized from benzene (IS), was dissolved in distilled water and made to a volume of 1 liter. Fifty milliliters of this solution were diluted to a volume of 1 liter. One milliliter is equivalent to 0.0005 mg. at. of silicon. POTASSIUM CHROMATE STANDARD.This solution was prepared according to Swank and Mellon (11).
Procedure To a 100-ml. sample of silicate solution are added PROCEDURE. 4 ml. of ammonium molybdate reagent and 0.5 ml. of 6 N sulfuric acid. After standing 5 minutes the transmission is determined with a Zeiss Pulfrich (gradation) photometer.
Experimental The measurement of t h e transmissions of the dilute solutions was determined with a 25-cm. absorption cell; distilled water in a similar cell was the reference standard. A 5-cm. cell, with air as the reference standard, was used with the more concentrated silicate solutions. The recorded transmissions are the averages of the drum readings before and after interchanging t h e standard and silicate solutions to avoid the possibility of error from unequal illumination. All experimental determinations were made in duplicate at least.
STANDARD SILICATESOLUTION. A stock solution was repared by dissolving 1.421 grams of NazSiOa.9HZ0(theoretical Formula) in silicate-free distilled water, diluting to 2 liters, and storing in a paraffined bottle. The silicon concentration was determined ravimetrically (5), acidimetrically (8), and colorimetrically 7) against picric acid standards. The gravimetric analysis, SELECTIONOF FILTER. The Zeiss Pulfrich (gradation) showing the silicon concentration to be 0.00281 mg. at. of silicon photometer is equipped with filters, each having an effective per ml., was considered to be the most accurate. (Milligram range of 250 8. A study of the light absorption by the yellow atom, abbreviated mg. at., is defined as the milligrams of the silicomolybdate in synthetic sea water containing 0.070 mg. at. element divided by its atomic weight.) The concentration of of silicon per liter showed that the maximum absorption occurs this stock solution was checked every few days. From this in the violet end of the spectrum. Therefore the 5-43 filter solution was prepared by dilution a standard solution containing with average wave length 4300 A. was used for the remaining 0.000281 mg. at. of silicon per ml. measurements. It has been shown (4, 5 ) that silicon in the colloidal form is not TIMEOF COLOR DEVELOPMENT.The intensity of the yellow estimated by the colorimetric method. The fact that the coloration has been shown to be somewhat dependent upon the analyses by the acidimetric and colorimetric methods checked time elapsed after development. Dienert and Wandenbulcke the gravimetric analysis indicated that the silicate was in the thought that the maximum color developed within 10 minutes, crystalloidal form. but that fading did not begin for 3 or 4 hours. Swarta (12) inSYNTHETICSEA WATER, SILICATE-AND PHOSPHATE-FREE. vestigated this factor and reported that the maximum color Synthetic waters of varying chlorinity were prepared by diluting developed instantly. The authors found that photometrically a synthetic water of a chlorinity of 19.00~/oo.This stock solution the full color development occurs within 3 minutes and does not was prepared by dissolving er liter 0.7455 gram of potassium fade for a t least 2 hours. chloride, 3.9066 grams of so&m sulfate, 23.38 grams of sodium TEMPERATURE EFFECT. The temperature at which reaction chloride, 5.000 grams of magnesium chloride, and 1.1097 grams takes place influences the intensity of color formed in certain of calcium chloride. Pure potassium chloride, sodium sulfate,
7
456
INDUSTRIAL AND ENGINEERING CHEMISTRY
.02 .04 .Ob .06 M G . RT. 51 PER L I T E R
.IC
FIGURE1. PHOTOMETRIC TRANSMISSION CURVES FOR POTABSIUM CHRONATE, PICRIC ACID, AND SODIUM SILICATE In distilled water and i n synthetic sea water, using a 25-om. cell. Concentrationsof potassium chromate and picric acid expressed in terms of silicate equivalencies.
VOL. 8, NO. G
to the work of King and Lucas (6) for picric acid and Swank and Mellon (11) for potassium chromate. The potassium chromate standards were buffered with sodium borate according to the directions of Swank and MelIon. Comparison of the various curves in Figure 1 showed that the chromate standards agreed more closely with the silicate solutions in distilled water than did the picric acid solutions. Swank and Mellon, after their spectrophotometric investigation, also advised the use of the buffered chromate standards in preference to the picric acid standards. Strohecker, too, observed that the chromate standards agreed more closely with the silicate than picric acid above 10 mg. of silicon dioxide per liter, though below this concentration he found satisfactory agreement. There are several other reasons for selecting potassium chromate rather than picric acid as the standard: (1) The equilibrium in the buffered solution between the chromate and the dichromate ions ensures a definite color intensity. No special means is taken to regulate the equilibrium between the two tautomeric forms of picric acid. (2) The purification of potassium chromate is more convenient. (3) Since the absorption per gram of potassium chromate is less than for picric acid, a larger weight is necessary which lessens the error invoIved in weighing. (4) Chromate solutions, if permitted to stand in glass containers, have little effect upon glass containers, while dilute picric acid solutions often become contaminated with large quantities of glass slivers.
colorimetric methods. Thompson and Houlton ( I S ) found that temperature had no appreciable effect on the development of the silicomolybdate in sea-water solutions; Brujewicz (S), also working with marine waters, asserted that 1 to 2 per cent less color formed with a 10' t o 15' C. lowering of temperature, though the authors do not understand how his method of determination possessed such an accuracy. The temperature effect was investigated photometrically by the authors for both distillei water and synthetic sea-water mediums at temperatures of 22 and 6" C. No difference in color intensity could be detected.
COMPARISON OF COLOR INTEXSITY WITH WINKLER A N D WANDENBULCKE REAGENTS.A repetition of the work of Swank
and Mellon (11) with fresh waters confirmed their findings that the Winkler reagent with hydrochloric acid yielded 1.09 times as much color for the same amount of silicate as the reagent of Wandenbulcke with sulfuric acid. In synthetic sea water the color intensity was essent'ially tjhe same with either reagent. HYDROGEN-ION CONCENTRATION AND COLOR INTENSITY. Dienert and Wandenbulcke studied the effect of acidity upon the reaction and noted that an excess of acid depressed the formation of the yellow siIicomoIybdate compound. When Atkins ( 2 ) first applied the method to marine waters, a blue coloration developed with the acidity that Dienert and Wandenbulcke recommended which was eliminated only when the hydrogen-ion concentration was decreased to a pH of 2. Photometric investigation of the effect,of pH demonstrated that the maximum color developed in the synthetic sea water between a H of 1.5 and 2.3. With increasing acidity, slightly less color was cfeveloped.
CALrBRmroN CURVES. Silicate in fresh water has been determined photometrically by Strohecker over a considerable range of concentrations with a 3-cm. cell. However, many waters contain less silicate than may be estimated accurately with a cell of this length. For such waters 5- and 25-em. cells possess more desirable ranges, Photometric calibration curves were prepared for both fresh water and synthetic seawater mediums in such cells. The data are recorded in Table I and Figure 1. Transmission data were also obtained for picric acid and pota,ssium chromate at these concentrations t,o permit coniparison with the silicate curves. The data have not been included in this paper, but the transmission curves for the 25-cm. cell are shown in Figure 1, where the concentrations of both the picric acid and potassium chromate standards have been plotted in terms of their silicate equivalencies. For the calibration of these permanent standards for the colorimetric silicate determination reference is made
BEER'SLaw. Sormally the yellow silicomolybdate color of the unknown solution is compared with permanent standDATAOF SODIUM SILICATE SOLUTIONS TABLE I. TR~NSMISSION Silicon M g . at / 1
0.000 0.004 0.010 0.016 0.030 0.044 0.058 0.072 0.086 0.100 0.114 0.128 0.142
n nnn 0,003 0,008 0.014
0.028
0.042 0.056 0.070 0.084 0.098 0.000 0,007 0.018 0.030 0.056 0,086 0.114 0.142 0.170 0.199 0,227 0 256 0.283 0 000 0,006 0,017 0.028 0.056 0.084 0.113 0.141 0,170 0.197 0.225 0.266 0.281
Reading I
-Log I/Io
Silicon Ic/mg. at.
Distilled Water Medium, 25-Cm. Cell 0.000 100.0 0.086 82.0 0.194 64.0 0,292 51.0 0.530 29.5 0.721 19.0 0.873 13.4 1.114 7.7 1,230 5.9 1.469 3.4 1.668 2.1 1.757 1.8 1.886 1.3 Synthetic 0,000 82.5 0.032 76.7 0.138 60.0 0.217 50.0 0,388 33.7 24.2 0,531 18.5 0.649 0.863 11.2 0.983 8.6 1.161 5 7 Distilled Kater, 5-Cm. Cell 80.0 0: 032 74.5 0.070 66.5 0.125 60.0 0.209 49.5 0,290 41.0 0.384 33.0 0.464 27.5 0.570 21.5 0.652 17.8 0.751 14.2 0.807 12.5 0.843 11.5 Synthetic Sea Water, Cl = 16.08°/oo,5-Cm. Cell 73.0 0'0i9 68.2 0,062 63.2 0,099 58.2 0.143 52.5 0.223 43.7 0.303 36.3 0.370 31.1 0.427 27.3 23.1 0.499 0,558 20.2 0.633 17.0 0.688 15.0
...
0.86 0.77 0.73 0.71 0.66 0.60 0.62 0.57 0.59 0.59 0.55 0.53 , . .
0.43 0.69 0.62 0.55 0.51 0.46 0.49 0.47 0.47
...
0.91 0.78 0.83 0.75 0.68 0.68 0.65
0.67 0.65 0.66 0.63 0.60
...
0.97 0.73 0.71 0.51 0.53 0.54 0.52 0.50 0.51 0.50 0.49 0.49
NOVEMBER 15, 1936 ards prepared by dilution of a more concentrated standard solution. Obviously Beer’s law is assumed to apply not only where the permanent standards are diluted, but also to the silicate solution; or if it does not apply, to deviate identically with both solutions. The applicability of Beer’s law may be tested by plotting the negative logarithm of the transmission against the silicate concentration. If the law is valid a straight line is obtained. I n the case of the 25-cm. cell a definite blank transmission was noted when the synthetic sea water was compared with the distilled water r e f e r e n c e s t a n d a r d . ’With the 5-cm. cell, characteristic blanks, lo,were obtained with both distilled water and synthetic sea-water mediums, since air was the reference standard. To bring these curves to a more comparable basis all the original transmissions, I , were divided by the blank transmissions, lo. It is FIGURE 2. NEGATIVE LOGARITHM OF TRANSMISSION PLOTTED AGAINST SILICATE the negative logarithms of these recalculated COKCENTR ATION (Using a 5-om. cell) t r a n s m i s s i o n s which have been plotted in Figure 2. R e c a l c u l a t i o n d i d n o t c h a n g e the slope of the curve, but merely the intercept. silicomolybdate color develops within 3 minutes and is conOn investigation, Beer’s law was found to apply only apstant for a t least 2 hours; (2) a variation in temperature of proximately, as the curves were not perfectly straight. This 10” to 15’ C. a t the time of development does not influence was particularly true for concentrations greater than 0.20 the color; (3) the maximum color is developed between a mg. at. of silicon per liter. For concentrations less than this, pH of 1.5 and 2.3; (4) in marine waters the reagents of the curves deviated but slightly from a straight line. The Winkler and Wandenbulcke both yield the samecolorintensity. applicability of Beer’s law may also be determined from an The maximum absorption cccurs with a filter having an examination of the specific extinction coefficient per milligram average wave length of 4300 A. Calibration data are given atom listed in column four of Table I. The specific extincfor 5- and 25-cm. cells. Beer’s law was found to apply only tion coefficients were calculated by dividing the negative approximately over the range of concentrations studied. logarithm of the transmission by the cell length and the silicate Less silicomolybdate color develops in a saline medium than . concentration. Beer’s law applies if the specific extinction in a fresh-water medium. To correct previous determinations coefficient remains constant throughout, which was not the which are somewhat in error, a factor of 1.16 should be used. case for silicate. The values for the distilled water medium Potassium chromate solutions are recommended rather than show general agreement with those of Strohecker when they picric acid if permanent standards are to be used. are converted to a common basis. SALTEFFECT.It was also noted that for the same silicate Literature Cited concentration less silicomolybdate color developed in a saltAm. Pub. Health Assoc., “Standard Methods of Water Analymater medium than in fresh water. The effect of varying sis,” p. 66, New York, 1933. was determined chlorinities, 7.00, 11.8, 16.08, and 19.00~/oo, Atkins, W. R. G., S. M a r m e Biol. Assoc. United Kingdom, 14, 89 (1926). using the 5-cm. cell. Though increasing chlorinity slightly Brujewicz and Blinov, State Oceanographical Institute U. S. diminished the intensity of the silicomolybdate color, the S.R., Bull. 14. effect was too small to be significant. I n view of this fact Dienert, F., and Wandenbulcke, F., Compt. rend., 176, 1478 only the data for a single chlorinity have been given with each (1923). Harmon, R. W., J . Phgs. Chem., 31, 629 (1927). cell-17.32°/~~ for the 25-cm. cell and 16.080/oofor the 5-cm. cell. King, E. J., and Lucas, C. C., J . Am. Chem. Soc., 50, 2395 A measure of the salt effect may be obtained from the ratio (1928), of the slopes of the curves demonstrating the validity of Beer’s Robinson, R. J., and Kemmerer, G., Trans. Wisconsin Acad. law in Figure 2. The fresh-water curve for the 5-cm. cell Sci., 25, 129 (1930). Scott, “Standard Methods of Water Analysis,” 4th ed.. D. 447. had a slope of 3.49 and the salt-water curve 3.01, which yielded New York, D. Van Nostrand Co., 1925. a ratio of 1.16. Approximately the same value was obtained Steffens, W., Chem.-Ztg., 54, 996 (1930). from the curves with the 25-cm. cell. This value is considerStroheoker, R., Vaubel, R., and Breitwieser, K., 2.anal. Chem., ably lower than that found by Brujewicz and Blinov by 103, 1 (1935). Swank, H. W., and Mellon, M. G., ISD.ENG.CHEar., Anal. Ed., another method. They reported a correction factor of 1.66 for 6, 348 (1934). the salt error, which is a value larger than might be anticipated. Swarts, M. C., Ibid., 6, 364 (1934). All previous colorimetric silicate values for ocean waters Thompson, T. G., and Houlton, H. G., Ihid., 5, 417 (1933). obtained by colorimetric methods are then too low. To be Winkler, L. W., 2. anorg. Chem., 27, 511 (1914). corrected they should be multiplied by the factor 1.16. RBCEIVED July 13, 1936 With low silicate values the correction would be insignificant. However, in the future when permanent standards are to be used for the determination of silicate in marine waters it is CORRECTION. In the article entitled, “A System for the recommended that the standard be adjusted to compensate Qualitative Analysis of the Alkaline Earth and Alkali Groups” the salt effect. [IND. ENQ.CHEM.,Anal. Ed., 8, 346 (1936)], the reagent employed to detect magnesium was wrongly called p-hydroxySummary benzene-azoresorcinol. The substance used was p-nitrobenzeneazoresorcinol, as is stated in the section on procedure. From a photometric study of the determination of silicate CHARLES H. GREENE in marine waters, it was concluded t h a t (1) the maximum \-
~
