1156
ANALYTICAL CHEMISTRY
Table I. Times Required to Obtain 2% Probable Error %K 0.1 1.0 10.0 50.0
Net Counts per Minute 2 20 200 1000
Time, Min. 6250 113 8 1
Table 11. Comparison of Chemical and Radioactive Assays Sample No. 1
2 3 4 5 6 7 8 9 10 11 12 13 14
Kz0, Chemical, %
Kz0, Counter, %
26.70 36.34 27.81 29.23 16.12 25.10 20.92 30.12 16.00 30.00 35.55 45.70 60.05 63.00
26.9 36.5 28.6 29.8 15.9 25.2 20.8 31.0 15.0 31.0 40.3 48.5 60.3 61.8
Difference, % -0.2 -0.6 -0.8 -0.6 0.2 -0.1 0.1 -0.9 1.0 -1.0 -0.7 1.2 -0.2 1.2
analyzed, but the apparatus is not recommended for such a use. The times necessary to count samples to 2y0 probable error, with a background of exactly 20 counts per minute, are given in Table I.
The figures in Table I1 show the accord in assays obtained by
’
chemical analysis and by the apparatus described above in the analysis of sylvite ores and concentrates. In order to show the proportionality existing between counting rate and per cent potassium in a low range, Figure 6 was determined experimentally for synthetic mixtures of sand and potassium dichromate. The sand, although radioactively inert, did not decrease the background when counted alone. Hence, a constant background prevailed for all mixtures. These mixtures were prepared from washed screened silica and C.P. screened potassium dichromate. Standards prepared thus are free from errors caused by segregation of the undersize fraction. However, it is desirable to use, as a standard, material that will not change in composition or decrease in uniformity. For this purpose the authors employ a sealed sample holder tightly packed with an ore of relatively high activity. This standard is used for frequent calibration of counter tubes to detect changes in sensitivity. LITERATURE CITED (1) Barnes and Salley, IND.ENGI. CHEM.,ANAL.ED., 15, 4-7 (1943). (2) Clarke, U. S. Geol. Survey, Bull. 770, 5th ed., p. 441 (1924). (3) Evans, “Science and Engineering of Xuclear Power. Chap. I. Fundamentals of Nuclear Physics,” p. 26, Cambridge, Mass., Addison-Wesley Press, 1947. (4) Evans and Goodman, Bull. GeoZ. SOC.,52, 464 (1941). (5) Feather, Proc. Cambridge Phil. SOC.,34, 599 (1938). (6) Gleditsch and Graf, Phys. Reu., 72, 640 (1947). (7) Henderson, Ibid., 71, 323-4 (1947). (8) Libby, ISD. ENG.CHEM.,As.41.. ED.,19, 2 (1947).
RBCEXYED February 21, 1948.
Photometric Determination of Magnesium in Water with Brilliant Yellow JIICHAEL T A R i S Department of Water Supply, Detroit, Mich. Magnesium hydroxide in strongly alkaline solution converts to red the normal orange color of Brilliant yellow dye. The positive effect of aluminum and calcium is compensated for by raising the concentration of these ions to a level where their influence is predictable and constant. Starch and Colloresin are permissible stabilizing agents. High concentrations of chlorine must be removed. The dye has been applied successfully to the analysis of typical natural waters, conventionally treated with filter alum or lime-softened.
B
RILLIANT yellow and several common azo indicators were first suggested by Kolthoff (5) for the colorimetric determination of magnesium, but no critical evaluation of these dyes on a quantitative basis was attempted a t the time. Accordingly, a study of Brilliant yellow was undertaken with a view to ascertaining its admissibility for the magnesium determination. The color characteristics of Brilliant yellow in the presence of magnesium hydroxide closely resemble those of Titan yellow (6). Although an instrument for measuring light absorption a t various wave lengths was not available, experimentation Tvith a series of Wratten filters disclosed that minimum transmittancy occurs in the spectral range covered by the green filter (approximately 525 mp). Despite the close parallel between the Brilliant yellom-magnesium reaction and the Titan yellow reaction, certain peculiar differences confer advantages on the Brilliant yellow system-for example, aluminum ion above 0.5 p.p.m. reduces the Titan yellow color (2) but has an intensifying effect on the Brilliant yelloL7
color. Therefore, in the presence of moderate amounts of aluminum, Brilliant yellow can be considered a more desirable reagent. A method has been devised for determining magnesium directly in a natural water sample without preliminary removal of common interfering ions like calcium and aluminum. Inasmuch 88 interference from these ions is relatively constant a t specific concentrations, compensation can be made by bringing the concentrations up to this uniform level, and a photometric curve can be plotted yith the ions in solution. APPARATUS AND REAGENTS
A Cenco-Sheard-Sanford Photelometer was used in this investigation, with green filters having wave lengths near 525 mp. Stock magnesium solution was prepared by dissolving 10.1353 grams of C.P. magnesium sulfate heptahydrate in 1 liter of dietilled water. Standard magnesium solution was prepared by diluting the stock solution 1 to 10. The solution wm equivalent to 100 p.p.m. of magnesium. The magnesium content was confirmed by gravimetric analysis.
1157
V O L U M E 20, NO. 1 2 , D E C E M B E R 1 9 4 8
50
E
m
4 PPM. MAGNESIUM
z40-
2
Stock aluminum solution was prepared by dissolving 3.09 grams C.P. aluminum sulfate octadecahydrate in 1 liter of distilled water. Standard aluminum solution was prepared by diluting the stock solution 1 to 10. Saturated calcium sulfate solution was prepared from the C.P. anhydrous salt and distilled water. Brilliant yellow solution, 0.05%, was prepared by dissolving 0.50 gram of the solid dye (Sational Aniline Co., Color Index S o . 364) in 1 liter of distilled JTater. Sulfuric acid, 0.037, by volume. Sodium hydroxide solution, 20'5. Starch solution, lrc, was prepared by completely dissolving 1 gram of soluble starch (Eimcr and Amend, Gram-pac) in 100 ml. of boiling distilled water. The solution was prepared fresh every 2 days. Colloresin reagent, 1%, was prepared by dissolving 1 gram of Colloresin 25 (General Drug Co., Aromatics Division, Brooklyn, N. Y.) in 100 ml. of distilled water. Dechlorinating agents consisted of 1% solutions of sodium nitrite, sodium sulfite, and phenol.
of
to a solution containing 2 p.p.m. of magnesium. Figure 1 offers an excellent picture of this property. Calcium exercises a similar influence, though not to the same pronounced degree as aluminum. Figure 2 illustrates the effect of calcium on the magnesium color in the presence and absence of aluminum. The slope of the calcium lines is slight in contrast with the incipient sharp inclination of the aluminum lines in Figure 1. Aluminum and calcium ions of themselves produce no color change in the dye. Coupled with magnesium ion, however, they exert a profound effect on the color intensity. As little as 0.05 p.p.m. of aluminum ion suffices to cause a noticeable intensification in the pink lake. The intensification increases up to a maximum of 1.0 p.p.m. of aluminum, leveling off a t this point up to a high of 10 p.p.m. Above 10 p.p.m. contrary forces begin to operate and the magnesium color is progressively attenuated. At 20 p.p.m. of aluminum, the color of the Brilliant yellow-magnesium system again reaches the equivalent of 0 p.p.m. of aluminum. Thereafter, the ion extinguishes the magnesium color until no magnesium is indicated a t 50 p.p.m. of aluminum. The safe aluminum .concentrat,ion may, therefore, be regarded as 0 to 10 p.p.m. for the purposes of Brilliant yellow analysis. If the water is knovn to contain more than 1 p.p.m. of aluminum, no aluminum standartl need be npplicd to the n-ater being analyzctl. EFFECT OF ELEMENTAL CHLORINE
Heavily chlorinated nater must be dechlorinated before analysis, or an off-color consequent upon the interaction of chlorine and dye will produce erratic readings. Samples containing 0.5 p.p.m. or less of chlorine require no dechlorination.
RECOMMEh-DED PROCEDURE
Starch Reagent. Measure into a 100-ml. volumetric flask a sample aliquot containing between 0.10 and 1.0 mg. of magnesium ion. Follow in order with 1 ml. of sulfuric acid, 20 ml. of calcium sulfate solution, and 5.0 ml. of standard aluminum eolution. .4t this point, bring the volume to 80 ml., and mix the contents of the flask. Then add 10 ml. of starch solution, 1.0 ml. of Brilliant yellow solution, and 4 ml. of sodium hydroxide. Dilute to the mark, shake vigorously, and read after 5 minutes in a photometer against a blank prepared in the same way. A cell depth of 50 mm. gives the best results. Colloresin Reagent. The magnesium content of the sample aliquot must be limited to the range 0.10 to 0.50 mg. Also, 5.0 ml. of Colloresin solution suffices for stabilization, and a cell depth of 10 mm. must be used for transmittancy measurements. Otherwise, the procedure remains the same as for the starch reagent. EFFECT OF ALUMINUAI AND C.iLCIUM IONS OX REACTION
Some photometric procedures based on Titan yellow reconimend the total elimination of aluminum ion before the magnesium analysis is undertaken (4). The recommendation is premised on the observation that in certain concentrations aluminum partially destroys the magnesium color, thus contributing to low readings. Contrasted with this striking phenomenon, aluminum in the presence of Brilliant yellow and magnesium exerts, up to a point, an opposite or positive effect. Aluminum in the amount of 0.5 p.p.m. produces color intensitier, corresponding to tviice the true magnesium value when added
1
1
I
1
25
50
75
r
J
I
100
PPM. CALCIUM Figure 2.
Effect of Calcium on 4 P.P.M. .Magnesium Starch stabilizer used
Three reagents found effective for dechlorination were: sodium sulfite, sodium nitrite, and phenol. One milliliter of a 1% solution of any one of these compounds is sufficient to dechlorinate residuals common in water works practice (up to 5 p.p.m.). Lesser volumes of the reagents may be employed for lower residuals. The conventional dechlorinating agent, sodium thiosulfate, yielded a colloidal sulfur precipitate almost immediately upon addition, and hence is unsuitable in this connection. -111 aliquots of Flat Rock tap water-chlorine residual, 1.5 p.p.m.-were dechlorinated with 1 ml. of sodium sulfite before other reagents rrere added. EFFECT OF OTHER IONS
Interference attributable to ions occurring in the water treatment field was studied in the presence of 4 p.p.m. of magnesium and 5 p.p.m. of aluminum. Iron was found to be without effect in amounts below 2.5 p.p.m., in excess of that concentration con-
ANALYTICAL CHEMISTRY
1158 Table I.
Brilliant Yellow-Magnesium Transmittancy
Magnesium M g . / 1 0 0 ml. 0.2 0.4 0.5 0.6 0.8 1 .o
Starch stabilizer
Transmittancya Colloresin stabilizer
%
%
57.3 42.7 37.0 32.3 28.4 26.0 a Cell depth of 50 mm. used with starch stabilizer. used with Colloresin stabilizer.
66.3 63.0 48.0 47.0
.... ....
Cell depth of 10 mm.
nesium. From 0.60 to 1.0 mg. the curve bows slightly. Irrespective of the divergence, analyses of natural water have been carried out successfully in this concentration interval by referring all readings to the calibration curve. The final results compared favorably with those obtained in the interval wherein Beer’s law prevailed. With Colloresin, on the ot,her hand, the Brilliant yellow-magnesium color system gives a straight line within the effective range of 0.10 to 0.50 mg. of magnesium. A4bove0.50 mg. negligible color development occurs ; therefore, analysis must be confined to this area alone. ,ANALYSIS OF NATURAL WATERS
Table 11. Analysis of Natural Waters
Water
Magnesium Found by Photometric Analysis Starch Colloresin stabilizer stabilizer P.p.m.
7.1
a
hfagnesium Found by Gravimetric Analysisa P.p.m
7.1
Gravimetric analyses on Detroit water performed by C. C. Rausch.
tributing to the magnesium color. Tolerable limits for other ions were: chloride 250 p.p.m., orthophosphate and fluoride 5 p.p.m. Manganic ion, on the other hand, must be entirely absent because of a destructive effect on the dye; it oxidizes the dye even before the addition of the alkali. Zinc ion interferes significantly with color development and likewise must be absent. COLOR STABILITY
A solution of magnesium ion and Brilliant yellow turns orangered or red upon the addition of strong alkali. The density of the red is proportional to the concentration of magnesium (Table I). Devoid of magnesium, the solution retains a typical orange color. The introduction of a protective colloid like starch imparts stability and reproducibility to the color system, inhibiting precipitation for days. Omission of the starch results in the speedy precipitation of magnesium hydroxide above 0.3 mg. of magnesium, and subtracts materially from the sensitivity and reproducibility of the reaction. However, the use of starch entails a recognition of its limitations. First, the stabilizing power of different lots of starch varies widely. Some starches possess practically no stabilizing action, whereas other grades have the power to inhibit precipitation for as long as a week. A starch of the latter grade was available for this investigation. Secondly, the fact that starch is subject to rapid bacterial deterioration necessitates the preparation of small volumes and fresh solutions when determinations are made several days apart. As a result of these drawbacks, new stabilizing agents have been sought for the magnesium-azo dye color system. The two most promising agents uncovered to date are Colloresin 25 (3) and Dupanol C (7). Of the two, Colloresin has been found the most satisfactory for the Brilliant yellow method. Colloresin intensified the red magnesium color to a greater extent than starch, requiring the substitution of cells of 10-mni. depth in place of the prescribed 50-mm. depth of the starch procedure. The magnesium concentration of the initial sample taken for analysis cannot exceed 0.5 mg. Careful control of the applied Colloresin volume must also be practiced. The outstanding advantages of Colloresin are the permanence of the reagent solution and the increased precision possible a t lower magnesium concentrations. In the presence of starch, the Brilliant yellow-magnesium color system yields a straight line in the range 0.10 to 0.60 mg. of mag-
Waters with a magnesium content of 7 to 30 p.p.m. were analyzed photometrically and gravimetrically. At least three, and in most instances four, sample volumes were taken for photometric analysis. Readings were made with a Wratten green filter and a Cenco green filter of wave length 525 mp. Regardless of the color filter or sample volume used, the final magnesium values checked very closely among themselves. Detroit tap water is effluent that has been coagulated with filter alum and sand-filtered. Flat Rock tap water receives similar treatment, additively being lime-softened chiefly for calcium reduction. Flat Rock raw water possessed a color value of 15. Well water 2 had 50 p.p.m. of very fine turbidity that was centrifuged out before analysis was undertaken The gravimetric analyses were conducted in the standard manner ( I ) , with silica, combined aluminum and ferric oxides, and calcium removal preceding magnesium precipitation. In the case of the natural waters the silica ranged between 0.1 and 5 p.p.m., the combined oxides between 0.5 and 4 p.p.m., total iron between 0.04 and 0.6 p.p.m., and calcium between 24 and 60 p.p.m. The data in Table I1 testify to the possibility of accurate analysis in this range of accompanying ions. Indeed, it is logical to presume that slightly improved results are possible with the photometer, since the accumulation of errors inherent in a series of routine gravimetric separations are obviated. This fact has been demonstrated repeatedly in Detroit where the gravimetric analysis seldom gives the same magnesium value for the raw and finished water whereas the photometric analysis shows the values to be identical, as might be expected. DISCUSSION
Under the conditions described in this paper, Brilliant yellow matches and resembles Titan yellow in the photometric determination of magnesium. Where aluminum ion is likely to constitute a problem, Brilliant yellow may be considered superior in some respects because of its predictable positive effect as opposed to the repressive action on Titan yellow. Such a situation often prevails during the winter season, when coagulation is impeded by lower water temperatures. Table I1 indicates that the choice of protective colloid is a matter for the individual analyst to decide. Either starch or Colloresin may be selected, if the starch is of the proper quality. Otherwise, Colloresin is preferable because of its dependability and stability in solution form. ACKNOWLEDGMENT
The suggestion of C. R. Johnson that this work be extended to include Colloresin and Dupanol, in addition to the regular starch stabilizing agent, is hereby gratefully acknodedged. LITERATURE CITED
Public Health Assoc., “Standard Methods for the Examination of Water and Sewage,” 9 t h ed., p. 60, New York, 1946. (2) Bengolea, D. J., and Amato, F. D., Rep. Ohras Sunitarias ( A r g . ) , (1) Am.
20, 7 1 (1947); J . Am. Water Works Assoc., 39,934 (1947). (3) Cox, J. R., and Johnson, C. R., private communication.
(4) Gillam, W. S., IND.ESG.CHEM.,. ~ N A L . ED..13, 499-502 (1941). Mikrochemie, Emich Festschr, 180-90 (1930). (5) Kolthoff, I. M., (6) L u d w i g , E. E., a n d J o h n s o n , C. R., IND.ENG.C H m f . , ANAL ED., 14,895-7 (1942). ( 7 ) Saifer, A , , and Clark, F. D., Ibid., 17, 757-9 (1045). RECEIVED December 3. 1947
