Further Studies of the Molybdenum Blue Reaction R. E. KITSON WITH M. G. MELLON, Purdue University,
Lalayette, Ind.
ml. Nitrate, sulfate, and acetate salts were used for the cations; and sodium, potassium, and ammonium salts were used for the anions.
As a result of a spectrophotometric study of the determination of phosphorus, a modiRed A.O.A.C. procedure is presented which gives solutions of greater color stability and of equal color intensity. The work includes a determination of the effects of the following variables: reagent concentration, temperature of color development, order of addition of reagents, phosphorus concentration, and sixty diverse ions.
STABILITY OF SOLUTIONS PREPARED BY A.O.A.C. METHOD AND STOLOFF’S MODIFICATION. To check the A.O.A.C. method, a series of molybdenum blue solutions of varying phosphorus concentration was prepared following A.O.A.C. directions (1). The transmittancy curves of the solutions were determined 0.5 hour after the addition of the reductant, and a t deiinite time intervals thereafter. The solutions show some increase in color intensity on standing. Although the error thus produced is not serious, it seemed that more stable solutions might be found. A similar experiment, using Stoloff’s modification of the A.O.A.C. method, showed the color to be slightly more stable than that eecured by the A.O.A.C. method. The maximum color is not developed, however; when it is developed by addition of more succinate solution, the color fades rapidly. Stoloff states that 3 ml. of 20% sodium sulfite may be substituted for the sodium succinate in his modified procedure. Maximum color is so develop,ed,but the solutions fade rapidly. The data in Table I show, in terms of per cent transmittancies a t 650 mp, the relative stability of three concentrations of solutions prepared under four different sets of conditions. EFFECTOF pH ON STABILITY OF COLOR. The following experimental procedure was used to study the effect of the final pH on the colorimetric stability of the molybdenum blue solutions.
0
NE of the most frequentlyused methods for the colorimetric determination of phosphates is the so-called molybdenum
blue procedure, which depends upon the formation of molybdiphosphoric acid, with subsequent reduction t o a blue system of uncertain composition. As the blue color formed is unstable, many modifications have been suggested in the effort t o get a more nearly stable system. Woods and Mellon (9) give a review of some past work, together with the results of a spectrophotometric study of some of the methods. In a recent article Stoloff (8) proposed a modification of the official A.O.A.C. method ( f ) , which he stated gave a stable color. He claims that both the stability and color intensity with the A.O.A.C. method are greatest when the final pH of the colored system is between 4.0 and 4.7, and proposes u8e of a sodium succinate buffer in preference to the sodium sulfite recommended in the official method. Although the work described here was started t o check Stoloff’s conclusions, the preliminary results led t o study of the A.O.A.C. method.
Table
EXPERIMENTAL WORK
APPARATUS AND SOLUTIONS.Transmittancy measurements were made in 1.000cm. cells with a General Electric recording spectrophotometer adjusted for a spectral band width of 10 mp. All pH determinations were made with a glass electrode. Since error due to small amounts of silica in the reagents waa alwa s possible, and since the molybdate reagent possessed a sliggt yellow color, the blanks in the reference beam of the spectrophotometer contained compensating amounts of such materials. A standard phosphate solution containing 0.1 mg. of phosphorus per ml. was prepared by dissolving 0.4394 gram of twicerecrystallized potassium dihydrogen phosphate in redistilled water and diluting to 1 liter. Two ammonium molybdate solutions were prepared differin only in acidity. The solution recommended by the A.6.A.C. (If was :p: ared by dissolving 5.0 grams of C.P. ammonium molybdate, &H4)t.Mor0~,.4H~0”, in 60 ml. of water, and adding to this solution one containing 15 ml. of concentrated sulfuric acid in 40 ml. of water. Stoloff’ssolution (8),1 N in.sulfuric acid, was prepared by dissolving 5.0 grams of C.P. ammomum molybdate in approximately 80 ml. of warm (50’ C.) water, adding 2.8 ml. of concentrated sulfuric acid to the cooled solution, and diluting to 100 ml. with water. This solution should not be used if a white residue has settled f r m it. To stabilize the 0.5% solution of hydroquinone, one drop of concentrated sulfuric acid was added per 100 ml. of solution. Two sodium sulfite solutions were prepared. One contained 200 grams and the other 110 grams of C.P. sodium sulfite per liter of solution. The former has been designated by some as a 20% solution; although this designation is not strictly correct, it was retained and extended to the other reagents. A sulfite-carbonate solution was made by dissolving 160 grams of C.P. sodium carbonate in 800 ml. of water, adding a solution of $J grams of C.P. sodium sulfite in 150 ml. of water, and diluting the whole to 1 liter. A solution of sodium acetate was prepared by dissolving 200 grams of C.P. sodium acetate trihydrate in water and diluting to 1 liter. A solution of sodium succinate (E.K. N?. 1219) was prepared similarly. An 0.8 M solution of boric acid was made from recrystallized acid. To observe the effect of 60 diverse ions on the color reaction, standard solutions were prepared containing 10 mg. of the ion per
1. Stability of Solutions Preprred under Different Conditions
Procedure
A.O.A.C.
(sul5te) Stoloff (nuccinatc) Stoloff (sulfite) Recommended (sulfite) 0
Transmittancy 5p.p.m.P 10p.p.m.P
pH
2p.p.m.P
%
%
%
1.8
80.4 59.0 73.0 72.5
27.5 a5.9 49.2 48.0
82.5 73.0 62.4 62.4
di.4
7.2 5.9 28.7 26.9 10.0 20.4 10.0 30.0 9.5 9.3
1.8
2.3 2.3 4.7 4.7 6.5 6.5 3.6 3.5
... ...
...
48.7 30.5‘ 30.1’
pH wan 4 . 5 for this concentration.
8a
466
Time Hour8 0.5 4 0 4 0 4 0 4 0 4
I
I
I
I
I
I
I 1
luly, 1944
ANALYTICAL EDITION
Five milliliters of the phosphate solution were pipetted into a 100-ml. volumetric flask, followed by about 25 ml. of water and 8 ml. of the 5 % molybdate solution 1 N in acid. This solution was thoroughly mixed and allowed to sit a few minutes. After adding 8 ml. of hydroquinone solution, mixing, and allowing the system to stand a short time, the desired amount of buffer was added the solution was diluted to the mark with water, and the spectral transmission curve was determined. The pH was then measured, and the remainder of the solution allowed to stand measured intervals of time, after which either the entire transmittancy curve or the transmittancy a t 650 mp was redetermined. When sodium sulfite was used as a buffer, 30 minutes were allowed to elapse between a4dition of the reductant and measurement of the transmittancy curve. In all other cases the transmittancy curve was determined immediately after final dilution. Sodium SuZ fe. Solutions pre ared with and without small amounts of su te are markedly Jfferent in color, although their H is the same (Figure 1). Solutions prepared without df l t e ave a greenish-blue color and their transmittancy curves show a maximum a t 465 mp. dolutions repared with sodium sulfite have a much more intense blue coror, and the maximum in the transmittancy curve shifts to 445 mp. If the amount of sulfite is increased, the color becomes slightly more intense, the pH increases and the solutions become more stable. Table I1 shows the stability of these solutions a t various H values. The high pH values were secured with the sulfite-car\onate reagent recommended by Bell and Doisy (9). This series of experxaents reveals a range of color stability from pH 2.3 to 5.2 for solutions standing 1.5 hours, and from pH 2.3 t o 4.7 for solutions standing 4.5 hours. The color intensity of all solutions containing sulfite ion is practically the same from pH 1.9 to 6.0. Above RH 6.0 the color intensity increases with increasing pH. Sodium Succinute. In a similar series of experiments with sodium succinate as b d e r the range of best color stability was from pH 2.1 to 2.7 for solutions standing one hour. Maximum color development occurs a t pH 4.1 to 4.9. Solutions having a pH below 2.1 or above 2.9 fade rapidly on standing. Their color changes gradually from a blue-green to deep blue as the pH changes from 2.1 to 4.1.. Sodium Acetate. Solutions prepared with this buffer show transmittancy curves and fading characteristics similar to those obtained using sodium succinate Other Salts. Similar studies were carried out with such buffern as sodium carbonate and potassium hydrogen phthalate. In
E
&
467
Table II. Effect of Sodium Sulfite on Color Stability (5 p.p.m. of phosphorua present throughout) Phosphorus Founds PH 1.5hours 4.5hours MI. P.p.m. P.p.m. Sulfite reagent. 20 per cent 0.0 1.9 6.11 5 67 1.0 i.9 6.25 5.73 2.0 2.1 5.22 5.17 3.0 2.3 5.0s 5.11 4.0 2.6 5.05 5.08 4.7 5.0 6.01 5.01 6.0 5.2 6.08 5.29 7.0 5.5 5.13 5.62 8.0 6.8 4.14 4.65 9.0 6.1 4.52 3.73 10.0 4.54 6.2 3.51 Sulfite-carbonate reagent 5.0 6.1 4.66 8.85 7.5 7.1 4.55 3.55 10.0 8.7 4.10 2.18 12.5 9.1 3.85 2.13 15.0 9.3 3.47 1.64 25.0 8.7 3.53 1.81 50.0 9.9 4.08 1.88 a Calculations based on readin a t 650 mr Transmittancy readingfi taken 0.5 hour after addition of reyuctant were iasumed to repreuent entire concentration of phosphorus.
Solution
general, they did not permit the gradual changes in pH possible with the other substances, or did not cover the entire pH range. The data secured, however, showed that these substances affected the stability of molybdenum blue in much the same manner as the sodium succinate and sodium acetate. Of the various buffers tried, sodium sulfite gave the most stable solutions, and was consequently chosen for use in the remainder of the work. Subsequent investigation showed that solutions prepared with sodium sulfite develop their full color within 5 minutes if the pH is between 4.0 and 4.7. Solutions having pH 3.0 to 4.0 increme in color intensity slowly through the first half hour. Although the error due to this increase is small, it becomes negligible if the color measurements are made a t a definite time ( 1 5 minutes) after reduction. Below pH 3.0, the solutions do not develop their full color for 0.5 hour, and color measurements should not be made before that time. EFFECTOF VARIABLESOTHERTHAN pH. I n the study of other variables, the solution containing the phosphorus waa pipetted into a 100-ml. volumetric flask, the diverse ion was added next, and then enough water to make the total volume 30 to 40 ml. After this, there were added in order and with mixing, 10 ml. of the 5% ammonium molybdate 1 N in sulfuric acid, 10 ml. of the hydroquinone solution, and 10 ml. of the 11% sodium sulfite solution. The solution waa thoroughly mixed after dilution to the mark. The spectral transmission curve and pH were then determined. Normally this procedure gave solutions with a pH of about 3.5. The exact pH varies slightly because of small errors in the addition of reagents and small differences in the strength of different lots of reagent. It is desirable to check the pH of the solutions occasionally, and if the proper value is not being secured, to make suitable adjustments in the strength or amount of reagent added.
Molybdate Concentration. The molybdate concentration was varied, without affecting the pH, by the use of two solutions, one containing 5.0 grams of ammonium molybdate in 100 ml. of solution, the other 1 N in sulfuric acid; 10 ml. of the latter solution were used in all cases. The amount of molybdate wah varied from 5 to 15 ml. without any effect on the color produced. Hydroquinone Concentration. Variation in the amount of hydroquinone from 5 to 15 ml. produced no change in the color. Sulfite Concentration. Except for the expected effect on the pH, variations in the amount of sulfite between 8 and 12 ml. have no effect on the color produced. Order of Addition of Reagents. The order of addition of any of the rea ents except the sulfite has little effect on the 6nal color. must be added last; otherwise the full color is not The developed. Time between Addition of Reogent8. AB much as 5 minutes may elapse between the addition of each reagent without aflectmg
Ate
468
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 16, No. 7
described by Kurtz (6). Five milliliters of 0.8 M boric acid, added before the color development, prevent the interference of 1000 p.p.m. of fluoride. The rolor can be developed immediately after the addition of boric acid. The use of larger amounts of boric acid to remove larger amounts of fluoride ion leads to small negative errors, the magnitude of the ermr increasing dightly with increasing boric acid and/or fluoride concentration. DISCUSSION
Table I1 shows the peculiar effect of raising the pH of these solutions above 5.8. Below this value the color intensity of the solutions either is constant or increases gradually on standing. Above p H 5.8, the solutions fade on standing. Similar results were obtained with l-amino-2-naphthol-4-sulfonic acid as reductant, following the procedure of Fiske and Subbartow (3). If the solutions listed in Table I1 are heated in boiling water for 30 minutes, those with pH 5.6 or lower retain their blue color, while those with pH 5.9 or higher give a colorless solution dt low phosphorus concentration, or an orange-red precipitate at higher phosphorus concentrations. A solution with pH 5.8 showed a slight blue color and a reddish turbidity. The exact cause of this behavior is not known; it is probably connected with the changes in conductmre of molybdiphosphoric acid solutions noted by Jander and Witsmltnn (6) a t pH 5.5 to 6.5. the color, provided blanks are used. If blanks are not used, the sulfite should be added immediately after the hydroquinone. I n the absence of sulfite, the hydroquinone slowly reduces the molybdate and introduces a positive error. Shbility. The stability of these solutions over short periods of time has been discussed. Solutions allowed to stand in glassstoppered bottles for 3 months show considerable increase in color intensity and a change in color. The maximum in the transmittancy curve shifts toward the purple on standing. Heating the Solution. If. the solutions are heated in boiling water after color development, the color intensity increases. The amount of increase is dependent on the lengtb of time of heating. The resulting solutions are no more stable than unheated solutions. Although greater sensitivity could be secured by heating the solutions, the increase was not thought large enough to justify the complications introduced into the procedure. Phosphorus Concentration. The range of phosphorus concentration which can be measured in 1-cm. thickness by this procedure is shown in Figure 2. Beer’s law is valid over the entire range for transmittancy measurements a t 650 mp. Diverse Zm. In observing the effect of diverse ions, 2.5 p.p.m. of phosphorus were used. The transmittancy curve of the solution was drawn within 5 minutes after color development, and the apparent phosphorus concentration calculated from transmittancy measurements made a t 650 mr. In keeping with previous studies of this type, a 2% error was assumed to be negligible. An error not exceeding 2% is caused by 500 p.p.m. 0 , -each of the fol!owing ions: beryllium cadmium, caicium liLhium, magnesium, manganese, potasdum, sodium, uranyi, acetate, benzoate, borate, bromide, carbonate, chloride, cyanide, formate, iodide, lactste, nitrate, oxalate, perchlorate, salicylate, sulfate, tartrate thocyanate, and thiosulfate. The following metals precipitate under the conditions used: barium, bismuth, gold, lead, mercury (-ic or -OW), silver, stannic, strontium, thorium, titanium, and zirconium. Strong oxidizing agents, such as dichromate and permanganate, oxidize the reducing agents faster than the molybdiphosphoric acid; and strong reducing agents, such as sulfide and chlorostannous ions, reduce the excem molybdate reagent to molybdenum blue. Therefore, all strong oxidizing and reducing agents must be absent. Silicates arsenates, and arsenites form hetero oly acids with Molybdlacid molybdate solutions just as phosphorus &ea. silicic acid is readily reduced under the conditions used here, and silicates muet be entirely absent. The heteropoly acid containing arsenic is not RO readily reduced, and 100 p.p.m. of arsenate or 50 p.p.m. of arsenite may be present without interference. The effect of other diverse ions is summarized in Table 111, the calculations being made as described earlier (7). A few curves are shown in Figure 3. Removal oj Flwride Interference. Although fluoride interference is not so serious in this procedure as in other molybdenum blue methods (9), it can be readily avoided with boric acid, as
Table 111. Ion
Added as:
NarHAsOi HIAS04 NarBkOi NarCsH,Oi KClOi NaF KIOI
IC104
KNOi Na4PtCh HIPtCli NaaSeOi
KVOI
Na2WOa
Effect of Diverse Ions Present
Error
Amount Permissible
P.p.m.
%
P.p.m.
50 100 50
0 0
50 100 50
2 2 0 3 0 2
300
10
200 10
300 100 10
500 100 10 100
1 3 New hue 0 60 10
300 10 100 10
300 100 5 0 100 0
20
RECOMMENDED PROCEDURE
SAMPLE. From a representative sample, suitably pre w e d , measure a sample containing at least 0.002 mg. of phosphorus, and dissolve it (if necessary) by appropriate means, Obviously, dissolution in phosphoric acid, or fusion with yro hos hate, is inap licable. Care should be taken to have t t e p\osp!orus in the L r m of orthophosphate. Interfering ions must be removed or their action inhibited to bring their effective concentrations within the limits given in Table 111. Make the resulting solution just acidic to litmus, and dilute to a definite volume in a volumetric flask. (In case of very small amounts of phosphorus it may be necessary to use all the sample. This step would then be omitted.) DESIRED CONSTITUENT.Transfer a suitable ali uot of this solution to a 100-ml. volumetric flask. If Nessler tuies are used for visual comparison, the aliquot should contain 0.002 to 0.20 mg. of phosphorus. For photometric measurement with 1-cm. cells, the aliquot should contain 0.1 to 1.5 mg. of phosphorus. Add enough watef to .make the total volume a t , least 30 ml. (Molybdiphosphoric acid precipitates unless water is added. In case of preci itation, increase the amount of water. If fluoride is present, a$d 5 ml. of 0.8 M boric acid at this point. If more than lo00 p.p.m. of fluoride ion is present, remove it by fuming with sulfuric acid. Too much boric acid causes a small negative error.) Add in order, and with constant mixing, 10 ml. of 5% ammonium molybdate solution 1 N in sulfurlc acid,. 10 ml. of 0.5% hydroquinone solution, and 10 ml. of 11% sodlum sulfite solution. Dilute to the mark, and mix well. The final p H of the solution should be between 3.0 and 4.7; otherwise make suitable adjustment in the amount of the molybdate or sulfite
ANALYTICAL EDITION
July, 1944
reagent. Allow the solution to stand 30 minutes and then measure the color by any of the usual means. (MLasurement can be made the fist half hour after color development, provided the time of measurement is controlled within - 5 minutes.) Visual comparison may be made with standards prepared with known amountsof phosphorus, or with a series of permanent s&,ndards (9). Filter photometric measurement should be made with a red filter. For spectrophotometric measurement in the visual region a wave length between 600 and 700 mp seems preferable. Measurement may be made beyond the color range if a suitable spectroradiometer is available. Thus, Fontaine (4) shows a peak in the absorption band near 820 mp for solutions reduced with chlorostannous acid. The writers' solutions did not show this band, but one much Rharper was found with R minimum near 340 mu.
489 LITERATURE CITED
(1) Assoc. Otficial Am. Chem., Oficial and Tentative Methods of (2)Be,l Analysis, and Daisy, SectionBioi. XII,Chem,, Subsections 44, 55 31, (1920). 32,33 (1940). J ,
66, 375 (1925). ANAL.ED., 14, 77 (1942). ( 5 ) Jandcr and WitZmann, 2.Qn'JW.&7m. C h a . , 215,310(1933). (6) IND* ED.* 14* 855 (1942)* J. Education* 19* 415 (1942). (7) ED.9 14@ 636 (1942) (') Sto'offt IND. (') 13* (lg41).
(3) Fiske and Subbarrow* J a
(4) Fontsine,
I N D - ENQ.CHEM.9
ABBTBACT~D from B thesis presented by R.E. Kitson to the Graduste school of Purdue University in partial fulfillment of the requiremente for the degree of dootor of philosophy, February, 1944.
Quantitative Determination of Mixtures of Methyl Vinyl Ketone and Diacetyl A Dropping Mercury Electrode Method ELLIS 1. FULMER, JOHN J. KOLFENBACH, AND L. A. UNDERKOFLER Chemistry Department, Iowa State College, Ames, Iowa
A dropping mercury electrode method has been described for determination of (1) methyl vinyl ketone alone and in mixtures with methyl vinyl carbinol and methyl ethyl ketone and (2) diacetyl alone and in mixtures with methyl vinyl ketone and methyl ethyl ketone.
9
UDIES are in progress in these laboratories on the catslytic vapor-phase oxidation of methyl vinyl carbinol and of 2,a-butylene glycol. I n the course of this work it became necessary to devise methods for determining methyl vinyl ketone in the presence of methyl vinyl carbinol and methyl ethyl ketone, and diacetyl in the presence of methyl vinyl ketone and methyl ethyl ketone. The present communication presents such meth& using the dropping mercury electrode. The methods are based on the fact that methyl vinyl ketone and diacetyl are reduced a t the dropping mercury electrode while methyl vinyl carbinol and methyl ethyl ketone are not so reduced.
The equipment employed was manually operated and was similar to that described by Kolthoff and Lingane (8)and Lingane and Kolthoff ( 3 ) . The voltage was measured against an external saturated calomel electrode. The current, in microamperes, was calculated from the IR drop across a resistance of 7000 ohms. The pressure on the mercury was maintained a t 80 cm. of mercury; the dro -time was 3.4 seconds. The value of m,in weight of mercury lowing from the capillary per second, wan 0.00134. Dissolved air was removed from the solutions by meanh of gaseoue nitrogen and all determinations were made a t 25" C Table I. Current-Voltage Data" for Methyl Vinyl Ketone Solutions
--
(Molarity X 102)
Current Microampmcr 0.56 0.57 1.31 2.21 2.46 2.46 2.36 2.36 2.36
1.23-
SOURCE OF MATERIALS
The methyl vinyl ketone was made by the catalytic vaporphase oxidation of methyl vinyl carbinol; the details of the method will be made available at a later date. The highly purified methyl vinyl ketone had the following characteristics: b.p. = 19-80" C. a t 760 mm., nL0 = 1.4081, and d:' = 0.862. The diarervl, purchased from the Eastman Kodak Company, was f u r t l i & purified and had the foflowing characteristics: b.p. = 86.5-S7.5'C. at75Omm.,ny 1.3910,,and di8 = 0.980. These constants, for both chemicals, agreed with the best data available in the literature. The methyl vinyl carbinol was purchased from the Shell Chemical Company and the methyl ethyl ketone from the Eastman Kodak Company. I n each case the stock solutions consisted of 1 ml. of the chemical made up to 100 ml. with 0.10 A' potassium chloride. These solutions were diluted with 0.10 .\'
-
Table
APPARATUS AND PROCEDURE
-4.494-
in an atmosphere of nitrogen. Potassium chloride, 0.1 .V, was used as the indifferent electrolyte.
-2.47------
Volts
Current Microamperes
Volts
Current Microamperes
Volts
1.245 1.338 1.412 1.478 1.545 1.615 1.680 1.745 1.820
0.53 0.71 1.71 3.43 4.71 5.20 4.96 4.99 4.99
1.250 1.330 1,400 1.454 1.515 1.581 1.650 1.718 1,800
0.57 0.57 0.90 4.93 7.74 9.07 9.50 9.57 9.57
1.080 1.215 1.290 1.407 1.462 1.527 1.577 1.640 1.720
0 Potential difference between dropping electrode and saturated calomel electrode.
II. Diffusion Currents and Half-Wave Potentials for Methyl Vinyl Ketone
Molarity X 10: ExperiCalcu!ated mental Equation 2 0.494 1.234 2.468
mental
1.43 1.42 1.41
1.80 4.43 9.01
I -
0.493 1.213 2.469
Diffusion Current Calculated Equation 1
Expen-
El/,
1.79 4.50 9.01
Table 111. Current-Volta e Datao for Mixtures of Methyl Vinyl Ketone, Methyl Ethy! Ketone, and Methyl Vinyl Carbinol
---
MVK 0.494 MEK 0.'448 MVC 0.469 Current Microamperea Vob 0.50 0.53 1.57 2.33 2.33 2.33 2.33
...
... ...
1.265 1.314 1.422 1.506 1.580 1.653 1.714
... ... ...
...
--
(Alolnrity X 103) MVK 1.23 MEK = 1.12 MVC 1.17 Current Microampere8 Volts 0.40 0.46 0.89 2.79 4.36 5.04 4.93 4.94 4.93
*.. ...
1.200 1.283 1.352 1.433 1.490 1.557 1.621 1.695 1.760
... ...
---
MVK 1.87 MEK 0.224 MVC 0.234 Current Microamperea Volts 0.53 0.57 1.06 2.86 5.14 6.71 7.07 7.71 7.43 7.37 7.37
1.220 1.270 1.346 1.418 1.470 1.530 1.594 1.655 1.725 1.789 1.850 ... ~
Potential difference between dropping electrode and saturated calomel electrode.
