Colorimetric Determination of Manganese Oxidation with Bromate in Sulfuric Acid Medium WILLIAM C. PURDY and DAVID
N. HUME
Department o f Chemistry a n d Laboratory of Nuclear Science, Massachusetts lnstitute of Technology, Cambridge 39, Mass.
(Monsanto Chemical Co.). All of the experiments were performed at the same acidity, 2.7M hydrogen ion, wi h several concentrations of manganese. The colors produced varied from amber to magenta. The spectra, together with the curve reported by Tomula and Aho. are shown in Figure 1. Distilled water was used as the blank. Sodium polymetaphosphate (Calgon), pyrophosphoric acid, metaphosphoric acid, and sodium dihydrogen pyrophosphate (Monsanto Chemical Co.), or solutions of pyrophosphate which had stood for some time a t about p H 3, gave eeeentially the same curves. Different absorption curves were obtained, however, with orthophosphoric acid, tetrapotassium pyrophosphate, and sodium tripolyphosphate. Since a colored complex could be obtained using orthophosphate instead of pyrophosphate, this system was investigated by a factorial experiment. The factors were the same as previously used, except for the substitution of orthophosphate for
.i method has been developed for the colorimetric determination of manganese in the centigram range based on the oxidation of manganous ion to a trivalent sulfate complex with bromate in 8 M sulfuric acid solution. The bromine color is discharged with cyanide ion. The complex, measured at 500 mp, obeys Beer's law over the range of 2 to 70 mg. of manganese per 100 ml. of solution, and the color is stable for one week. Interfering substances are reducing agents, chromic ions, and ions forming insoluble sulfates. The effect of the latter can be removed by filtration.
T
HE present investigation was the result of a need for a simple
colorimetric method for measuring therecovery of added manganese carrier in the determination of trace manganese through radioactivation. One characteristic desired for the method was that it be applicable to samples containing 10 to 50 mg. of manganese without extensive dilution or subsampling. The high sensitivity of the conventional colorimetric determination of manganese by oxidation to permanganate, although highly desirable with samples in the microgram range, becomes more inconvenient when centigram amounts of manganese are involved. Alternative methods were considered, and the method of Tomula and Aho (5, 6 ) was investigated because it appeared to be a simple procedure TTith satisfactorily low sensitivity and relatively few interferences. I n it, manganous ion in a sulfuric acid solution was oxidized with bromate to the tripositive state which was stabilized by complexation x i t h "sirupy pyrophosphoric acid." According to the authors, the complex had an absorption peak at 500 mp and obeyed Beer's law from 2 to 40 mg. of manganese per 100 ml. of solution. I n attempting to study the reproducibility and eensitivity of their method, the authors were unable to obtain consistent results. However, it was found that manganese could be determined satisfactorily as the trivalent sulfate complex in strong sulfuric acid.
"1
I
4w
453
I 500
I
5w
600
X in M p
Figure 1.
Spectra of phosphate complexes
I. 23.5 mg. manganese per 100 ml. of solution. Obtained
by
Tomula and Aho also with metaphosphoric acid, pyrophosphoric acid, dihydrogen pyrophosphate, and Calgon 11. 23.5 mg. manganese per 100 ml. of solution, tripolyphosphate as complexing agent 111. 33.5 mg. manganese per 100 ml. of solution, tetrapotassium pyrophosphate as complexing agent 1V. 33.5 mg. manganese per 100 ml. of solution, orthophosphoric acid as complexing agent
EXPERIhl ENTA L
When the procedure of Tomula and .4ho was followed with the substitution of tetrapotassium pyrophosphate for sirupy pyrophosphoric acid, no color was obtained. Only lvhen the acid concentration was increased 2.5-fold did a color appear The reproducibility under these conditions was very poor, with duplicate samples often differing by 10%. A factorial experiment was run in order to determine the factors affecting complex formation. These factors, each at two levels, were acidity, concentration of manganese, concentration of pyrophosphate, and the temperature of the reagent* at the time of color development. The results of this esperinient indicated that acidity was an important positive factor and that there was a strong positive interaction between acidity and manganese concentration. The effect of using different phosphates in place of pyrophosphate was then investigated: orthophosphate from orthophosphoric acid, metaphosphate from orthophosphoric acid heated until the appearance of fumes, polymetaphosphates from Calgon, pyrophosphate from tetrapotassium pyrophosphate (City Chemical Co.) and from orthophosphoric acid heated at 200" C. for 1 hour, and tripolyphosphate from sodium tripolyphosphate
Table I. Effect of Final Concentration of Sulfuric . h i d and Time on Color Produced in Ortho- and Pyrophosphate Solutions
0.1 0.16 0.26 0.33
Color Obtained Pyrophosphate Orthophosphate Standing Standing Initial overnight Initial overnight Colorless Colorless Pale pink Colorless Colorless Magenta Colorless Brown ppt. Broirn ppt. Colorless Magenta Colorless Yellow amber (10 Brown ppt. Magenta (IO min.) Magenta
1
Magenta
Magenta
hfagenta Magenta Magenta Magenta Dark red Dark red Red t o yellow (2 min.)
Magenta Magenta Magenta Magenta Dark red Dark red Yellow
Final
HzS04
Concn.,
.vr
1.5
2 3 4 4.5 A 9
12
256
Brown Brown Brown Brown Blood red Dark red Red t red Dark o yelloir ( 2 rnin.)
ppt. ppt.
ppt. ppt.
Blood red Dark red Dark red Yellow
V O L U M E 2 7 , NO. 2, F E B R U A R Y 1 9 5 5 pyrophosphate. The results were the same-i.e., acidity was an important positive factor and there was a strong positive interaction between acidity and manganese concentration. Intermediate curves between the absorptfon curves of the pure phosphate forms were obtained when mixtures of those phosphates were used. Attempts to determine the combining ratio of manganese with ortho- and pyrophosphate by the method of continuous variations or a molar ratio plot proved futile, because of the formation of manganese dioxide in the absence of excess phosphate. The results of test tube experiments to determine the effect of the final sulfuric acid Concentration on the color produced are given in Table I. The color of the solution on dilution depends on the final acidity. Dilution with sulfuric acid of the same concentration as in the solution resulted in no change of color. The amber-red color of the orthophosphate gave the same spectrum as that for orthophosphate (Figure 1). The spectra of the dark red colorsobtainedin 631and S i l l sulfuric acid with both the orthoand pyrophosphate solutions were the same. This was also the case v,-ith the yellow colors in 12M sulfuric acid. The ultraviolet region of the spectrum was examined briefly. Solutions of the manganic orthophosphate complex showed no ahsorption, but those made from a dihydrogen pyrophosphate in 1.11 sulfuric acid exhibited a peak a t 255 mB, which was fiftyfold higher than the visible peak. A plot of the sulfuric acid concentration us. absorbance a t two manganese concentrations for all of the various phosphates is shown in Figure 2. The comparatively flat region from 7 A l to 9.11 sulfuric acid and the overlapping of all curves in this region suggested that only one complex was present a t this concrntration of sulfuric acid, regardless of the starting phosphate. The work of Bell ( 2 ) on the composition of strong phosphoric acids. suggested that all phosphates would be converted to orthophosphate in 8-11sulfuric acid. I n order to test this hypothesis, samples of meta-, pyro-, and tripolyphosphate, containing equal amounts of phosphorus, were dissolved in 8M sulfuric wid. .%fter neutralizing with ammonia, the orthophosphate present \\as determined by the colorimetric method of Fiske and
1
0
METAPHOSPFATE PRESENT PIROPmOSPHATE PRESEVT
e NC
PHOSPBATE
PRESENT
ORTbOP3OS1111TE PRESENT U4VE.EYGTw
XCmp
0
I
I
i
1
REAGENTS AVD APPARATUS
I
W 06-
0 T91POLYPH3SP~ATE PRESENT
Subbarow (S), and calibrated against orthophosphate in 8"kZ sulfuric acid. All phosphates showed evidence of complete conversion to orthophosphate. The meta-, pyro-, and tripolyphosphate samples gave no test for orthophosphate unless previously made 8 X in sulfuric acid. As further confirmation, two solutions of metaphosphate were prepared in which one solution was made 8.M in perchloric acid, and the other not. Upon neutralizing the acid solution with ammonia, both solutions were analyzed for metaphosphate according to the Jones method ( 4 ) for precipitating barium metaphosphate a t pH 3. The solution which had been acidified showed that metaphosphate was absent.
Because it now appeared that an orthophosphate complex exists in 8J1 sulfuric acid, calibration curves u ere made and proved to be reproducible. The complex obej-ed Beer's law over the range of 2 to 50 mg. of manganese per 100 ml. of solution. Color formation \vas rapid and suggested the possibility of a molar ratio plot to determine the combining ratio of manganese to orthophosphate. When this R as tried, however, the calibration curve wa3 obtained indicating that the complex did not depend on the presence of orthophosphate. A plot of the sulfuric acid concentration us. the absorbance of the complex a t two manganese concentrations and containing no phosphate also is included in Figure 2. No red color could be obtained in 8 X perchloric acid. Absorption spectra and calibration curves run on solutions 8.11 in sulfuric acid and containing no phosphate gave the same curves as obtained under similar conditions with phosphate present. I t waq therefore concluded that a t the high acidities of the method, the sulfate complex predominates.
I
e
257
-I -
FINAL MOLARITY OF HzSO4
Figure 2. Effect of final sulfuric acid concentration on absorbance of complex Lower curve, 7 mg. manganese per 100 ml. of solution; upper curve, 35 mg. per ml. of solution. Phosphate concentration, 2.4 millimoles. Solutions above 9 M in sulPuric acid were measured against reagent blank: in all other cases, the blank was distilled water. Absorbance measurements were made at 500 m p
Sulfuric acid, 18X and 8'21. Potassium cyanide, 0 5 M . Potassium bromate, 0.17.11. llanganese stock solution (approximately 3.5 mg. of hIn per ml.); 10 grams per liter of manganous sulfate monohydrate tvere dksolved in distilled water, and standardized gravimetrically as manganous pyrophosphate. All measurements of absorbance were made on a Bcckman Model DC spectrophotometer using 1-em. cells. PROCEDURE
The sample for the determination should contain between 2 and 70 nig. of manganese and should be dissolved (preferably in dilute sulfuric acid) in a volume not over 50 ml. T o this sample in a 100-ml. volumetric flask is added an amount of concentrated sulfuric acid calculated to make the solution, upon addition of all reagents. 851 in sulfuric acid. Two milliliters of 0.5M potassium cyanide and 5 ml. of 0.17JIpotsssium bromate are then added
ANALYTICAL CHEMISTRY
258 and the contents shaken. The flark should be kept stoppered to avoid loss of hydrogen cyanide. The solution is then diluted t,o the mark with 8.11 sulfuric acid and the absorbanre of the solution measured a t 500 nip in a spectrophotometer.
The temperature at which the color was measured did not have any effect on the absorbance obtained. Most measurements were made with the solutions a t room temperature, but even a 20" rise did not affect the dading.
DISCUSSION
The spectrum of t'he manganic sulfate complex is given in Figure 3. Beer's law is obeyed over the range of 2 to 70 mg. of manganese per 100 ml. of solution. and the absorbance index is about. two times that obtained in the Tomula and Aho pyrophosphate method. The standard deviation of the method was estimated to be 0.006 absorbance units from 12 replicate 15-mg. samples run over a period of a month. In the method bromine is produced both by the oxidation of manganese and by the decomposition of bromate in strongly acid medium. The bromine causes an interfering color which is discharged by the addition of cyanide to form cyanogen bromide. The colored coniplex wts stable for a week without loss of intensity. Three solutions, containing the same amount of manganic sulfate complex, were prepared using lead dioxide as the osidizing agent. After excess oxidizing agent Tvas removed, cyanogen bromide was added to one of the solutions, bromate and cyanide to the second solution, and nothing to the third. The solution containing bromate and cyanide was stable for a week, whereas the other two solutions rapidly lost intensity. These findings agree wit,h those of Belcher and West ( 1 ) and indicate that the stability of the complex is the result of excess bromate in the solution rather than the increased acidity or the presence of cyanogen bromide. The manganic pyrophosphabe complex was stable for 12 hours. The order of reagent addition had no effect on the absorbance of the complex with onc notable exception. The manganese must he added to the solution before t.he bromat,e, for the cyanogen bromide is not a strong enough osidizing agent to oxidize manganese to the plus three state.
INTERFERENCES
Of the interfering ions bromide. chromic, and cobalt in bhe original pyrophosphate procedure, only chromic ion is a sourcp of difficulty in the sulfate method. When present at concentrations equal to that of manganese (15 mg. per 100 ml. of solution) arsenous, ferrous, chromic, and stannous ions show a 4y0 int'ci.ference. Ions forming insoluble sulfates cause difficulty. but this can be prevented by filtmt,ion through a sintered-glass filter funnel, Ions which have been investigated and show no int,ciference at the same concentration as the manganese are robalt, nickel, zinc, ferric, aluminum. copper, stannic, arsenic, cadniiLini, bismuth, cerous, ceric, nitrate, fluoride, bromide, and dichromate. For applying the determination in the measurement of recovcwd carrier, this extent of freedom from interference is more than adequate. ACKNOWLEDGMENT
The authors are indebted t o t8he;\tomic Energy CommiPsion for partial financial support,. LITER 4TURE CITED
Belcher, R., and West, T. S.. Aual. Chim. Acta, 6 , 322 (195'13. Bell, K. N., Ind. Eng. Chevr., 40, 1464 (1948). J . B i d . Chem., 66, 375 (19'15). (3) Fiske, C. H., and Subbarow, T., (4) Jones, L. T.,TKD. ESG.CHEY.. - 4 s . i ~ED., . 14, 536 (1942). (5) Tomula, E . S., and .4ho, V.,A u a . -4cad. Sei. Fennieae, A52, T o . 4 (1939). (6) I b i d . , A55, S o . 1 (1940) R E C E I V E for D review M a y 14, 1'334. .\wepted Sovernber 2 , 1954.
Molybdenum Blue Reaction and Determination of Phosphorus in Waters Containing Arsenic, Silicon, and Germanium HARRY LEVINE, J. J. ROWE, and F. S. GRlMALDl U. S.
,
Geological Survey, Washington 25,
D. C.
hlicrograiii amounts of phosphate are usuall~ cletermined by the molybdenum blue reaction, but this reaction is nqt specific for phosphorus. The research established the range of conditions under which phosphate, arsenate, silicate, and germanate give the molybdenum blue reaction for differentiating these elements, and developed a method for the determination of phosphate in waters containing up to 10 p.p.m. of the oxides of germanium, arsenic(V), and silicon. With stannous chloride or l-amino-2-naphthol-4-sulfonicacid as the reducing agent no conditions were found for distinguishing silicate from germanate and phosphate from arsenate. In the recommended procedure the phosphate is concentrated by coprecipitation on aluminum h>droxide, and coprecipitated arsenic, germanium, and silicon are volatilized by a mixture of hydrofluoric, hydrochloric, and hydrobromic acids prior to the determination of phosphate. The authors are able to report that the total phosphorus content of several samples of sea water from the Gulf of \lexica ranged from 0.018 to 0.059 mg. of phosphorus pentoxide per liter of water.
T
HE literature on thc detci,iiiitiatiori of phosphorus hy tlic
molybdenum blue react,ion is voluminous. The method5 are based on the formation of mol!hdophosphoric acid and its subsequent reduction to a blue ronipound. The original Denigba method ( 1 ) has been riiodified for the determination of phosphorus in sea water. Important papers on water analysis include those of Zinzadze ( 8 ) ,Kalle (4))Redfield et al. ( 5 ) , Woods et nl. ( 7 ) , Harvey (3),and Robinson et a!.( 6 ) . The molybdenum blue reaction is not specific for phorphoruF because arsenic (V), germanium, and silicon also form heteropoly acids with molybdenum, which also yield blue compound5 on reduction. Some selectivity for phosphorus may be obtained by control of acidity. For example, at' high acidity the heteropoly acids of phosphorus and arsenic may be reduced without iriterference from small amounts of silicon and germanium. The interference of arsenate in sea water may be eliminated by reducing the arsenate to arsenite before t,he addition of ammonium molybdate. Various agents-such as sodium hydrogen sulfite--have been proposed for the reduction of arsenic, but the literature rontains conflicting statements as to their effectiveness. It ie also reported that arsenite may enhance the intensity of the blue color of the reduced molybdophosphoric acid.
