274
INDUSTRIAL AND ENGINEERING CHEMISTRY
Although solid tetraphenylarsonium chloride precipitates rather slowly in 3.0to 3.5 M sodium chloride solutions, it is best to use no larger excess than can remain in solution in the aqueous sodium chloride-i. e., about 9 ml.of 0.015M reagent in 3.5 M sodium chloride. With 4.0 M sodium chloride good results are obtained, but the excess of reagent is rather critical. With more than 45 mg. of zinc the precipitate becomes inconveniently bulky, and with less than 0.3 mg. the solubility error is too great and the time required is too long. The error in a series of experiments on pure solutions was *0.09 mg. The acid concentration supplied only by hydrochloric acid should not be greater than 0.4 M . If the solution is made about 0.3 M a slightly larger excess of reagent may be used safely and the precipitate allowed to stand longer before filtering. No zinc precipitates in alkaline solution. INTERFERINQ SUBSTANCES. Zinc is subject to the same interferences as cadmium. These interferences may be aggravated somewhat by the higher sodium chloride concentration re-
VOL. 11, NO. 5
quired to effect complete precipitation of the zinc complex. The means whereby, and the extent to which, interference may be avoided are the same as for cadmium. If the concentration of other substances is unusually high the concentration of sodium chloride should be reduced to 2.5 to 3.0 M to avoid salting out the excess tetraphenylarsonium chloride. Interference by mercury and cadmium cannot be avoided to any extent, although interference by as much as 50 mg. of stannic ion may be eliminated by the addition of 2 to 4 grams of an alkali tartrate. Interference by manganese may be avoided by use of acetate, phosphate, or tartrate. (Table VIII.)
Literature Cited Lamprey, H., thesis, University of Michigan, 1935.
(1) (2) Willard and Smith, IND.ENQ.CREM.,Anal. Ed., 11, 18B (1939). FROMa thesis presented by G. M. Smith to the Graduate School of the University of Michigan in partial fulfillment of the requirements for the degree of dootor of philosophy.
Colorimetric Determination of Manganese with Periodate A Spectrophotometric Study J. P. MEHLIG, Oregon State College, Corvallis, Ore.
A
NUMBER of early investigators (2, 6, 10, 11) studied the reaction between manganous and periodate ions with more or less conflicting results, but WilIard and Greathouse (16) were the first to use it as the basis of a colorimetric method for the quantitative determination of manganese. Their method depends upon the oxidation in acid solution of manganous salts to permanganate by potassium periodate and they state that it is free from all the faults of the other methods for manganese and yields results of a high degree of accuracy. More recently it has been successfully applied to water ( I ) , to animal and vegetable tissues (4, 12, I S ) , and to salt solutions (3). The purpose of the work described in this paper was to make a study of this method by means of the photoelectric recording spectrophotometer (9), with particular attention to the effect of other ions upon the color system. Similar studies of other colorimetric methods have recently been made (5, 7, 14, 16). The recording spectrophotometer is an ideal instrument for making such an investigation, because if two solutions give identical spectral transmission curves they will have the same color under any condition of illumination and t o any observer. By such curves very small differences in color intensity and in hue can be detected.
Apparatus and Solutions All spectrophotometric measurements in the present work were made with the instrument built for the Department of Chemistry of Purdue University by the General Electric Co. (8).
A stock solution of potassium permanganate containing a p proximately 10 mg. of manganese per ml. was made by dissolving the proper amount of the salt in redistilled water and makin the volume up to 1 liter. After 4 days this solution was liltere3 through asbestos and standardized against ferrous ammonium sulfate of known iron content. A 100-ml. volume of this solution was accurately measured into a beaker, 25 ml. of concentrated
sulfuric acid and 6 grams of potassium metaperiodate were added, and the solution was boiled for 2 minutes. After 15 minutes on the hot plate, the solution was cooled to room temperature, accurately diluted t o 1liter in a volumetric flask, thoroughly mixed, and transferred to a brown, glass-stoppered bottle. Each milliliter contained 0.09865 mg. of manganese. Because of the presence of the periodate this solution was stable and could be used as a standard throughout the work. Standard solutions of the metals were prepared from the nitrate or sulfate salts, while the sodium, potassium, or ammonium salts were used for the preparation of standard solutions of the anions. Redistilled water was used throughout. Each milliliter contained 10 mg. of the ion in question. For the stannous, stannic, and antimonous solutions it was necessary to evaporate a known volume of the chloride solution with concentrated sulfuric acid to remove all chloride and then make up to the original volume with dilute sulfuric acid. The resuIting solutions gave negative chloride tests. For producing the color system the procedure of Willard and Greathouse (16) was followed. To 5 ml. of the standard mansolution, representing 0.4933 mg. of manganese, in a tallorm 250-ml. beaker about 25 ml. of water were added followed by 10 ml. of concentrated sulfuric acid, and the solution was diluted to 100 ml. After the addition and solution of 0.3 gram of potassium metaperiodate, the solution was carefully boiled for 1 minute, kept warm for 10 minutes, transferred t o a 250-ml. volumetric flask, cooled to room temperature, made up to the mark, and thoroughly mixed. The spectral transmission curves were determined for a solution thickness of 4.983 cm. The absorption of the glass cell was comensated for by lacing in the rear beam of light a similar cell Elled with redistiaed water.
8”””””
The transmittancy curves for varying amounts of manganese are shown in Figure 1. The peak of the absorption band is located a t 522 mp.
Conformity to Beer’s Law That the color system follows Beer’s law, a t least up to a concentration of 20 mg. of manganese per liter, is apparent from Table I, in which are shown the observed transmittancies a t 522 mp, the wave length of maximum absorption, for six
MAY 15,1939
ANALYTICAL EDITION
275
tion of acid to prevent the precipitation of manganic periodate or oxide and that the minimum concentration of acid required increases with the concentration of manganese. A number of metals, such as silver, lead, bismuth, and mercury, form iodates or periodates which are insoluble in dilute acids, but soluble in high concentration of acids. Because of the high concentration of acid needed, the p H values of the various solutions were so low-less than 1.0-that no attempt was made to determine them. I n studying the possible effect on the color of the amount and kind of acid present, the curve producted by the solution containing 5 ml. of the standard manganese solution (repre senting 0.4933 mg. of manganese), 10 ml. of concentrated sulfuric acid, and 0.3 gram of potassium periodate per 250 ml. was compared with the curves produced bjr a series of similar manganese solutions containing in place of the 10 ml. of sulfuric acid 15, 20, 25, and 30 ml. of concentrated sulfuric acid, 20, 30, and 40 ml. of concentrated nitric acid which had been decolorized by drawing air through it by suction, and 5, 10, 15, 30, and 50 ml. of concentrated phosphoric acid, respectively. From the transmittancies a t 522 mp of the standard solution and each of the other solutions and from the known manganese concentration of the standard solution. by use of the formula employed above in the Beer’s law calculations, the apparent concentration of manganese in each of the other solutions was calculated by the aid of 1 I 1 1 I I I the special color slide rule. The difference 400 420 4 4 0 460 4 8 0 5 0 0 520 5 4 0 560 580 600 620 6 4 0 660 680 between this value and the actual concentraWave Length tion multiplied by 100 and divided by the FIGURE 1. TRANSMITTANCY CURVES FOR MANGANESE actual concentration gave the percentage error. In accordance with the procedure of a manganese solutions of different concentrations, together former study (7) a 2 per cent error was arbitrarily set as the with the transmittancies, calculated by means of a special maximum allowable for negligible interference. Although color slide rule from the transmittancy of the solution convisual methods of color comparison often have a precision of taining 5 mg. of manganese per liter. The calculations were not less than 5 per cent, it was thought advisable to set a made by use of the formula which expresses Beer’s law lower figure to provide for other possible factors. In all cases the curves were practically identical and the error was CI negligible except for the solutions containing 30 and 40 ml. of TZ = T?’ nitric acid, where it amounted to -4.0 and -5.0 per cent, where T I represents the observed transmittancy, expressed respectively. Possibly these large amounts of nitric acid as a decimal, for the solution of concentration c1, and Tz the produced enough nitrogen oxides during the boiling and dicalculated transmittancy for the solution of concentration c2. gestion to influence the color. The observed and calculated values check satisfactorily. Varying the amount of potassium periodate added has no appreciable effect upon the color. Curves for a series of soluTABLE I. VALIDITYOF BEER’SLAW tions containing in 250 ml. 0.4933 mg. of manganese and 10 (1.961-om. cell) ml. of concentrated sulfuric acid with 0.2, 0.4, 0.5, and 0.6 ManTransmittancy at 522 m)r Man- Transmittancy at 552 mp ganese Observed Caloulated ganese Observed Calculated gram of potassium periodate, respectively, showed a negligible MO./l. % % Mo./l. % % error when compared to the standard curve made from the 18.4 84.5 10 18.4 1 84.7 15 7.7 7.9 solution containing 0.3 gram of potassium periodate. There60.2 3 60.2 20 3.7 3.4 *. 5 43.0 fore it is possible to add an excess of periodate when reducing agents are present and thus prevent reduction of the perAdditional evidence that Beer’s law is followed is given by manganate and consequent fading of the color of the solution. the fact that a straight line results when the logarithm of the observed transmittancy is plotted against the concentraS t a b i l i t y of the Color tion. Concentrations above 20 mg. per liter give solutions too Because of the presence of periodate in the solution there deeply colored for visual determination. is practically no tendency of the color to fade on standing. Effect of Reagents Willard and Greathouse (16) found it stable for 3 months when measured by a visual colorimeter and Clark (3) reported it Willard and Greathouse (16)state that the success of the stable for 7 months. reaction depends upon the presence of a sufficient concentra-
INDUSTRIAL AND ENGINEERING CHEMISTRY
276
Curves made a t intervals for six solutions containing, respectively, 0.25,0.75,1.25, 2.5,3.75, and 5.0 mg. of manganese with 10 ml. of concentrated sulfuric acid and 0.3 gram of potassium periodate per 250 ml., after the solutions had stood in glass-stoppered Pyrex bottles in diffuse light, gave no evidence whatever of fading or other change in the color over a period of 2 months. Lack of time prevented tests over any longer period. Such marked stability makes possible the use of a series of permanent standards.
Effect of Anions I n the ion interference studies the curve produced by the standard manganese solution, containing 0.4933 mg. of manganese, 10 ml. of concentrated sulfuric acid, and 0.3 gram of periodate per 250 ml., was compared with the curve produced by a similar manganese solution containing a known amount of the added ion. Calculations of the percentage error were made as explained above. A considerable number of the common anions, cyanide, thiocyanate, iodide, bromide, chloride, oxalate, tartrate, citrate, sulfite, nitrite, and arsenite, have the ability to reduce the permanganate and thus decrease the color intensity. However, by using larger amounts of periodate this reducing action may be prevented for moderate amounts of the ions and the permanganate may be kept in the oxidized state with the color unchanged. In the usual procedure for determining manganese these reducing ions will be removed during the boiling with nitric acid and the subsequent evaporation with sulfuric acid. In general it is preferable to remove them. In Figure 2, curve 4 shows the effect of 10 mg. of oxalate per 250 ml., curve 5 the effect of 3 mg. of nitrite per 250 ml., and curve 6 the effect of 50 mg. of sulfite per 250 ml. In each case 0.3 gram of periodate was used. Certain colored ions, as dichromate, cause a change in hue. In Figure 2, curves 2 and 3 show the respective effects of 10 and 2.5 0.4933 mg. mg. of dichromate per 250 ml. Thiosulfate
VOL. 11, NO. 5
in addition to its reducing action also causes the precipitation of sulfur in the acid solution, Tungstate causes turbidity because of the precipitation of tungstic acid in the acid solution. The effects of the common anions and their approximate limiting concentrations are listed in Table 11.
Effect of Cations In the tests involving barium, calcium, lead, and strontium ions it was necessary to substitute concentrated phosphoric acid for the sulfuric acid, and a standard permanganate solution to which no sulfuric acid had been added was used. I n each case except for lead, where 10 ml. of phosphoric acid were necessary to prevent precipitation, 5 ml. sufficed.
0
Wave Length
FIGURE2. EFFECTOF ANIONS of manganese, diyerse ion, 10 ml. of concentrated sulfuric acid, and 0.3 gram of potassium iodate in 250 ml. of solution. 4.983-om. cell
ANIONSON THE COLOR DEVELOPED BY MANGANESE TABLE11. EFFECTOF COMMON I on
Concentration Mg./250 ml.
Acetate Arsenate Arsenite Borate Bromide Carbonate Chlorate Chloride Citrate Cyanide Dichromate Fluoride Iodide Molybdate Nitrate
(0.4933 mg. of manganese in 250 ml. of solution) Apparent Change Approximate in Manganese Limiting Ion Concentration Concentration Concentration
%
60 100 (As) 50 (As)
Negligible Negligible Change in hue 20 (As) -63.7 10 (As) -36.1 150 (BzOs) Negligible 50 -7.0 50 (0.6 gram KIOI) Negl!g/bIe 200 Negligible 100 Negligible 100 -35.0 50 - 3.0 50 Decolorjzed 50 (0.6 gram KIOd Decolorized 10 - 2.6 100 -10.0 50 - 3.5 10 Change jn hue 2.5 Change in hue 50 Negllgjble 50 (0.6 gram KIOd Negligible 10 10 (Mo)
200
-- 53:4 .0
Negligible
M g . / B B O mi.
.. 0:0
..
Orthophosphate Oxalate
..
Perohlorate Pyrophosphate Silicate Sulfate Sulfite
30
.. ..
7
..
25
010
.. 50 3 5
%
Mg.1B.50 ml.
Nitrite
10 60
Apparent Change in Manganese Concentration
Tartrate Thiocyanate Thiosulfate Tungstate
Decolorized 50 5.9 50 (0.6 gram KIOa) Decolorized 10 10 (0.6 gram K101) - 4 . 4 -43.6 3 Negligible 200 (PZOS) Decolorized 50 50 (0.6 gram KIOd Decolorized -1s.0, 10 10 (0.6 gram KIOI) Negligible
-
100 200 (PZOS) 100 (SiOz)
300 50
10 (0.6 gram KIO4
50 (0.6 gram X I 0 4 10 100 (0.6 gram KIOd 50 1
30 10
Negligible Negligjble Negligible -71.6 -12.5
- 3.6 Decolorized 3.1
-- 2 . 0 Change in hue -- 65.,6 .2
Turbidity Precipitates Precipitates
A proximate eimiting Concentration Mg./26O ml.
.. 0:0
.. ..
6:O 10
..
.. 0:0 3
..
25 10
..
10
25 0.0 0:0
MAY 15, 1939
ANALYTICAL EDITION
277
ing for the color imparted by these colored cations by adding the same amounts to the color standard as are present in the sample. Silver, lead, and mercuric ions form iodates or periodates which are insoluble in dilute acids, but dissolve in high concentrations of acids. Bismuth and stannous ions give turbidity even in strongly acid solution and should be removed. Although antimonous ion readily reduces permanganate, the amount of the former left in solution after evaporation with sulfuric acid is so small that the periodate will easily oxidize it and prevent any reduction of the permanganate. Because of its reducing action mercurous ion causes a decrease in the color intensity (Figure 3, curve 4). Perrous ion reduces the periodate to free iodine which colors the solution and causes a changein hue. It should, therefore, be oxidized by boiling with nitric acid. In Table I11 are listed the effects of the common cations and their approximate limiting concentrations.
Summary A spectrophotometric study shows that the periodate method for the determination of manganese colorimetrically is a most satisFIGURE3. EFFECTOF CATIONS factory one with very few limitations. 0.4933 mg. of manganese, diverse ion, 10 ml. of concentrated sulfuric acid, and 0.3 gram of potassium periodate In 250 ml. of solution. 4.983-om. cell Sulfuric, nitric, or phosphoric acid may be present in widely varying amounts. A large excess of periodate may be used without interfering with the color. The most common interference caused by cations is due to The color system follows Beer's law. The color is stable in those which impart a color of their own to the solution and diffuse light for a t least 2 months. thereby cause a change in hue. Examples are cupric, nickelA study of the effect of fifty-six of the common ions was ous, cobaltous, chromic, uranyl, and ferric ions (Figure 3, made. A few seriously interfere with the color, but in the curves 2 and 3). The color caused by ferric ion may be recourse of the determination most of these would be removed. moved by the addition of sufficient phosphoric acid to form the colorless complex ion. The curve then coincides with the standard curve. Willard and Greathouse (16) suggest correctAcknowledgments The writer wishes to express his sincere appreciation to TABLE111. EFFECTOF COMMON CATIONS ON THE COLOR DEVELOPED BY MANGANESE Ion
(0.4933 mg. of manganese in 250 ml. of solution) Apparent Change Approximate in Manganese Limiting Concentration Concentration concentration
Aluminum Ammonium Antimonous Barium Beryllium Bismuth Cadmium Calcium Chromic Cobaltous
Mg./160 ml. 100 100 100 100 50 50 (20 ml. HSSOP)
100 100 50 10 50
Cupric Ferric Ferrous Lead Lithium Magnesium Mercuric Mercurous Nickelous Potassium Silver Sodium Stannio Stennoua Strontium Thorium Uranyl Zinc Zirconium
106 (10 ml. HaPOa) 100 100 100 (20 ml. HsSOa) 100 50
tin
% Negligible Negligible Negligible Negligible Negligible Turbidity Neglig!ble Negligib!e Change in hue Change in hue
++18.1 3.0 Change in hue
Change in hue Negligible Change in hue Negligible Negligible Negligible Negligible -50.7
-
1.1
Change in hue Neglig!ble Negligible Negligible Negligible Negligible Turbidity Negligible Negligible
-
2:o
Negligible Negligible
Mg./850 m2.
... . ....
0:0
.. ..
0:0
.. ..
5 5
0:0
.. .. .. .. .. 50 .. 10 .. ..
..
0:0
.. ..
50
... .
M. G. Mellon of Purdue University, in whose laboratory this investigation was conducted, and to thank him for the privilege of using the Purdue spectrophotometer. Thanks are also given to J. T. Woods and D. H. Byers for their aid in adjusting the spectrophotometer.
Literature Cited (1) Bartow, E., and Thompson, H., Proc. Iowa Acad. Sci., 36, 245 (1929). (2) Benedict, S.R., Am. Chem. J., 34, 581 (1905). (3) Clark, N. A., IKD.ENQ.CHEM.,Anal. Ed., 5, 241 (1933). (4) Davidson, J., and Capen, R. G., J . Assoc. Oficial Agr. Chem., 12, 310 (1929); 14, 547 (1931). ENQ.CHEM.,Anal. E d . , (5) Fortune, W. B., a n d Mellon, M. G., IND. 10, 60 (1938). (6) Langlois, Ann. chim. phys., [3] 34, 257 (1852). (7) Mehlig, J. P., IND.E N G .C H E W ,Anal. E d . , 10, 136 (1938). (8) Mellon, M. G., a n d Kasline, C . T.. Ibid.. 7, 187 11935). (9) Michaelson, J. L., a n d Liebhafsky, H. A., Gen. Elec. Rea., 39, 445 (1936). Price, W. B:, Am. Chem. J., 30, 182 (1903). Rammelsberg, Ann. Phys. Chem., 134, 528 (1868). Richards, M. B., A n a l y s t , 55, 554 (1930). Skinner, J. T., a n d Peterson, W. H., J . Biol. Chem., 88, 347 (1930). (14) Swank, H. W., a n d Mellon, M. G . , IND. EXQ.CHEM.,Anal. Ed., 9, 406 (1937); 10, 7 (1938). (15) Willard, H. H., and Greathouse, L. H., J . Am. Chem. Soc., 39, 2366 (1917). (16) Wright,'E. R'., and Mellon, M. G . , IND.EKG.CHEM.,Anal. Ed., 9, 251, 375 (1937).
