V O L U M E 28, N O . 4, A P R I L 1 9 5 6 bromide addition compounds. The largest disagreement was apparent in the visual titrations using only methyl red indicator; the mixed indicator visual titration mas only slightly better, Data obtained by the various analysts checked very well in both the spectrophotometric and potentiometric titrations. Conductometric titrations were not checked. While potentiometric and spectrophotometric methods are equally satisfactory, the spectrophotometric method appears to be more satisfactory for general use, because it is not necessary to plot the curve to obtain the end point. Once the apparatus has been standardized, the titration can be run to 50% transmittance and the buret reading taken. The needle deflection is also much greater on the spectrophotometer, making i t easier to take readings. Spectrophotometric titrations are especially suitable to visual titrations in which the end point is difficult to see.
507 LITERATURE CITED
Brunishols, G., Bonnet, J., H e h . C‘him. Acta 34, 2074 (1951). (2) Henry, 11. C., Ph.D. dissertation, University of Pennsylvania, (1)
1955.
(3) Johnson, H. It., J . Phys. Chem. 16, 3 (1912). (4) Iiinney, C. IT., ;\Ishoney, C. L., J . Org. Chem. 8, 526 (1943). (5) Kolthoff,I. AI., Sandell, E. B., “Textbook of Quantitative Inorganic Analysis,” 3rd ed., p. 534, Macmillan, Yew York, 1952. (6) RIandell, K. C., Ph.D. dissertation, University of Pennsylvania, 1951. (7) llartins, C. A , Ann. Chem. Justus Liebigs 109, 79-82 (1859). (5) Smeetser, P. B., Bricker, C. E., A x . 4 ~ CHEJI. . 25, 253 (1953). RECEIVED for review August 5 , 1955. Accepted January 18, 1956. Delaware Valley Regional Meeting, ACS, Philadelphia, Pa., February 16, 1956. Abstracted from a portion of a dissertation presented by William J. Bchuele to t h e Faculty of t h e Graduate School of the University of Pennsylvania in partial fulfillinent of the requirements for t h e degree of doctor of plulosophy.
Available Oxygen in Manganese Dioxide M. J. KATZ,
R.
C. CLARKE’, and W. F. N Y E
Signal Corps Engineering Laboratories, Fort Monmootb, N.
Examination of analyses of manganese dioxide for available oxygen indicated very poor agreement among different laboratories. An appraisal was made of the t w o most important methods used in this analysis-the ferrous sulfate method and the sodium oxalate method. It was discovered that good agreement between the methods could be achieved only with samples of natural origin. It was shown that in the analysis of synthetic m’anganese oxides, the oxalate method is subject to a positive error because of air oxidation of the oxalate. The methods gave good agreement when a protective atmosphere was provided in the oxalate procedure.
J. Exactly 6 grams of ferrous ammonium sulfate is introduced into the flask and a 0.5-gram sample of the manganese dioxide to be analyzed is added. Carbon dioxide is passed into the flask, the contents are brought to a boil, and thereaction is allowed to proceed until all of the sample is decomposed. Then 5 ml. of sirupy phosphoric acid is added and the excess ferrous ammonium sulfate is titrated with potassium permanganate. At the same time a blank is run with 6.000 grams of the ferrous ammonium sulfate under identical conditions. The available oxygen, calculated as per cent manganese dioxide, is given by the equation
where
TY = weight of sample B = volume of potassium permanganate required for blank
F
OR the past several years this laboratory has been concerned with the evaluation of manganese oxides for use as depolarizers in dry cells. As a part of this program, the analysis of the oxides for available oxygen is of importance. The ferrous sulfate method (6) was used in specifications defining the minimum available oxygen ( 4 ) . This procedure proved useful in establishing working hypotheses for the correlation of particle morphology and x-ray diffraction powder patterns with manganese to oxygen ratios. Difficulties arose, however, when interlaboratory comparisons were made. Even when the same methods of analysis were used, there were serious discrepancies in results. A survey of the literature showed this to be a problem of long standing. I n 1917, for example, Barnebey (1) pointed out that chemists could obtain discrepancies of 5% in the manganese dioxide content of a given sample, while smaller differences were not a t all uncommon. This has also been the authors’ recent experience. The ferrous sulfate and oxalate methods seemed t o offer most promise as potential referee methods. They are given by the National Bureau of Standards in their certificate for the analysis of manganese dioxide (standard sample No. 25b). Therefore, an investigation was undertaken regarding the general applicability of these methods to manganese oxides with different genetic histories. EXPERIMENTAL
Ferrous Sulfate Method. A 100-ml. portion of 10% (by volume) sulfuric acid is transferred to a 300-ml. Erlenmeyer flask.
* Present address, Chemistry N. Y.
Laboratory, 84th Police Precinct, Brooklyn,
T
N
= sample titer = normality of potassium permanganate
Sodium Oxalate Method. A 100-ml. portion of 10% (by volume) sulfuric acid and 1.000 gram of sodium oxalate are added to a 300-ml. Erlenmeyer flask, followed by a 0.5-gram sample of manganese dioxide for analysis. The reaction is allowed to proceed on the steam bath until the sample is decomposed. The excess oxalate is determined by titrating the hot solution with 0 . W potassium permanganate. Available oxygen as per cent manganese dioxide is calculated from the equation
% M n 0 2 = 4.346 (1/0.067 - T N ) TY
where the symbols have the same meaning as in Equation 1. DISCUSSION
Effect of Phase Type and Stoichiometry. For some samples, results obtained with both methods were consistent; for others there was considerable disagreement. I n the early stages of the work good agreement had been obtained for a number of naturally occurring pyrolusites. Of the various manganese oxides the pyrolusites @-manganese dioxide) make the closest approach to stoichiometric manganese dioxide. On the other hand, for other phase types usually having appreciable oxygen deficiencies, considerable discrepancies had been obtained. Phase type ( 2 ) means a commonly recurring manganese oxide, defined by its x-ray diffraction pattern and by a more or less distinctive morphology. Thus, it seemed that the phenomenon was related in some may to the manganese-oxygen ratio and/or the phase type. In order to test the dependence of analytical inconsistencies on stoichiometry and phase type, two materials, Samples D and 0, were prepared. Sample D is a synthetic P-manganese dioxide.
ANALYTICAL CHEMISTRY
508
Table I. 1
Analyses of Synthetic and Naturally Occurring Manganese Oxides 3
2
Sainplc
NaiCtO4, COI
NazCpOc, NBS
5
6
Av. Diff.,
AV. Diff.,
4
% Manganese Dioxide
FeSO4, NBS
+
C01.284
C01.384
Synthetics
B D
I ,J
0
Na
78.08 78.91 99.34 99.34 90.02 89.63 89.94 89.71 89.35 82.29 81.83 102.2 101.6
75.96 75.65 97.59 97.78 89.69 89.18
93,45 93.75 91.14 90.91 90.43 90,59 90.67 85.38 84.92 80.64 80.29
93.11 92.95
88.56 87.90 78.49 78.68 99.60 99.67
’
74.37 75.00 97.30 97.35 88.94 88.33 88.87 88.24 88.00 77.88 78.10 99.37 99.38
3.80
1.11
2.00
0.30
1.15
0.73
1.41
0.11
4.17
0.70
2.52
0.28
0.99
0.42
Naturals C
E F b
H
L
92.75 92.47 90.59 90.55 90.30 90.56 90.57 85.13 85.13 80.08 79,56
“ ,J. T. Baker manganese dioxide, Lot
0.45
0.06 0.02
0.66
h-0. 8271.
5 NBS standard sample of manganese dioxide, 2513.
It was prepared by thermal decomposition of manganous nitrate. Analytical discrepancies were again obtained. This was confirmed by analysis of Sample AT, a special J. T. Baker reagent, which became available subsequently. The second preparation, Sample 0, was the product obtained from the Volhard reaction (5) used in the determination of total manganese. This method off ered a convenient means of obtaining a nonpyrolusitic illno,, where n is almost 2 and can be computed independently from a known empirical factor (3). Once again the methods gave inconsistent results (Table I). Thus, the effect could be demonstrated by controlling the phase type or n-value independently. Naturals us. Synthetics. Typical results are given in columns 2 and 4 of Table I. For the oxides from natural sources, there is reasonably good agreement, except for a slight tendency of the oxalate results to be high with respect to the ferrous sulfate results. On the other hand, for the synthetic materials the oxalate results were uniformly too high, the differences varying from about 1 to 4%. It is noteworthy that the extent of the difference is reproducible for any given sample but varies from one sample to another. Extraneous Loss of Oxalate. I n the oxalate method high results would be caused by a disappearance of oxalate. Two alternative explanations were available. The first possibility was decomposition of oxalate; the second, that it was subject to air oxidation. Because manganous ion may promote the decomposition of oxalate, it was decided first to check this possibility experimentally. This reaction proceeds according to the equation (3) On the other hand, the reaction with manganese dioxide is given bs
+ HG01
>In02 +,H2S04
-
MnSOa
+ 2CO2 + 2Hz0
(4)
Carbon monoxide is a reaction product only if there is decomposition of the oxalic acid. Accordingly, an apparatus was set
up to permit quantitative measurement of any carbon monoxide which might be evolved during the reaction. I n several runs, no carbon monoxide could be detected. Furthermore, when a carbon monoxide generator, consisting of a hot solution of sodium oxalate in concentrated sulfuric acid, mas put in series with the reaction vessel, the theoretical amount of carbon monoxide was recovered. The conclusion was reached, therefore, that no catalytically promoted decomposition of oxalate occurred. This agrees with Barnebey’s findings (1). Only the second alternative remained to explain the disappearance of an excessive amount of oxalate-namely, by air oxidation. If an air osidation did occur one would expect the effect to be time-dependent. This, in fact, was found to be the case. The oxalate results on the synthetics were very sensitive to digestion time. Results for the natural materials, on the other hand, were insensitive to variations in digestion time. For a given sample the largest analytical discrepancies were obtained for longest digestion periods, For this reason the oxalate data given in Table I are to be regarded as illustrative only, because digestion periods of 4 hours were used in all cases. Various values intermediate between the ferrous sulfate and the oxalate results may be obtained for shorter oxalate digestion periods. Some samples were in solution long before they were removed from the steam bath. Therefore, the continued disappearance of oxalate must be accounted for, a t least in part, by effects of soluble reaction products. I t may be inferred that the time needed to dissolve a sample may influence the extent of the positive error. No quantitative data are available on this point. I t is noteworthy that the pyrolusites which generally require longer digestion times do not show the largest difference. Use of Protective Atmosphere. Each of the reaction vessels was fitted with an inlet tube for carbon dioxide (or nitrogen) and an outlet tube. A flow of gas was maintained in the reaction vessel during the entire reaction and up to the time of titration. Some typical iesults are given in column 3 of Table I. The use of carbon dioxide brings the methods into reasonable agreement, although there is still a residual positive difference in favor of the oxalate method. CONCLUSIONS
The standard oxalate procedure is subject to a positive error because of air oxidation of oxalate. The extent of osidation is variable; it depends on sample origin, and is influenced by iron content. For best results, therefore, the sodium oxalate method should be revised to provide an inert atmosphere before it is generally applicable for the determination of available oxygen in manganese dioxide. No evidence was obtained for the decomposition of oxalate under conditions of the reaction. ACKNOW LEDGRI ENT
h4. S. Fink of these laboratories set up the train for the detection of carbon monoxide and prepared the Volhard product. Thanks are due also to other members of the staff who supplied supporting data for this work. LITERATURE CITED
(1) Barnebey, 0. L., J. Ind. E ~ QChem. . 9 , 961 (1917). (2) Cole, 1%’F., . Wad5ley, A. D., WF’alkely,A., Electrochemical Society, Preprint 92-2 (1947). (3) Kational Bureau of Standards, Circ. 26, 4th ed., 1921. (4) Signal Corps Specification, “Synthetic Manganese Dioxide,” SCL-3117-C (Feb. 27, 1952). (5) Treadwell, F. P., Hall, W. T.. “Analytical Chemistry,” vol. 11, 9th Eng. ed., pp. 550, 559, Wiley, New York, 1942. RECEIVED for review June 4, 1955.
Accepted December 28, 1955.
