The reaction between potassium permanganate and hydrogen peroxide

The reaction in which n=5 is the generally accepted one. ... lent to those given by the thiosulfate method of deter- mination of ... prepare approxima...
0 downloads 0 Views 2MB Size

THE REACTION BETWEEN POTASSIUM PERMANGANATE AND HYDROGEN PEROXIDE A Special Project in Quantitative Analysis MAX I. BOWMAN University of Louisville, Louisville, Kentucky


shown that, using procedures in standard textbooks of quantitative analysis, this method gives results equivalent to those given by the thiosulfate method of determination of peroxides. Although the equation in which n=5 holds very well under the conditions used in analysis it is not valid for all possible conditions. Several investigators have shown that when Hz02 is added to a large excess of XMnOI thc amount of oxygen evolved is greater than 2 K M n 0 ~ nHpOn 4H-SO4 2MnSOI that pmdicted by the accepted equation. Some early Or (n 3)HtO (1) experimenters thought fhat this was due to the presence of some higher oxide of hydrogens but that view is not in which n may be any odd integer. Even values of n generally held a t present. The increased amount of oxygen could he accounted may be used provided the invariant coefficients are for by allowing n in equation (1) to equal some number doubled. The reaction in which n=5 is the generally accepted less than 5. If n=5, one rnol of H202 will yield one rnol one. Huckaba and Keyes2 have recentdy shown the of 02,if n=3, one rnol of Hz02 yields one and one-third accuracy of this method of determining Ha02 as com- mols of 02,and if n=l , one rnol of the peroxide gives ~ a r e dwith the rtasometric method. It can also he three mols of oxygen. %EI,WR, J. W., "A Comprehensive Treatise on Inorganic STEINBMA,0.F,.J O ~ AOFLCKEM.EDUC. 21,66 (1944). and Theoretical Chemistry," Longmans, Green and Company, 2 Hucxaen, C. E.,AND F. G. KEYES,J . Am. Chem. Soc., 70, potassium perman&nate-hydrogen peroxide equation is one of several that were described by Steinbachl as "nonstoichiometric equations" which could he balanced in several ways. The following general equation will confirm his statement that there are theoretically an infinite number of solutions to this equation, although it does not include all of the possibilities listed by Steinbach:


1640 (1948).


-rq) ++


New York, Vol. I., 1922, p. 245.


Ramsay4 noted also that the quantity of HaOa needed to react with a given amount of KMn04 was less than the amount calculated from the accepted equation in the case in which the Ha02 was added to the acidified KMn04, but he did not investigate this problem in detail. The titration aspect has been rather thoroughly emlored by Rilis and Gomeza? However,' their work suffers from the fact that they used an in: sufficient amount of acid in many of their titrations, alloning the precipitation of oxides of manganese. They recognized this fact, but stated that vigorous stirring caused these oxides to go into solution. Our experience has been that it is difficult if not impossible to get results which check under these conditions. The amount of acid needed to suppress the oxides of manganese is roughly ten times as great when the KMn04is added to the flask &st as when the HsOe is added first. It is possible to explain this in terms of the mass action effect. Addimg KMnO4 first will mean that KMnO4 will be in excess a t all times during the titration until the equivalent point is reached. If alternate reactions are possible, those would be favored which require relatively more KMn04 for a given amount of HzOZ. Substitution in equation (1) of a number less than five will give us equations of this type. Another equation ~ q u i r i n ga higher KMli04/H20~ratio is the following:

meyer flask, then add concentrated acid from a buret or Mohr pipet. After the acid solution has cooled to room temperature 20 ml. of one reagent is added and titrated with the other. For purposes of uniformity, we used a rapid rate of titration and only moderate shaking. Our , results are shown in Table 1. TABLE 1 Minimum Amounts of Acid to Sunress Oxides of Manganese -1 N S o l u t i o n s 4 . l N S o l u t i o n s 4 . 0 2 5 NSolutions 18 N 36 N 36 N 18 N 36 N HnSO, HNOa, HBOI, HNOJ. HaSOa, ml. ml. ml. ml. ml. HnOl first 1.7 4;5 0.6 1.2 0.3 KMnO~first 14 7 14 3


* With up to 60 ml. of concd. FINO$,oxides of manganese were still formed. It may also be of interest to note that with 0.1 N solutions and below, the normal titration (i. e., with HzOa added first) may be carried out with the use of acetic acid as the acidifying medium. However, when the KMnO, is added first it is impossible to add enough acetic acid to prevent the formation of the brown oxides. C . To show the variation in the stoichiometric amounts of reagents titrations-are carried out as above except that the amount of acid used should be a t least 30 per cent in excess of the minimum amount as shown in Table 1. The volume ratios of HzOzto KMnO4 are to be determined in both ways; that is, when the Ha02 is added first and when the KMnO4 is added first. These ratios will be about 5 to 11 parts per thousand less in the latter instance, i. e., with the KMnO4 first. The differential increases somewhat with increased concentration of solutions. Nitric acid may be used as well as sulfuric for the lower concentrations of solutions but i t must first be freed from reducing agents by titration to a slight pink with KMnOd before being used in the KMn04-HaOz titrations. The fact that nitric acid gives results similar to those given with sulfuric acid is evidence against Ramsay's theory that the abnormalities in this titration might be due to the formation of persulfuric acid. After the main points had already been ascertained by the instructor, this project was assigned to two students in the elementary quantitative analysis course for confirmation. These students had previously not shown much interest in the course, but the project served to increase their interest greatly and one of them put in many hours outside the regular laboratory period.

We may assume that adding permanganate first would increase the tendency toward both reaction (2) and reaction (1) with n < 5. Therefore it should require a larger amount of acid to suppress reaction (2) when the titration is carried out with the KMn04ad ' .d first. Suggestions for Student Work. The student should prepare approximately 1 N solutions of KMnO4 and H201. These may be diluted to obtain solutions of lower normality, such as 0.1 N and 0.025 N. A. The 0.1 N KMnO4 may be used to compare the - permanganate method with the thiosulfate method for determiniig HzOz. The sodium thiosulfate solution is made up and standardized against the KMnO4. Both solutions are then used to determine the concentration of a sample of H20p, following procedures given in standard textbooks! If the work is carefully done the results will agree within 1or 2 parts per thousand. The fact that the thiosulfate is standardized against the permanganate makes it unnecessary to know the normality of the permanganate with a high degree of accurzcy to demonstrate the close agreement of these two methods in the determination of hydrogen peroxide. B. The minimum amounts of acid needed to prevent the precipitation of the oxides of manganese may be determined for various concentrations. A typical procedure is to add 50 ml. of distilled water to an Erlen- ACKNOWLEDGMENT 6

RAMSAY, W., J. hem. Soc.,79,1324 (1901). Rlfrs, A., AND J. M. GOMEZA, Anales FZs. y gutm., 37,442-58

f,-"*A,. lOAl> 6 TREADWEL~F. P., AND W. T. HALL,"AnalytimlChemistry," John Wiley & Sons,Inc., New York, 1924,6th ed., pp. 53.F576.

The author wishes to acknowledge the assistance of these former students, Mr. Luis G. Romero and Mr. Robert Lee Davis for their assistance in checking part of this work.