V O L U M E 2 2 , NO. 5, M A Y 1 9 5 0
691
in water can be determined in this way with a precision of 0.03 percentage unit, and that larger concentrations can be determined with a precision of 0.03 to 1 percentage unit, depending on the dilution necessary to bring the deuterium oxide content to within the measurable range (about 3% for greatest Precision Of measurement of the absorbance in a 0.043-mm. cell).
LITERATURE CITED
(1) Halford, J. o., Anderson, L. C., Bates, J. R., and Swisher, R. D.. J . Am. Chem. Soc., 57, 1663 (1935). (2) McMurry, H. L., Thornton, V., and Condon, F. E., J . Chern. P h y s . , 17, 918 (1949). R~~~~~~~
November
18, 1949.
Report 774-49R, Phillips
Petroleum
Company.
Ferrate Oxidimet ry Oxidation of Arsenite with Potassium Ferrate(VI) J. M. SCHREYER, G. W. THOMPSON1, A N D L. T. OCKERMAN University of K e n t u c k y , Lexington, K y . Methods of analyzing potassium ferrate(V1) are described. The methods are based upon the oxidizing property of the ferrate(V1) ion and the determination of the total iron present in the compound.
T
H E availability of potassium ferrate(V1) in a state of high purity ( 4 ) and its potentiality as a new and powerful oxidizing agent indicated the necessity of developing methods of analysis for the compound. The impurities usually present in samples of potassium ferrate (VI) are potassium chloride and hydrous ferric oxide. For the analysis of impure as well as highly purified samples, use of the oxidizing property of the ferrate(VI) ion seemed the most logical approach for the development of suitable methods. De Mollins (3) reported the quantitative reduction of the ferrate(V1) ion by means of the iodide ion in acid solution. 2FeOa--
The alkalinities of the solutions were varied between approximately neutral and 10 molar in sodium hydroxide. As shown in Figure 1, higher percentages of potassium ferrate(V1) were found in those determinations conducted in the more alkaline arsenite solutions. One of the difficulties encountered in the use of potassium bromate for the standardization of highly alkaline arsenite solutions was the fact that the concentration of hydrochloric acid a t the end point was found to be critical. Reproducible results in standardization of alkaline arsenite solutions were obtained only if the concentration of hydrochloric acid was 1.5 to 2.0 N.
+ 81- + 16H+--+-2Fe++ + 8H20 + 412
In view of the expected reduction of any ferric ions present as
an impurity and the instability of the ferrate(VI) ion in acid solution, it appeared impossible that this reaction would be a quantitative measure of the ferrate(V1) ion present in a sample. The method developed in this laboratory makes use of the increased stability of the ferrate(V1) ion in strongly alkaline solution and is based upon the reduction of the ferrate(V1) ion to ferric ion in alkaline arsenite solution. A weighed sample of potassium ferrate(V1) is added to a standard alkaline arsenite solution containing a quantity of arsenite in excess of that required for the reduction of the ferrate(V1) ion. The excess arsenite is back-titrated with standard bromate or standard cerate solution. The following equation represents the chemical reaction upon which the method is based: 2Fe04--
+ 3AsOa--- + 11H20----t 2Fe(OH)a(H20)3
+ 3kSO4--- + 4 0 H -
For confirmation by an independent method, the samples under investigation are analyzed to determine the total iron present in the compound. The hydrous ferric oxide impurity is removed by solution of the sample in sodium hydroxide solution and subsequent filtration. After the filtrate is acidified, the ferric ions are reduced to the ferrous state and titrated with a standard cerate solution. DEVELOPMENT OF METHODS
Preliminary studies showed increased stability of the ferrate (VI) ion in alkaline solutions. Investigations were undertaken to determine the optimum alkalinities of arsenite solutions used in the determination of potassium ferrate(VI) in a sample. 1
Present address, 2632 Dominguez St., Long Beach, Calif.
2650
I
'
I O
'
2 0
' 30
4D
5 0
60
70
BO
90
100
I I1
N Q 0 H ( M oI e s/ LIt er )
Figure 1. Average Per Cent Potassium Ferrate us. Molarity of Sodium Hydroxide
Smith (5) found that Gyory's method ( I ) for the determination of arsenite with bromate need not be conducted in heated solutions. He reported that if methyl orange was used as an indicator in the titration conducted a t room temperature, the response to the first excess drop of oxidant required a t least 30 seconds. Gyory's original method seemed preferable because the solutions were already hot as a result of the neutralization of the strongly alkaline arsenite solutions that were employed. No difficulty was encountered in standardizing strongly alkaline arsenite solutions with standard cerate solutions. In the investigation of the method based on the determination of total iron present in a sample, results were obtained which were not in agreement with those obtained by the arsenite methods. The error was found to be caused by the presence of certain reducible substances in the caustic solution used to dissolve the potassium ferrate(V1). Heinemann and Rohn (2) have reported
ANALYTICAL CHEMISTRY
692 anal1 quantitirs of reduced and reducible sulfur compounds, sodium sulfide and sodium thiosulfate, in commercial caustic soda. Blanks carried through the entire procedure were found to be significant, and satisfactory agreement with the results of the' arsenite methods was obtaind only when blanks were employed.
Table I. Sample No. 1
SOLUTIONS REQUIRED
Alkaline Arsenite Solution, approximately 0.375 A' AsOs---. Weigh out 0.27 grams of pure dry arsenious oxide and dissolve in 200 ml. of 1 X sodium h!,droxidc. Mix with approximately 240 ml. of saturated sodium hydroxide solution. When using the alkaline arsenite solution in the arsenite-bromate method (see procedure), standardize the arsenite solution as follows: Pipet 10 ml. of the alkaline arsenite solution into a 500-ml. Erlenmeyer flask and add 225 ml. of distilled water and 65 ml. of concentrated hydrochloric acid. Heat to 70" to 80" C., add 2 drops of methyl orange, and titratc immediately x i t h standard bromate solution using a 75-m1. buret. An additional drop of methyl orange map be required tiefore the end point is reached because of fading of the color. and 0.02 Standard Ccr:irr Solutions, approximately 0.075 ~ oxide. X. Standardize against p u i arsenious Osmium Tetroxide Solution, 0.01 31. Dissolve 0.25 gram of osmate in 100 ml. of 0.1 .\'sulfuric acid. o-Phenanthroline Ferrous Complex Solution, 0.025 M . Stannous Chloride Solution. Dissolve 150 grams of iron-free stannous chloride dihydrate in 1 liter of 1to 2 hydrochloric acid. Mercuric Chloride Solution, 5y0solution in water. Sulfuric Acid Solution, 1 to 5. Methyl Orange Solution, 0.5 to 1 gram per liter of water. Sodium Hydroxide Solution, saturated. Sodium Hydroxide Solution, 8 Jf. Hydrochloric Acid, C.P. ANALYTICAL PROCEDURES
Procedure for Arsenite-Bromate Method. Weigh a sample, containing approximately 0.1 gram of potassium ferrate, into a 500-ml. flask containing 10 ml. of the alkaline arsenite solution. Standardize the alkaline arsenite solution against the standard bromate solution immediately prior to use. Add the weighed potassium ferrate sample to the alkaline arsenite solution carefully, and particularlr if the flask is wet with water, do not allow the sample to strike the side of the flask. Add 2 drops of methyl orange solution, 225 ml. of distilled water, and 65 ml. of concentrated hydrochloric acid. Heat the solution to 70" to 80" C., and titrate immediately Rith standard bromate solution. Introduce another drop of methyl orange near the end point, and approach the end point cautiously while vigorously swirling the solution. The end point is marked by a sudden change from golden yellow to greenish yellow. From the known titer of the arsenite solution and the volume of the standard bromate solution used, calculate the per cent of potassium ferrate as follows:
Per. cent K2FeO4 =
(10 x 1%' AsOa--- - ml. of BrOa- X N BrO,-) X K2Fe04
3000 X weight of sample
Procedure for Arsenite-Cerate Method. Weigh a sample, containing approximately 0.1 gram of potassium ferrate, into a 500-ml. flask containing 10 ml. of the alkaline arsenite solution. Standardize the alkaline arsenite solution against the standard cerate solution immediately prior to use. Observe the same precautions as in the arsenite-bromate method in adding the sample to the arsenite solution. Add 100 ml. of water and acidify with 100 ml. of 1 to 5 sulfuric acid. Cool to 20" to 25" C. and add 2 drops of osmate solution. Add 1 drop of o-phenanthroline ferrous complex indicator and titrate with the standard cerate solution. From the known titer of the alkaline arsenite solution and the volume of standard cerate solution used, calculate the per cent potassium ferrate as follows: (10 X N A s O q - - - - ml. of Ce+4 X ATCe"+l)X K2FeO( Per cent K2FeOa= 3000 X weight of sample
loo
Procedure for Total Iron Method. Weigh a sample, containing approximately 0.1 gram of potassium ferrate, into a clean, oven-dried fritted-glass filter of 50-ml. capacity and fine porosity. Pipet 15 ml. of 8 M sodium hydroxide solution into a 25-m1. graduate. Add the sodium hydroxide solution in three or four
2
3
Comparison of Methods for Analysis of Potassium Ferrate Total iron 26.28 26.35 26.32 36.30 26.34 63.37 63.35 63.34 63.45 04.56 94 2.5
Potassium Ferrate, % bromate Arsenitemethod 26 30 26.29 26 35 2fi.32 , . .
-4r~enitecerate method 26.32 26.31 26.30 26.33 ?6,2!)
63.40 63.35 63.43 63.39
fi3 ,J4
fi3.42 6 4 , 37 C3 4 0
portions to the potassium fwrat(b(\'I) sample, stir witti a g1:is.~ rod, and filter continuously at the full c
