Factors Affecting the Stability of Aqueous Potassium Ferrate(V1) Solutions W. F. WAGNER, J. R. GURIP',
'
ECEST interest in the use of ferrate compounds as strong oxidizing agents prompted an investigation of some factors influencing the stability of potassium ferrate solutions. In a study of the st,ahilityof ferrate ions in aqueous solutions, Schreyer and Ockerman (2j found that the more dilute solutions of ferrate are more stable. They found also that some added salts, after increasing the initial deconiposition rate of ferrate solutions, apparently staliiljze the remainder; and that ferrate ions are more stable in buffer solutions of p H 8 than those of pH 7. These authors believe that the major factor influencing stability is the alkalinity of the solution. From qualitative experiments Schreyer ( 1 ) reported that solutions of potassium ferrate were partially decomposed photochemically over a period of 9.5 hours. Since i t seemed desirable to establish the factors influencing the stability of ferrate compounds in connection with other xork, a quantitative study of t,he effects of light, temperature, alkalinity, and concentration, on the decomposition of aqueous solutions of potassium ferrate was made under carefully controlled conditions. PROCEDURE
The potassium ferrate was prepared by the method reported by Thompson, Ockerman, and Schreyer (4). To determine the effect of light on the decomposition rate of solutions of potassium ferrate under varying conditions, each of the experiments described below was performed using identical solutions in three flasks placed in a thermostat. One flask was exposed to the daylight in the laboratory, one was placed in the path of a beam of light from a 150-m-att G.E. spotlight, and one was painted black to exclude light. The latter was fitted with a rubber stopper containing a release valve to permit the escape of oxygen. As it was impossible to maintain a constant temperature with the splotlight shining directly on the bath, a 3000-ml. beaker of water was inserted between the bath and the spotlight to absorb most of the heat radiation. In this manner the temperature was successfully maintained constant to within +O.l O C. The solutions were analyzed by pipetting 10-ml. portions and determining the ferrate concentration by the chromite method ( 3 ) . Analyses were performed a t varying intervals over a 2hour period after the potassium ferrate solutions were prepared. Preliminary experiments indicated that several factors such as stirring, vibrations, and exposure of dry samples to the atmosphere influenced the decomposition. To minimize these factors the solutions were not stirred, and every effort was made to maintain constant conditions except for the one condition varied in 1
Present address, University of Cincinnati, Cincinnati, Ohio.
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AND
E. N. HART
University of Kentucky, Lexington, K y .
30
60 TIME, MINUTES
Figure 1
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each series of experiments. obtained.
I n this way reproducible rcwults were RESULTS
Effect of Temperature. To study the effect of temperature on stability, quantitative determinations 15-ere made on aqueous solutions of potassium ferrate at' temperatures of 25' and 0.5' C. The results in Figure 1 shox that at 25" C. the concentration of potassium ferrate had decreased 10% by the end of the 2-hour period. At a temperature of 0.5' C. the solution was relatively stable, decreasing only 2% over the same time. The solutions used to obtain curves A in Figure 1 were prepared using water at 0.5" C. and consequently showed no initial decomposition. Those for curves B , which were prepared from water a t room temperature and placed in the bath a t 0.5' C., showed an initial decomposition of about 5% before attaining the temperature of the bath. In one experiment using 0.010 jiO.001) ilf potassium ferrate, the beaker of FTater was not placed between the spotlight and the bath; consequently, the temperature of the flask exposed to the spotlight \\-as from 1.5" t,o 2" C. higher than the other two flasks, As is shown in curves C in Figure 1, this solution decomposed a t a more rapid rate than the others. A sharp break occurred in the curve at the end of 60 minutes, whereas a similar hreak for the other two occurred after 120 minutes. This demonstrated the effect of even a small temperature difference over a period of 2 hours Effect of Alkalinity. The influence of alkalinity on the stability of potassium ferrate vxis determined by analyzing, at successive intervals of time, 0.010 df solutions of potassium ferrate in 3 11' potassium hydroxide and in 6 M potassium hydroxide at 25' C. Figure 2 shows that in 6 Af potassium hydroxide the concentration of potassium ferrate decreased only 5% over the 2-hour period. The solution of potassium ferrate in 3 AI potassium hydroxide decomposed n:ore rapidly than the water solution over a 2-hour period. Upon the completion of this series a 6 ilf potassium hydroxide solution of potassium ferrate was placed in a low temperature refrigerator a t a temperature of -20' C., and an analysis after 31 days showed a decrease in concentration of only 29.3%, Another 6 ill potassium hydroxide solution decomposed completely a t room temperature in 7 days. These results indicate clearly the pronounced effect exerted by both temperature and alkalinity on the stability of potassium ferrate solutions. Effect of Concentration. Stability of 0.010 M and 0.0019 Jf aqueous potassium ferrate solutions a t 25" C. was studied by the procedure dcqcribed above. As reported by Schreyer and Ockerman (a), the more concentrated solution of potassium ferrate was less stable. The concentration of a 0.010 AI solution decreased 79.5% over a 2 5-hour period, while the concentration of a 0.0019 -11 solution decreaaed only 3i.4% after 3 hours and 50 minutes. Effect of Light. The above experiments were designed primarily to determine the effect of light on the decomposition rate at different temperatures, alkalinities, and concentrations. Figure 1 shows that the effect of liLht on the I20 decomposition of 0.010 111 solutions of potas3ium ferrate in water a t 25' and 0.5" C. is negligible over a 2-hour period.
1497
ANALYTICAL CHEMISTRY
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instability of solutions of ferrate in 3 J f potassium hydroxide, the authors believe that the differences are not significant. The influence of light on the decomposition of the 0.010 11 and 0.0019 M aqueous potassium ferrate was negligible within the limits of the experimental error (approuniately 1%) over a 2hour period.
effect on the decomposition rate over at, least a 2-hour period. The greater stabilit'y of more dilute solutions was confirmed.
I
~~
The result^ in Figure 2 show that light, has a negligible effecton and 6 Jf potassium hydroxsolutions of potassium ferrate in 3 ide. The authors have no satisfactory explanation for the greater differences in curves B of Figure 2, but in view of the
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(3) Schreyer, J. M., Thompson, 6. IT.,and Ockerman, L. T., Ibid.,
22,1426 (1950). (4) Thompson, G. IT,, Ockernlan, L, T., and Schreyer,J. Chem. SOC., 73, 1370 (1951).
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R~~~~~~~ for review lRIrtrch 27, 1 s ~ ~Accepted . 11ay 22. 1952.
Rapid Approximate Method for Determination of Moisture In Pressed Beet Pulp R . H. COTTON, W. A. HARRIS, L. P. ORLEANS,
AND GUY ROR4BIIUGH Holly Sugar Corp., Colorado Springs, Colo.
E E T pulp, the by-product from the diffusion of sugar from beet cossettes during sugar manufacture, comes from the diffusion battery with a moisture content of a t least 95%. Usual practice involves dewatering over a screen and, where dehydration of the pulp is practiced, final pressing t o reduce the moisture content to 80 to 86%. Old presses, or presses improperly adjusted, will deliver pulp having moisture contents as high as 90%. Where several presses operate t o remove water from beet pulp prior t o dehydration, a rapid means of evaluating individual press performance is needed. A press which one day mill produce pulp at 85% moisture may give pulp a t 88% the nevt dav. This corresponds t o a change of 5.66 to 7.32 in ratio of water t o dry matter. In other words, the amount of water to be evaporated per pound of dry matter increases 29y0 when pressed pulp moisture goes from 85 to 88%. The present paper deals with a simple control method allowing operators to deteiniine moisture in approximately 6 minutes. Data are given on the correlation between the rapid method and the oven-dry method, ab well as on comparisons of reproducibility of the two procedures. The basis of the present analytical method is the release of moisture from pressed pulp by contact with molasses. This is followed by a final pressing in a simple potato ricer. The volume of liquid released by the ricer is directly related to the moisture content of the pulp. PROCEDURE
Keigh 200 grams of pressed beet pulp into a beaker and weigh into the beaker 200 grams of molasses a t room temperature. Xix manually n-ith a V-shaped wire for 1 minute. Place contents of the beaker in the potato ricer (see Figure 1). Press for exactly 2 minutes, using the weight as a source of pressure. Catch the press liquor in a 250-ml. graduate. Record volume of press liquor Read moisture content of pulp from a plot of press volume versus moisture, such as Figure 2.
APPARATUS
The basic unit of the apparatus is a household potato ricer consisting of a perforated basket ( erforations 2 mm. or 3 / / : 9 inrh in diameter), a plunger with attacged lever, and fulcrum. Attached to the end of the lever is a 20-pound weight so t h a t constant pressure can be applied during each moisture determination. The ricer is mounted on a stand. A funnel below the screen leads the press liquor into a graduate (Figure 1). RESULTS AND DISCUSSION
Figure 2 represents data from 95 determinations of pressliquor volume versus moisture determined by oven drying at 100O C. with a vacuum of 15 inches of mercury to constant weight. The coefficient of correlation ( 2 ) betm-een press-liquor volume and per cent moisture obtained from the 95 comparisons !vas 0.9664, a very high degree of correlation. Figure 2 is based on the regression calculated from the correlation coefficient, y = 60.23 0.140iz where y = moisture content and x = press volume ( 3 ) . Once a correlation was established the data n+ereanalyzed in order t o compare the precision of the new method compared to the oven-dry procedure. Table I gives an experiment comparing press volume with moisture determined by oven drying. Table I1 gives standard deviations ( 1 ) from 10 experiments, including the one in Table I, comparing press volume with ovendry moist,ure determinations; each experiment consisted of from 4 to 12 replicate determinations on a given sample of pulp. The average of the standard deviations for moisture content by the two methods was approsimately the same, 0.1594 and 0.1489. The rapid met,hod is, t,herefore, as precise as the oven-dry procedure. I t has the advantage of speed. Furthermore, a foreman can use it right in the pulp dryer building and immediately spot a press that is improperly adjusted. Only occasional checks
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