ENGINEERING RESEARCH INSTITUTE UNIVERSITY OF MICHIGAN ANN ARBOR PROGRESS REPORT APPLICATION OF THE POLAROGRAPH TO ANALYSIS OF TITANIUM-BASE ALLOYS By - PHILIP J:AELVING CHARLS L. RULSFS ROBERT W. PARRY GRAHAM A. STONER Project 2075 AIR RESEARCH AND DEVELOPMENT COMMAND, U. S. AIR FORCE CONTRACT AF 18(600)-397, E.O. No. R606-60 SR-3z February, 1953

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ENGINEERING RESEARCH INSTITUTE ~ UNIVERSITY OF MICHIGAN PROGRESS REPORT APPLICATION OF THE POLAROGRAPE TO ANALYSIS OF TITANIUM-BASE ALLOYS SUMMARY The present report covers the period from December 7, 1952, to February 7, 1953. During this period an extensive study was made of the changes in half-wave potential and in diffusion current of ferric ion with respect to change in the concentration of the potassium fluoride supporting electrolyte and change in pH of the polarographic solution. SUMMARY OF RESULTS A. Experimental Several stock solutions were prepared, each 0.40 mM in ferric ion but varying in potassium fluoride concentration from 1 M to 035 M. The pH of each of these solutions was measured and polarograms were taken of each at 25 + 0.1~C. The pH was then adjusted to values above and below the original values by the addition of small amounts of hydrochloric acid or potassium hydroxide solution, and the polarograms of the resulting solutions were recorded. B. Interpretation of Data Analysis of the polarograms obtained yielded the following data:

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN Potas;sium Ferric Ion Species Fluoride pH Molarity Id), Fpal: E1/2, volts 0.3 5.8.52 -0o455 0.4 5.8 1.76 -0.467 0.5 5.8 1.47 -0.471 o.6 5.8 1.72 -0.475 0.75 5.8 1.65 -0.481 1.00 5.8 1.55 -0.494 1 00. 63 0.95 -0.492 o.6 5.4 0.45 -0.478 0.5 6.2 0.11 -0.471 0.5 5.4 0.68 -0.471 From these data, we see that a pH of 5.8 seems to be the optimum pH for the polarographic determination of ferric ion in a fluoride medium. Values of pH on either side of this value show a marked decrease in the diffusion current. This phenomenon could very well explain the discrepancy in the literature as to the E1/2 for the ferric-fluoride system. Values of pH very far from 5.8 would completely obliterate the wave, which may explain the fact that West and Dean1 claimed to have found no reduction wave for ferric ion in fluoride prior to the discharge of sodium ion. Our results are in complete disagreement with those of Stackelburg and Freyhold,2 who reported that ferric ion produced a reduction wave with E1/2 of -1.36, which remained constant when the concentration of potassium fluoride was increased from 0.04 to 0.8 M. The wave they observed was probably due to hydrogen discharge rather than to ferric ion.* Figure 1 is a graphic representation of the change in El/2 with the concentration of potassium fluoride at pH 5.8. From the graph it can be seen that El/2 varied logarithmically with fluoride concentration. This is in accord with the Nernst expression for the reversible reduction of ferric ion to ferrous ion in the presence of fluoride: E = E1/2 + 0.0591 log (F-) When the fluoride concentration is 1 M, the E for the system equals E1/2, which is the standard electrode potential for the reduction. We have found this value to be -0.494 v... This logarithmic relationship between fluoride concentration and E1/2 indicates that the reduction at the drop interface involves the loss of one fluoride ion from the ferric-fluoride complex. -- ~2

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ENGINEERING RESEARCH INSTITUTE ~ UNIVERSITY OF MICHIGAN Fse [F?- 3-P + e -- Fe -CF' p-1 p P-1 We propose the above expression to represent the electrode reaction. The use of the Ilkovic equation enables us to calculate the diffusion coefficient for the ferric-fluoride complex. For an aqueous medium at 250~C which is 1*0 M in potassium fluoride, we found that D = 5.81 x 10-5 cm2 - sec-1. FUTURE WORK Next the chromic system will be investigated in fluoride at pH 5.8. It is desirable to obtain suitable polarographic waves for iron and chromium from the same solution at the same pH. It is, however, possible that the chromium wave may be best defined at a different pH, in which case it will be necessary to adjust the pH of the solution between polarograms for iron and chromium. BIBLIOGRAPHY 1. West, P. W., and Dean, J. F., Ind. Eng. Chem., Anal. Ed., 17, 686 (1945). 2. Von Stackelberg, M., and Von Freyhold, H., Z. Electrochem., 46, 120 (1940). 4

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