HI I I IHIHIHHWIHIHH ‘1 I ____—— ___-_—— ,___'—- ____—. ___d— ’— ___.—_ __'_— —__—4 ______— _____——_ ____’ ——— ELECTROLYSJS CF CHROMIC ACID SOLUTIONS CONTAINING HYDROFLUOSILJCEC ACID ’7‘ J hesis for the Degree of M. 3. Kurt Kanncwski f937 w 5 ‘1 .47 I . I \. '1}~ . U I V l 1' .I l ‘ | J .. .3 C i" . ' o I~ Q I .1 ' II.— 0 ". ELECTROLYSIS 0F CHRCMIC ACID SCLUTICNS CCNTAINING HYDRCFLUOSILICIC ACID by KURT/KANNOWSKI A THESIS Presented to the Graduate School of Michigan State College of Agriculture and Applied Science in Partial Fulfillment of Requirements for the Degree of Master of Science. Chemistry Department East Lansing, Michigan ’ 1937 Chromium is electrodeposited from baths containing chromic acid and small amounts of sulphate or hydrofluorosilic acid and a variable concentration of tri velent chromium. The latter is derived from the partial reduction of the chromic acid. When Faraday made his famouns investigation on the electrolysis of aqueous solutions, he and his contemporaries did not fail to notice that complications will often arise, not only at the anode, but also - and perhaps more commonly - on the cathode. These purely chemical complications are caused by side reactions, due to the new products formed on the electrodes and the changes which occur in the composition of the solution in the immediate neighborhood of the cathode under the influence of the current. Liebrisch(l) states that these chemical reactions can arise in two ways: first, the electrolyte may react with the metal of the cathode, even when no current is flowing, or second, - and this is the more important case - the pH value of the solution may increase near the cathode owing to passage of current, thus producing complexes of a colloidal character in the solution, which are catephoretically conducted to the cathode. The increased hydroxyl ion con- centration can give rise to the formation, on the surface of the cathode, of basic salts or hydroxides of the metal. An increase of polarisation follows in consequence of an increased transfer resistance, together with a larger con- sumption of energy for cleaning the cathode by chemical reduction or mechanical removal of the products by the hydrogen bubbles 1 090812 2. leaving the surface. All of the above effects have a direct influence on the form of the metallic deposits. Since the current efficiency of the process for the electrodeposition of chromium depends on the metallic deposit all variables effecting the current efficiency are of importance. The effects of such factors as current density, temperature, and concentration on the current efficiency have been studied for the sulphate bath by D. T. Ewing, J. O. Hardesty and Te Hsia Koa(2). The effect of the same variables of current density, temperature and concentration for hydrofluosilic acid baths on the current efficiency is reported in the following work. APPARATUS AND MATERIALS. The apparatus with the exception of some changes is the same as that used by J. O. Hardesty(2). Diagram 1 shows the apparatus used. A porous cup (I) separates the anolyte from the catholyte. Sheet lead (10 x 17.5 cm.) bent to fit the cup loosely was used fer the anode. The cathode (L) was a steel plate, (.1 sq. ft.) coPper and nickel plated with a mirror finish. The plate is fastened by means of a small bolt to a heavy copper holder ( 1 cm. wide ) which is insulated with an acid resisting lacquer. The holder is bent in a Veshape to keep the plate under the funnel (K). In series with the chromic acid cell is a carbon resistance (R), 30 ampere ameter (A), a copper coulometer (H), a switch (T) and the connection to 15 volt D. C. line. Across the cell is a 50 vole voltmeter ( ). The solutions of varying concentrations consisted of 5. ‘c. p. commercial chromic anhydride and hydrofluosilicic acid (29.8%). Constant temperature was maintained within .50 C of each determination during the eXperiment. The cell was partially immersed in a sixteen gallon aquarium of water. The temperature was raised by means of a resistance heating coil, and lowered by a COpper cooling plate. The flask (A) has a calibrated capacity of 537.35 cubic centimeters. This capacity includes both tubes, (x) and (y) to the marks indicated by (O) on the drawing. StOpcocks, (B), (F) and (E) are of pyrex. The manometer (mn) is attached to the leveling tube (S). The Jar, (Z), surrounding flask (A) is a constant temperature bath so arranged to determine the temperature of the collected gas. The coulometer consisted of a four liter jar with a cOpper cathode (56 x 18 cm.) hanging between two copper anodes. The coulometer solution is composed of 1000 parts by weight of water, 150 parts of c0pper sulphate, 50 parts of concentra- ted sulfuric acid and 50 parts of alcohol. EXPERIMENTAL PROCEDURE. While procuring data for this report 222 determinations have been made. All of this data has been recorded, but some has been discarded as unreliable because of temperature changes during the determination. Other data will not be used because it has no direct bearing on the principles to be discussed. Since the accuracy of the determinations depended 4. on the method of weighing Special emphasis has been put on the preparation and cleaning of the cathodes. The steel plates were copper and nickel plated, polished and buffed to a mirror like finish. Each plate was electrocleaned in an alkaline cleaner, rinsed and examined for water breaks. The plate is then rinsed in hot distilled water and dried at 1100 C, and weighed. The cOpper coulometer plate is rinsed in hot distilled water, dried at 1100 and weighed. The time for each determination is determined by means of a stopwatch. Amounts of current are regulated approximately by the use of the variable resistance and the ammeter. After the cathode is introduced into the solution the gas collecting funnel is placed over the former. By lowering the leveling bulb the plating solution is drawn into tube (x). Adjusting stopcocks and raising the leveling bulb all of the air in (A) is displaced by water. As soon as the current is passed this water will be displaced by hydrogen evolved at the cathode. when the hydrogen reaches the graduation mark at (0) the circuit is broken. The manometer stopcock is then opened and by means of the leveling bulb the exact difference of manometer readings is determined. This difference in manometer levels, read in millimeters, divided by the density of mercury is the deviation from atmospheric pressures. 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In this investigation determinations were made for the following concentrations of CrO3 and H281F6 respectively 100 parts to 1 part, 50 to l, 30 to 1 and 20 to l. 250 g. of CrO5 were dissolved in 1 liter of water. The range of current densities considered was from 75 amps. per sq. ft. to 250 amps. per sq. ft. The temperatures for each concentration were 40°C, 45°C and 50°C. Cna general determination for the effect of a temperature change from 20°C to 60°C with the concentration of 100 parts CrO5 to 1 part of H281F6 has been made. In order to understand some of the facts brought out by the eXperimental procedure one of the theories of chromium deposition from a chromic acid - sulphate bath will be given. Sargent(3) recognized a number of important variable factors namely, that the cathode film is only slightly acid at high current densities, that reduction of chromic acid at high current densities must occur through the medium of the chromic - chromous couple, that sulphate is necessary for metal deposition, and that pure chromic acid can be reduced at low but not at high current densities. The latter factor was attributed to a colloidal layer which was designated as non-permeable, though Sargent tacitly assumed it to be permeable to hydrogen ions. The mechanism of the action of sulphate has not been set forth. Keeper(4) advances the following theory. Deposition of bright chromium l6. depends upon maintaining in the cathode film a definite hydrogen ion concentration. this must be sufficiently low to permit metal deposition and yet sufficiently high to prevent hydrolysis and thus yield oxide-free deposits. At low current densities direct reduction of hexavalent or tri- valent chromium undoubtedly occurs. At high current densities, which require cathode potentials sufficient to liberate hydrogen gas, the conditions are vastly different. On account of the high migration velocity of the hydrogen ion in the cathode film, the latter becomes relatively alkaline. If the hydrogen ion concentration drOps, a basic electro- positive dispersoid of Cr(OH)3 ' Cr(OH)CrO4 existso In the absence of the sulphate ion the dispersoid migrates to the cathode and covers it with a colloidal layer which prevents reduction of the constituents except hydrogen to which it is permeable. The action of the sulphate is to decrease the electrOphoretic velocity of the colloid by adsorption; which causes a reduction of the positive charge and prevents the formation of a dense, adherent colloidal layer. Other ions can reach the cathode and be reduced. Liebreich(l) has advanced a theory which is identical with the latter. He has investigated the effects of acids on the colloid film surrounding the cathode. The nature of the deposit depends largely on the degree of solubility of the film. He states that it is important that by a certain degree of solubility, the film should form and dissolve 17. continually, maintaining a certain definite thickness. This follows from the fact that in cases where, at ordinary temperatures grayish deposits are obtained, bright deposits are obtained at higher temperatures (30°, 400 or 50°C). This form of deposit, again is confined to certain limits of current density. Miller(5) discusses the above named film formation in relation to current densities. According to his determination the continual formation of the film is effected by the concentration of the 804 ion. With an increase of the concentration the film formation slows down hence a lowering in current density. The above fact will explain the lowering in efficiency of the sulphate bath investigated in this work. Table (6) and graph (6) show the decrease in efficiency with increase of sulphate from the ratio - 100 parts CrO to 1 part 804 ' to 5 the ratio of 50 parts Cr03 to 1 part 804. The temperature effects shown in the graph were explained by D. T. Ewing, J. 0. Hardesty and Te Hsia Kao.(2) From the data obtained in this work it can be concluded that the effect of H251F6 is the same as the SO4 for chromic acid bath of the same composition. The following effects of the chromic acid bath with hydrofluosilicic acid are the same as that of the sulphate bath. 1. A decrease of current efficiency with an increase of temperature. 18. 2. A decrease of current efficiency with an increase of HZSiFe' 3. A low efficiency at a low current density with a relatively sharp rise in efficiency at a higher current density. A leveling off of the efficiency curve at higher current densities. From these facts it may be concluded that the previously stated theories hold true also for chromic acid solution with hydrofluosilicic acid although the magnitude of effect of varying factors gives relatively different results. LITERATURE CITED (1) E. Liebreich, Trans. Faraday Soc. 2;, 1188-1194, 1935. (2) D. T. Ewing, J. O. Hardesty, and Te Hsia Kao, l2, Mich. Eng. EXpt. Station, 1928. (3) G. J. Sargent, Trans. Am. Electrochem. ooc.‘§Z, 479, 1920. (4) Ch. Kasper, B. S. Jour. of Res. 9, 353 - 375, 1932. (5) E. Miller, Trans. Faraday soc. .2_1_, 1194 - 1203, 1935. . g ' r _.~~< ‘v‘ ‘ - ‘ - 9'." ‘ . .5.’ fl ' . ‘3; . . . , fr ‘2 Q. - $7 \ ,2. '2 - 'A' swoon; w' GRCO. ' ,- \ canal-u ..‘! U h}??- I15 109082 Iannowski W - . at! #4}- _*p. . EEEEEEEEEEIEEEEIIEEEIEEEEEEIEEIEEEEEIEEEEEEEIEEEEIEEEEIEEEEEE 31293 02446 8179