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Simultaneous Coulcmetric Analtsis Dissertation

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Simultaneous Coulcmetric Analtsis Dissertation SIMULTANEOUS COULCMETRIC ANALTSIS DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By RICHARD DONALD McIVER, B«A. The Ohio State University 195k Approved by Adviser ACKNGWLED GMENTS I wish to express ny sincere appreciation to Dr® William MacNevin, a most competent adviser and friend, to the Proctor and Gamble Comparer and the Al­ lied Chemical Company both of which awarded me fellow­ ships, and to u$r wife who was both an inspiration and help during ray graduate studies® I also wish to thank Dr. Bertsil Baker for his assistance and suggestions and to the Department of Chemistry of The Ohio State University in which I served two years as a Graduate Assistant. i A. 4 8 2 7 2 TABLE OF CONTENTS page THE PROBLEM.............................................. 1 INI R QDUCTI O N ............................................ 2 The Faraday and Electrolysis.......................... 2 Instruments for Measuring the Number of Coulombs * . • • 2 Electrolysis at Constant Current ............... 3 Electrolysis at Controlled Potential ......... .... 5 i Devices for Controlling Electrode Potential ....... 6 Coulometric A n a l y s i s ................................. 7 Constant Current Coulometry ...................... * . 8 Controlled Electrode Potential Coulometry.............. 10 Coulometry Applied to Indirect Analysis ......... 11 Indirect Analysis and Simultaneous Equations........... 12 Propagation of Errors in Simultaneous Equations ......... 1U Applications of Coulometry to Indirect Analysis ..... 18 EXPERIMENTAL........................................... 20 General Apparatus and Materials ........ 20 Apparatus for the Automatic Control of Electrode Potential •• ........................... 20 Balance and Weights ......... ....... ... 2h Reagents ......... 2U Saturated Calomel Electrode ............. 2$ Barometer......... 25 Hydrogen-Ctxygen Coulometer...................... 25 ii Table of Contents (continued) Page Investigations with the Rydrogen-Qxygen Coulometer . 0 . 29 Construction . • • . .......................... 29 Investigation of Potassium Dichromate Solution for the Coulometer Electrolyte................... 29 Investigation of Current Density and its Effect on Accuracy of the Coulometer................... 30 Results and Conclusions..................... o . 32 Simultaneous Determination of Chloride and Bromide Ions . 3U Preliminary Considerations.................. • « 3k Special Apparatus............................. 36 Preliminary Investigations..... ............. • 38 Experimental Procedure • . ... ............... 39 Results and Conclusions.............. • ......... 1*0 Simultaneous Determination of Zinc and Cadmium . • 1*1* Preliminary Considerations ................ 14* Special Apparatus............................. 1*5 Preliminary Investigations................ 1*8 Experimental Procedure ........... £1* Results and Conclusions.................... 55 Simultaneous Determination of Lead andTi n ......... 58 Preliminary Considerations...... .............. * 58 Special Apparatus......... 60 Preliminary Investigations............. 62 Experimental Procedure .......................... 65 Results and Conclusions ......... • 67 iii Table of Contents (continued) Page Simultaneous Determination of Silver and. Copper .<>.<>. 69 Preliminary Considerations ............. 69 Special Apparatus ........... ........... 70 Preliminary Investigations ...................... 72 Experimental Procedure • . ...................... 75 Results and Conclusions , ..................... 76 The Investigation of Residual Currents ......... 78 Preliminary Considerations ..... ............. 78 Special Apparatus . .......... 78 Experimental Procedure ..................... 80 Results and Conclusions ................ ..... 82 SUMM&HY................ » ............................. 86 BIBLIO GR A P H Y .......................................... 88 AUTOBIOGRAPHY......... 92 iv THE HtOBLEH The objective of this research was to extend controlled potential electrolysis and coulometric analysis to indirect methods of analysis of two or mare substances which occur together and which are difficult to separate* This method of determination, although limited by the relative concentrations of -the two substances, sometimes is a more rapid method than conventional electrolytic or coulcmetric methods and in some cases gives good results where electrolytic separation of the substances is not possible. 1 INTRODUCTION The Faraday and Electrolysis In the study of phenomena associated with electrolysis. Michael Faraday (22,23) recognized the equivalence between the quantity of electricity passed through an electrolyte and the amount of chemical reaction brought about at the electrodes. These observations led to the postulation of the now famous laws of electrolysis which bear his name. To produce one equivalent of chemical change at an electrode there is required a definite number of ampere-seconds which are called coulombs. The value of this number, the far ad ay, has been precisely determined to be 9 6,1*9k *2 .3 coulombs in studies with the silver cou­ lometer (55,56,37,58)o Studies with other coulometers give values ranging from 9 6,1*90 + 1*0 for the copper coulometer to 9 6 ,3 9 0for the hydrogen-octygen coulometer. After surveying these results, Birge (2, 3,U) estimated the best value to be 9 6 ,5 0 1 + 1 0 coulombs. Instruments for Measuring the Number of Coulombs The number of coulombs passing through a circuit is most con­ veniently measured by maintaining a constant current and accurately measuring the time elapsed. Since this is often neither possible nor advisable, it is best to use a coulometer, or voltameter, to measure the total current. A coulometer is simply an instrument of one of two types, the most common of which is an electrolysis cell in which the reaction at one or both electrodes proceeds with 100 percent current efficiency and can be quantitatively measured by weighing, titration, observation of gas volume, stripping a deposit at constant current, etc . Examples of this type are the silver, the copper, the cadmium, the iodine, the hydrogen-axygen, and the coulometric (2 1) coulcmaters . The other type of coulometer is the electromechanical type© This is in the farm of an automatic recorder which describes a curve under which the area is proportional to the current. This area may be meas­ ured by several methods including the paper weight method (1 8), con­ verting to the slope-intercept form on sendlogarithmic graph paper followed by suitable calculation (1,U7), or by mechanical or electronic integration (£,3 6,UUjUE>) • Electrolysis at Constant Current In the most common method of electrolytic deposition, the total applied potential across the electrolysis cell is adjusted to maintain the current at some suitable value which is sometimes arbitrarily chosen. Under such conditions, only the most noble metal is deposited first but, as the concentration of this metal is reduced, the oxidation potential gradually decreases and the next less noble metal is depos­ ited. In this method there is no good way of determining just when to stop in order to obtain a deposit of only the most noble metal. This subsequent reduction of less noble metals continues until hydrogen is liberated, at which point the deposition of the metals below hydrogen in Table I may be considered essentially complete. The deposition may be extended to seme metals above hydrogen in the table by reducing the acidity of the solution and thus lowering the oxidation potential of the hydrogen or by taking advantage of the higher overvoltage exhibited by hydrogen toward some electrode materials such as copper, silver, 3 and mercury® Table I (37) STANDARD REDUCTION POTENTIALS 1C5* + e" cs K -2*92£ volts Na+ + e“ S3 Na -2 .7 lit + 3e“ B A1 -1*66 Zn+* + 2e“ a Zn —0 *7 63 Fe** + 2e~ as Fe -O.LtitO Cd** + 2e“ a Cd -0.2t03 Ni 2e” C3 Ni ~0.2£0 Sn** + 2e“ Q Sn -0 .1 3 6 Pb++ + 2e“ a Fb -0 .1 2 6 Normal Hydrogen Electrode 0 .0 0 0 Saturated Calomel Electrode 0 .2U£8 Cu *{* 2e~ C Cu 0.337 Ag* ■{* e“ B Ag 0.7991 Au + 3e“ O Au 1 .5 0 This constant current method is generally limited to the separa­ tion of single metals frcm solution or to the separation of metals be­ low hydrogen in Table I from those above® In seme cases selective deposition of one metal in the presence of another may be effected without controlled electrode potential even though both are below hydrogen in Table I. This is brought about by altering seme chemical property of one or all of the cations by cdu­ plexing them or by other means* If only one cation forms a complex with the addition of a ccmplexing agent, or if the stability of the complexes formed by different cations is sufficiently different, it is sometimes possible to deposit selectively one cation in the pres­ ence of the others* Examples are the separation of cadmium from copper in cyanide solution (2 6) and nickel frcm zinc as the ammina complexes discussed in Willard, Merritt, and Dean (67) * k Furman (28) has studied the use of 'potential buffers' to limit cathodic reduction to a certain potential range. These buffers are of such nature that they are reduced at a certain potential and thus act in the same manner as lydrogen to limit the reduction at the cathode to ions with cocidation potentials above the oxidation potential of the buffer. Electrolysis at Controlled Potential The second method of electrolytic deposition is that in which the total applied potential*
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