Eugene D. Olsen and Roger D. Wolfon University
of South Florida Tampa
II
Automation of Amperometric Titrations with Rotating Platinum Electrodes
Recently, simple and inexpensive equipment was described which allows students to automate potentiometric pH titrations, thereby saving time without sacrificingaccuracy.' I n this article it is shown that the above equipment can be adapted with only minor modifications to automation of amperometric titrations with rotating platinum electrodes, further extending the scope of automatic titrations for students. The apparatus used for automatic delivery of titrant solutions was the same as that described previously,' with one minor modification and one addition. First, a 50-ml buret with a Teflon stopcock was used instead of the Hempel distilling column, thereby eliminating the rubber pinch cock valve, since it was found that cerium(1V) ammonium sulfate titrant decreased in titer value rapidly in contact with rubber tubing (0.1 M Ce(1V) solution lost about half its value as an oxidant on standing in a weighing bottle in contact with rubber tubing for about 2 hr). Also, Tygon tubing was used instead of rubber wherever a flexible connection had to be made, since Tygon was found to be much more resistive to attack by Ce(1V) than was rubber. I n addition, a simple drop counter was devised which caused a pulse on the recorder chart each time the siphon pipet discharged. This counter was necessary since the nonequilibrium current pulses were usually indiscernible, whereas in potentiometric pH titrations each increment of titrant was clearly marked by a nonequilibrium pH pulse.' The counter consisted of two platinum wires, separated by approximately in., mounted horizontally and parallel to each other, and positioned immediately below the siphon pipet discharge point. Approximately 11/2-in. lengths of thin platinum wire were used, with electrical connections made to insulated copper wire by simply twisting and taping, and the copper wires were connected to thc tirn~iualsof 1111. rot:trin# p~:~ritnlm 31111 ? I I I I I ~ I I ~ P ~ 1~aIotnt.1 C~PCTII)&~S. .\s ~ 1 ' 1 1drop fvll it wo111d nlonlPtItarily make contact across the two platinum wires, thereby "shorting" across the working electrodes. To prevent the retention of any droplets on the platinum wires, a tip on each wire was bent downward 90 degrees. Any liquid adhering to the platinum wires 1
OLSEN,E. D., J. CHEM. ED., 43.310 (1966).
indicated that they were dirty; they were conveniently cleaned by heating in a flame. The remaining equipment and solutions have been de~crihed,~ except that a Sargent Model XV polarograph was used instead of a manual polarograph, and the salutions were tenfold more dilute. The platinum electrode was set 0.M.9 volts positive of the saturated calomel electrode, and a current sensitivity of 0.030 microamps/mrn, with no damping, was used. The capillary flow restrictor was chosen to give a flow rate of about 1 ml/min, and a 0.5-ml siphon pipet was used. Sipon volumes are calibrated as described elsewhere.' The rate of stirring with a magnetic stirrer was found to be uncritical. The figure showing the automatically recorded titration curve for the titration of M Fe(I1) with 0.01 M Ce(IV), at an applied potential of 0.6v, illustrates the type of titration curve obtained. Each time the siphon pipet discharged, an easily discernible pulse was recorded, followed by rapid attainment of equilibrium cell current. Before the equivalence point the current is due to the Fe(I1)-Fe(II1) oxidation current, which decreases as the titration proceeds, whereas the current after the equivalence point is due to the Ce(1V)-Ce(II1) reduction current, which increases as the titration proceeds. Straight lines can be drawn connecting equilibrium current values near the end point, and graphical extrapolation permits the
NUnBtROFSIPHON INCRtMtNTSLDOW
Automatically recorded titration curve for the litration of 10-8 M Fe(llJ with 0.01 M Ce(lV)
Volume 43, Number 12, December 1966 / 659
number of siphon increments and partial increments to the end point to be measured. I n the figure the end point is 24.7 increments. A series of four such titrations had a coefficient of variation of 0.30%, with a difference of +0.5% between the average amount of Fe(I1) found and the amount taken. For maximum teaching effectiveness, it is recommended that automation of amperometric titrations %REILLEY, C. N., AND SAWYER, D. T., "Experiments for Instrumental Methods," McGraw-Hill Book Co., New York, 1961, Exp. 4-5, p. 80.
660 / Journal o f Chemical Education
follow a manual exercise such as Experiment 4-5 in Reilley and S a ~ y e rwhich , ~ includes the manual measurement of voltages at zero current (potentiometry). Once the manual titration has been performed, the student can quickly set up the simple automation equipment described here, and perform a set of triplicate titrations in less than 45 mins, thereby introducing him to the principles of automation, as well as permitting the reproducibility of the experiment to be determined, the latter of which would be too onerous an undertaking manually.
