Sfanford 1. Tacketl Indiana University of Pennsylvania Indiana, Pennsylvania 15701
Comparison of Endpoint Methods A codometric titration experiment
constant current coulomctric titration is an extremely attractive method of quantitative analysis. It is at the same t,ime simple to perform with readily available equipment, and yet can he both highly precisc and highly accurate. The calculation of results is also desirable since the method is an absolute one. Calculations are based on Faraday's laws of electrolysis and thus standards need not be run. The value of the faraday has recently been determined to be 96,487.0 coulombs per equivalent by careful measurements performed at thc Nat,ional Bureau of Standards (1). Coulometric t,itrations arc being studied at the National Bureau of Standards as methods of high precision and high accuracy for the analysis of high purity Standard Reference Materials. Reports from NBS include coulo'mctric titrations of acids and bases (s), halides (S), potassium dichromate (4), uranium (5), and arsenious trioxide (1). Marinenko and Taylor reportcd conditions which permitt,ed the coulometric generation of iodine at an efficiency better than 99.9999% (1). Rnoeck and Diehl reported the coulometric assay of primary standards potassium dichromate and ammonium hcxanitratocerate with standard deviations of 0.002% and 0.005%, respectively (6). Practically any titration that can be performed by conventional means can be performed coulometrically with equal or greater precision and accuracy (7). Laboratory experiments based on coulometric titrat,ions have been popular for courses of instrumental methods of analysis for some t,ime (7-12). With the recent advances in electronics, especially with the development and ease of use of operational amplifiers, constant current power supplies arc easier to assemble and more reliable than ever (13-15). The new ease associated wit,h instrumentation, and the new high accuracy, high precision applications, coupled with the already desirable features of coulometric titration make it an attractive laboratory experiment for many chemistry courses. '
60-IOOY Rauulofad DC Power SUPPIY
Figure 1.
Conrtont current power supply.
tenance. If the components are not available for the construction of power supplies suggested by Morgenthaler (IS), Vincent and Ward (lh), and Stock (15), a very good one can be built with a line operated regulated dc power supply as shown in Figure 1. The voltage needs to be high enough so that changes in cell resistance during titration will be small compared to the value of the load resistor. An electrically operated seconds timer is a convenient addition and can be operated simultaneously through a double-pole douhlethrow switch (7). An accurate current value can be determined by using a 1% precision resistor as load resistor (or any exactly known value) and measuring its potential drop with a potentiometer during operation. Current level can also be established by titrating standard substances. The titration cell must contain the generator electrodes, a device for isolating them in most titrations, an endpoint detector system, and an efficient stirring device in addition to containing the sample. The cell used in this experiment is shown in Figure 2. Numerous other cell designs have been reported [7,14,15). For amperometric detection of the endpoint, approxi-
Apparatus
Constant current sources used in this experiment were Metrohm Model E211 coulometers. They have switch selected current levels, a builhin timer and a remote control switch. Other companies such as Sargent-Welch and Leeds Northrup have sold very fine coulometers for a number of years. Early improvised constant current supplies consisted of a high voltage dc battery with a high resistance in series (7). For example, a 100-V battery in series with a 10,000-ohm resistor would deliver a current of 10 m.4 (Ohm's law calculation). High voltage dc batteries can he fairly expensive if they are not already available. Rechargeable batteries are good, but they require main52
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Journal o f Chemicol Educotion
~ u n e t i s Stirrer
Figure 2.
Coulometris titrotion cell.
mately 20 mV dc was applicd to two identical platinum electrodes, and a box type lamp and scale galvanometer was used to detect current flow. The circuit is shown in Figure 3. 1.5 Voif DIY Cell
E
voi,a(ls Divider
Lesda N a t h r v p Modal 2435-C G~lvonornstsr. Sanaitivlly 0 025pll/mm
to ~ i o ~ i n u m indicala Elcctroder
Figure 3.
Circuit for ornperomefric endpoint detection.
Titration
The reaction used in this experiment was the oxidation of arsenite by iodine AsOa- + Iz HnO AsOs212H+
+
-
+
+
The iodine is generated by the electrolysis of aqueous iodide solution
+ + -
generator anode: 21-
generator cathode: 2H20
20-
1% 20-
H*(.,
+ 20H-
The generator cathode was placed inside the isolation tube to prevent interferences with I2generation. Prior to the analysis, 50 ml of 0.1 M I
