Evaluation - Coulometric Methods of Analysis

Evaluation

Scale of Operation 

Coulometric methods of analysis can be used to analyze small absolute amounts of analyte. In controlled-current coulometry, for example, the moles of analyte consumed during an exhaustive electrolysis is given by equation 11.32. An electrolysis carried out with a constant current of 100 μA for 100 s, there- fore, consumes only 1 x 10–7 mol of analyte if n = 1. For an analyte with a molecu- lar weight of 100 g/mol, 1 x 10–7 mol corresponds to only 10 μg. The concentration of analyte in the electrochemical cell, however, must be sufficient to allow an accu- rate determination of the end point. When using visual end points, coulometric titrations require solution concentrations greater than 10–4 M and, as with conven- tional titrations, are limited to major and minor analytes. A coulometric titration to a preset potentiometric end point is feasible even with solution concentrations of 10–7 M, making possible the analysis of trace analytes.

Accuracy 

The accuracy of a controlled-current coulometric method of analysis is determined by the current efficiency, the accuracy with which current and time can be measured, and the accuracy of the end point. With modern instrumentation the maximum measurement error for current is about ±0.01%, and that for time is ap- proximately ±0.1%. The maximum end point error for a coulometric titration is at least as good as that for conventional titrations and is often better when using small quantities of reagents. Taken together, these measurement errors suggest that accu- racies of 0.1–0.3% are feasible. The limiting factor in many analyses, therefore, is current efficiency. Fortunately current efficiencies of greater than 99.5% are ob- tained routinely and often exceed 99.9%.

In controlled-potential coulometry, accuracy is determined by current effi- ciency and the determination of charge. Provided that no interferents are present that are easier to oxidize or reduce than the analyte, current efficiencies of greater than 99.9% are easily obtained. When interferents are present, however, they can often be eliminated by applying a potential such that the exhaustive electrolysis of the interferents is possible without the simultaneous electrolysis of the analyte. Once the interferents have been removed the potential can be switched to a level at which electrolysis of the analyte is feasible. The limiting factor in the accuracy of many controlled-potential coulometric methods of analysis is the determination of charge. With modern electronic integrators, the total charge can be determined with an accuracy of better than 0.5%.

So what is to be done when an acceptable current efficiency is not feasible? If the analyte’s oxidation or reduction leads to its deposition on the working elec- trode, it may be possible to determine the analyte’s mass. In this case the working electrode is weighed before beginning the electrolysis and reweighed when electroly- sis of the analyte is complete. The difference in the electrode’s weight gives the ana- lyte’s mass. This technique is known as electrogravimetry.

Precision 

Precision is determined by the uncertainties of measuring current, time, and the end point in controlled-current coulometry and of measuring charge in controlled-potential coulometry. Precisions of ±0.1–0.3% are routinely obtained for coulometric titrations, and precisions of ±0.5% are typical for controlled-potential coulometry.

Sensitivity 

For a coulometric method of analysis, the calibration sensitivity is equivalent to nF in equation 11.25. In general, coulometric methods in which the an- alyte’s oxidation or reduction involves a larger value of n show a greater sensitivity.

Selectivity  

Selectivity in controlled-potential and controlled-current coulometry is improved by carefully adjusting solution conditions and by properly selecting the electrolysis potential. In controlled-potential coulometry the potential is fixed by the potentiostat, whereas in controlled-current coulometry the potential is deter- mined by the redox reaction involving the mediator. In either case, the ability to control the potential at which electrolysis occurs affords some measure of selectiv- ity. By adjusting pH or adding a complexing agent, it may be possible to shift the potential at which an analyte or interferent undergoes oxidation or reduction. For example, the standard-state reduction potential for Zn2+ is –0.762 V versus the SHE, but shifts to –1.04 for Zn(NH₃)₄2+. This provides an additional means for controlling selectivity when an analyte and interferent undergo electrolysis at simi- lar potentials.

Time, Cost, and Equipment 

Controlled-potential coulometry is a relatively time- consuming analysis, with a typical analysis requiring 30–60 min. Coulometric titra- tions, on the other hand, require only a few minutes and are easily adapted for au- tomated analysis. Commercial instrumentation for both controlled-potential and controlled-current coulometry is available and is relatively inexpensive. Low-cost potentiostats and constant-current sources are available for less than $1000.