Potentiometric Titration of Iron with Mohr's Salt

Potentiometric Titrations

Accuracy

The accuracy of a potentiometric analysis is limited by the measurement error for the cell's potential. Several factors contribute to this measurement error, including the contribution to the potential from interfering ions, the finite current drawn through the cell while measuring the potential, differences in the analyte's activity coefficient in the sample and standard solutions, and liquid junction potentials. Errors in accuracy due to interfering ions often can be eliminated by including a separation step before the potentiometric analysis.

Precision

The precision of a potentiometric measurement is limited by variations in temperature and the sensitivity of the potentiometer.

Time, cost & equipment

In comparison with competing methods, potentiometry provides a rapid, relatively low-cost means for analyzing samples. Commercial instruments for measuring pH or potential are available in a variety of price ranges and include portable models for use in the field.

Classical Titrations

Accuracy

An important limitation when using a visual indicator is the need to observe the change in color signaling the end point. This may be difficult when the solution is already colored. When working with macro-major and macro-minor samples, acid-base titrations can be accomplished with relative errors of 0.1-0.2%. The principal limitation to accuracy is the difference between the end point and the equivalence point.

Precision

The relative precision depends primarily on the precision with which the end point volume can be measured and the precision of the end point signal. The relative precision can be improved by using the largest volume buret that is feasible and ensuring that most of its capacity is used to reach the end point. The precision of the end point signal depends on the method used to locate the end point. With a visual indicator, the precision of the end point signal is usually between ±0.03 mL and 0.10 mL.

Time, cost & equipment

These titrations require less time than most gravimetric procedures, but more time than many instrumental methods of analysis, particularly when analyzing many samples. With the availability of instruments for performing automated titrations, however, concerns about analysis time are less of a problem. When performing a titration manually the equipment needs are few (a buret and possibly a pH meter), inexpensive, routinely available in most laboratories, and easy to maintain. Instrumentation for automatic titrations can be purchased for around $3000.

Derivative methods are particularly well suited for locating end points. The precision with which the end point may be located also makes derivative methods attractive for