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Beginners Guide to ISE Measurements, Chapter 10.

METHODS OF ANALYSIS

a) Direct Potentiometry

Direct potentiometry is the simplest and most widely used method of using ISEs as described above in the Basic Theory and Calibration sections of this work. Simply measure the electrode response in an unknown solution and read the concentration directly from the calibration graph (either manually or using special computer graphics and calculations - see later) or from the meter display on a self-calibrating ion meter. A big advantage of this method is that it can be used to measure large batches of samples covering a wide range of concentrations very rapidly without having to change range, recalibrate or make any complicated calculations. Moreover, if ISAB is not being used, it is not necessary to measure the volume of the samples or standards. Quite acceptable results can be obtained for some elements by simply dangling the electrodes in a river or pond or effluent outflow without the need to take samples in small beakers.

b) Incremental Methods

There are three main types of incremental methods in general use:

  • Standard (or Known) Addition,
  • Sample Addition,
  • Sample Subtraction.

    Standard Addition and Sample Addition Methods.
     These methods involve measuring the voltage in a relatively large, accurately measured, volume of sample (for standard addition) or standard (for sample addition) then adding a much smaller volume of standard (or sample) and taking a second reading after the voltage has stabilised in the mixture. For full details see: www.nico2000.net/datasheets/staddl.html

    Summary of Advantages over Direct Potentiometry

    • The electrodes remain immersed throughout the process so that there is little change in the liquid junction potential of the reference electrode (which can often be changed by several millivolts when the electrodes are removed from one solution and placed in another) between calibration and sample measurement - and therefore this source of measurement error is virtually eliminated.

    • Calibration and sample measurement are both made essentially at the same time and in the same solution so that ionic strength and temperature differences between standard and sample are not significant and ISAB is not normally required.

    • Once the approximate concentration for the samples is known, the calibration (slope) can be "fine tuned" by analysing a standard with a concentration that lies within the range of the samples (and is at the same temperature) and then adjusting the slope and re-calculating the results until the standard gives the correct answer. This "fine tune" procedure is very quick and easy using the ELIT ISE/pH Ion Analyser Software.

    • Measuring the slope at or very near to the sample concentration means that these methods can be used with old or worn electrodes which may not be completely linear over their whole range, as long as the slope is stable and reproducible over the limited range of the samples.


    Sample Subtraction method.
    This involves adding a small amount of sample solution to a standard solution of an ion with which it will react stochiometrically to form a complex or precipitate, thus reducing the concentration of both ions. The ISE used will be sensitive to the reactive ion in the standard, not the sample. The big advantage of this method is that it can extend the range of ions measurable by ISEs to others for which no ion-sensitive membranes are available. For example, there is currently no ISE capable of detecting the sulphate ion. However, sulphate can be removed from solution by precipitating as barium sulphate, and there is an ISE which is sensitive to barium. Therefore, sulphate can be measured by first measuring the voltage in a pure barium chloride standard. Then adding a known volume of a sample containing sulphate, wait for precipitation to be completed, and measure the voltage on the barium electrode again. The amount of barium used can then be calculated using a similar equation to that used for Sample Addition and the sulphate content in the sample will be the same as this - since each sulphate ion will combine with one barium ion.

    c) Potentiometric Titrations

    Potentiometry is generally valuable as a technique for detecting the end-point of titrations where there is often a drastic change in the concentrations of the reactants and thus a big shift in the electrode potential. These end point determinations can often be made more precisely than other ISE methods because they depend on the accuracy of the volumetric measurements rather than the measurement of the electrode potential. For example, when a calcium solution is titrated against the complexing reagent EDTA there is a gradual decrease in the Ca concentration as more EDTA is added until the end point when all the Ca disappears from solution. The progress of this titration can be monitored using a calcium electrode.

    This method can also be used to extend the range of ions measurable by ISEs. For example aluminium cannot be measured by direct potentiometry but it can be titrated by reacting with sodium fluoride and monitoring the reaction using a fluoride electrode. It can also be used for elements for which it is difficult to maintain stable standard solutions or which are toxic and it is undesirable to handle concentrated standard solutions. For example, cyanide solutions can be titrated against a hypochlorite solution which forms a complex with the cyanide ions and effectively removes them from solution. The amount of cyanide in the original solution is proportional to the amount of hypochlorite used from the start of the titration until the end-point when there is no further change in the cyanide electrode potential.

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