When a sample solution contains interfering ions that may have an impact on the results of measurement, those ions are removed from or masked out of the measurement system using treatments such as distillation and masking; the pH and ion intensity of the sample are then suitably adjusted in preparation for measurement. If required, pretreatments such as dilution, concentration, and extraction are also carried out.
This method obtains the target ion concentration from a calibration curve, following measurements of the difference in potential between an ion selective electrode and reference electrode that are immersed in the sample solution. This method requires that the reference solution used for preparation of the calibration curve be similar in composition to the sample solution. The difference in potential is usually measured after the ion intensities of the reference and sample solutions have been made equal by the addition of an ionic strength control solution.
If the target ion forms a complex, the difference in potential should be measured after dissociating the complex by adding a complex dissociating agent solution or adjusting the pH. The following is an example of the measurement procedure:
(1)Prepare reference solutions with a series of stepped concentrations and add an ionic strength control solution to each before transferring an appropriate amount into beakers. If necessary, add a pH-buffer solution or complex dissociating agent solution.
(2)Thoroughly wash the tip of the ion electrode and reference electrode with water then wipe off any water that remains (see “Caution” below).
(3)Connect the ion electrode and reference electrode to the potentiometer, immerse them in the solution prepared in step (1) that has the lowest concentration then, whilst stirring the solution, read the value indicated by the potentiometer after it has become stable.
(4)Carry out measurements of the difference in potential for the other reference solutions of higher concentration.
(5)Assign reference solution concentration to the logarithmic axis and difference in potential to the linear axis of a sheet of semilog graph paper and plot a calibration curve.
(6)After carrying out the operations in step (2), add an ionic strength control solution to the sample solution and measure the difference in potential. If necessary, add a pH-buffer solution or complex dissociating agent solution to the sample solution.
(7)Obtain the ion concentration using the calibration curve created in step (5).
Caution: Unless completely removed by cleaning with water, target ions attached to the response membrane may produce errors in measurement.
Note 1: Where the target ion concentration becomes very low, the calibration curve will lose linearity. In this case, the following method is effective. Add a certain amount of low-concentration reference solution to the sample solution, obtain its ion concentration using a linear region of the calibration curve, then subtract from it the increase in concentration resulting from the added reference solution to obtain the target ion concentration of the sample solution.
Note 2: When using a potentiometer that indicates measurements as ion concentrations, calibrate the potentiometer using two types of reference solutions before obtaining the target ion concentration of the sample solution.
The following are other measurement methods, explanations of which have been omitted.
(1)Standard addition-potential difference measurement method:
1．Known amount addition method
2．known amount diminution method
(2)Potentiometric titration method
(3)Gran’s plot method
1．Use in standard addition method
2．Use in potentiometric titration method
Transfer the required amount of reagent containing the target ions to a container, prepare a solution by adding water or the necessary reagent then transfer this solution to a measuring flask before adding further water (or necessary reagent) up to the marker level to give the undiluted solution. When using reference solutions, prepare one for each target ion concentration by taking a series of stepped amounts of the undiluted solution and diluting each to a certain volume. Ensure that each reference solution has approximately the same pH level and ionic strength.
Note that the ion concentration of some undiluted solutions may change during storage due to decomposition. If the concentration has changed to a great extent, prepare the solution at the point of use.
To obtain the accuracy of measurement, refer to JIS Z 8402: Accuracy (reliability and precision) of measurement methods and results. Note that the ion selective electrode method requires the utmost care since it tends to produce errors. Perform repeated accuracy and reproducibility evaluations using reference solutions to determine the minimum and maximum measurement limits for each ion selective electrode.
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