The second part of this blog series focuses on the different ways of carrying out a potentiometric ion measurement with an ion-selective electrode (ISE). The two most common ways are standard addition and direct measurement.

In the past, you may have doubted which of these two determination methods was most suitable for a sample. To make it easier for you to answer this question, I will introduce the two measuring principles and compare their advantages and disadvantages in this blog article. In addition, there are two handy checklists provided within to ensure the best reproducibility.
 

Click to go directly to a topic:

• Standard addition
• Direct measurement
• Advantages and disadvantages: standard addition vs. direct measurement
• Summary

If you want to know about the basic principles behind ion measurement, check out the first part of this series.

Ion-selective electrodes: General tips – Part 1

Standard addition

During a standard addition, defined volumes of a standard solution of the ion of interest are added to a known volume of the sample solution in several increments. After each addition of the standard solution, the potential of the solution is measured. The ion concentration of the original sample solution can then be calculated from the difference between the initial potential and the potential measured after each addition. 

An illustrated example of a typical standard addition curve is given in Figure 1. Here the initial concentration of the sample is calculated from the measured potential difference which depends on the added volume increment. The first measuring point (in red) corresponds to the measured potential of the sample solution, whereas the subsequent measuring points (in green) correspond to the measured potential after each addition of the standard solution.

Traditionally, fixed volumes of a standard solution are added during the standard addition process. This choice is often used during quality control of products with a defined ingredient concentration for time saving purposes. Modern devices use a fixed potential difference and add variable volumes of standard solution until the difference is achieved. This is useful for products with fluctuating or inconsistent ingredient concentrations. Table 1 compares the two different types of standard addition.

According to Table 2, the standard concentration (c_std) for different buret volumes (V_buret) must be chosen as a function of the sample concentration (c_smpl) to ensure the most accurate execution of the standard addition. Thereby any sample dilution must be considered (e.g., dilution with TISAB).

We recommend using a fixed potential difference for standard addition as it is less prone to errors and gives more reliable results. Modern Metrohm devices such as the OMNIS titrator can carry out these addition methods automatically. Just press a single key and the addition of your standard solution is automatically controlled from the titrator or the ion meter. The same applies to the evaluation since the calculation of your result is done iteratively by the device itself. 

That’s quite easy, isn’t it? This solution reduces human error and saves time for more important laboratory tasks.

Checklist: Standard addition

Having reproducible results is important for everyone working in the lab. To achieve optimal results, we have prepared this checklist for you. Can you answer all of these questions with YES?

Direct measurement

When performing a direct measurement, the ion-selective electrode is calibrated with standard solutions of the ion to be measured. This is done before the ion measurement itself, similar to the calibration of a pH glass electrode. The calibration can then be used for several determination series.
 

Check out our previous blog article for more information about pH calibration.

FAQ: All about pH calibration

When performing an ion measurement with the direct determination method, you should consider the following three points:

1. Composition of the calibration standards
   In general, standard solutions must have the same ionic background as the sample solution. Therefore, the calibration standards should consist of a particular concentration of the measuring ion as well as the same proportion of deionised water to ISA/TISAB that is used afterwards in the determination itself. If the sample has a high ionic background, we recommend emulating this in the standard solution. For example, to measure fluoride in fluorinated table salt, it’s recommended to add highly pure sodium chloride to the standard solutions to mimic the sample.
2. Concentration range of the calibration standards
   The expected ion concentration in the sample should lie somewhere in the middle of the concentration range of the standard solutions (Figure 2). Therefore, calibration standard concentrations should be chosen in such a way that they cover (bracket) the expected concentration of the measured ion in the sample.
3. Order of the calibration standards
   To reduce the influence of carryover, the standard solutions should be measured from the lowest to the highest concentration.

Checklist: Direct measurement

Having reproducible results is important for everyone working in the lab. To achieve optimal results, we have prepared this checklist for you. Can you answer all of these questions with YES?

Advantages and disadvantages: standard addition vs. direct measurement

Both determination methods discussed here have advantages and disadvantages. To help you decide which determination method to choose, the pros and cons are listed in Table 3.

Summary

The choice of the procedure depends on the sample matrix, the number of samples to be analysed, and the concentration range of samples.
 

Direct measurement is better if you have:

• a high sample throughput or
• a known sample of a simple composition

Standard addition is recommended whenever:

• your determination only needs to be carried out occasionally or
• the composition of your sample is unknown

Your knowledge take-aways