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.
 

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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.

Figure 1. Standard addition. a) Schematic representation of the signal given after each standard addition; b) example of a typical standard addition curve.

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.

Table 1. Standard solution addition by fixed volume or fixed potential difference.

Step Fixed Volume Fixed Potential Difference
1 Add the sample solution into the beaker.
2 Add ISA/TISAB.
3 Add defined volumes of a standard solution using a buret or pipette. Add standard solution until a defined potential difference is obtained using a buret.
4 Measure the potential after the addition.

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

Table 2. Recommended ratio of standard concentration and sample concentration dependent on the chosen buret volume.

Vburet in mL cstd : csmpl
5 40 : 1
10 20 : 1
20 10 : 1
50 5 : 1

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?

Yes / No Before measurement:
(  ) Fresh ISE/TISAB is added to my sample solution.
(  ) The number of my additions is at least four.
(  ) The added volume does not exceed 25% of the sample volume. So, there is no risk of a dilution error.
(  ) My defined potential difference is at least 12 mV per addition.
(  ) There is no temperature difference between my standard and sample solution.
Yes / No During / after measurement:
(  ) The total volume of my standard added lies in the region between 10 and 90% of the buret volume.
(  ) My solution is stirred while adding standard.
(  ) There are no air bubbles in the tubing and the dosing unit is tight.
(  ) My potential measurement reaches a stable potential between additions.
(  ) My obtained slope is acceptable.

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 deionized 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.
Figure 2. a) Schematic of the direct measurement concept. b) Calibration with standards of increasing concentrations.

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?

Yes / No Before measurement:
(  ) The amount of fresh ISA/TISAB is the same for all of my standards and for my measuring solution.
(  ) I have used the same ratio of ISA/TISAB to sample/standard plus water for the standard and sample measurements.
(  ) The calibration range sufficiently covers my sample concentration range.
(  ) My sample and calibration standards are measured under identical conditions.
(  ) Matrix composition is emulated as good as possible in my calibration standards.
Yes / No During / after measurement:
(  ) I have properly conditioned my electrode between measurements.
(  ) I have measured my standards in the correct / defined order.
(  ) The slope of my calibration is acceptable.

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.

Table 3. Advantages and disadvantages of standard addition and direct measurement.

  Standard addition Direct Measurement
Advantages
  • Matrix-independent
  • No calibration necessary
  • Only a single standard is needed
  • Nearly no user interaction necessary
  • Electrode is tested with each measurement
  • Fast measurement (parameters determined quickly, on average in approximately 60 seconds of measuring time)
  • Good reproducibility, also at lower concentration ranges

Disadvantages

  • Possibility of poor  reproducibility in lower concentration ranges
  • Longer determination times (parameters determined within approximately 300 seconds of measuring time)
  • A series of standards for calibration must be prepared
  • Matrix-dependent
  • Change of electrode properties is only seen during calibration

Summary

The choice of the procedure depends on the sample matrix, the number of samples to be analyzed, 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

Overcoming difficulties in ion measurement: Tips for standard addition and direct measurement

Click here to download

Ion measurement can be conducted in several different ways, e.g., ion chromatography (IC), inductively coupled plasma optical emission spectrometry (ICP-OES), or atom absorption spectroscopy (AAS). Each of these are well-established, widely used methods in analytical laboratories. However, the initial costs are relatively high. In contrast, ion measurement by the use of an ion-selective electrode (ISE) is a promising alternative to these costly techniques. This White Paper explains the challenges which may be encountered when applying standard addition or direct measurement, and how to overcome them in order for analysts to gain more confidence with this type of analysis.

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Hoffman

Doris Hoffman

Product Manager Titration
Metrohm International Headquarters, Herisau, Switzerland

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