Four-channel pH measurements using an ADC-16 Data Logger - results and answers

Explain why pH measurements based on glass electrodes are exceptionally more challenging in comparison to measurements with other electrode types such as metallic electrodes.

Because the resistance of the glass membrane used in the construction of glass electrode is exceptionally high. This necessitates a measuring device of very high input impedance of 10¹² Ω or more—one million times that of typical voltmeter or data logger.

Even if the cell resistance is relatively low, which may allow data loggers with 1 MΩ input impedance to measure the cell potential, explain why it is still preferred to use high input impedance device. 

In potentiometry, we measure the cell potential (potential difference between two electrodes). In principle, when the cell resistance is low (eg with metal electrodes), we can use a data logger or regular voltmeter to measure the cell potential. However, an appreciable current will flow through the cell. This current causes the cell potential to decrease with time in a way similar to discharging of a battery. Therefore, even though with cells of low resistance, it is preferred to use high input impedance device to monitor the cell potential.

Explain why the high input impedance voltmeter is commonly referred to as a pH-meter.

Because pH measurements are the most common form of potentiometric analysis, and the most common test carried out the chemical laboratories. Note that a pH meter is a voltmeter but with very high input impedance.

What will happen if you attempt to measure the cell potential of the glass/reference cell electrode directly using the ADC-16 data logger or the regular voltmeter?

Assume that the potential of the glass/reference cell is 0.5 V and its internal resistance is 100 MΩ. The circuit can be considered to consist of a potential source (Es) and a resistance (Rs) on series. Now let us calculate the relative error in the measured potential if the measuring device has a resistance of 1 MΩ. This example is similar to that found in “Principles of Instrumental Analysis” by Skoog et al, Saunders College Publishing, fifth edition, 1998 (page 611.)

From Ohm’s law: Es = IRs + IRm

Since Rs is almost 100 times Rm, the measuring device will measure only about 1% of Es. Consequently, the measured potential will be very small and close to zero. When we substitute these values in the equation pH = 7.00 - ( X / 59.1 ) — where X is very small (close to zero), we can understand why the ADC-16 will measure a pH of around 7 with the glass electrode connected to it directly whatever the pH of the solution. This prediction is verified in the experiment shown in Figure 5. In this experiment, two glass combination electrodes are immersed together in different buffer solutions. Where one electrode is connected to Ch1 in the 4–Ch module and the other is connected directly to channel 5 of the ADC-16. Since the ADC-16 will measure only a very small fraction of the cell potential, the measured mV will be close to zero and the displayed pH will be around 7 regardless of the pH of the solution.

Can you think of another advantage of using such multi-channel recordings?
Measuring the same parameter eg pH or other ion concentration using more than electrode could provide a mean to enhance the signal to noise ratio. The “calculated parameters” feature of PicoLog helps to plot directly the average of the 4 channels.

Figure 5: comparison of the glass–electrode pH response measured directly with the ADC-16 and through the high input impedance module.

Teachers’ notes

General notes

Two standard buffers of pH 4.0 and 7.0 are commonly used in pH calibration.

The equation shown in this experiment can be used in a single point calibration. However, for more accurate measurements, A “table look up” option in the scaling feature of Picolog is recommended to carry out 2 or 3 point calibration.

Sampling rate can be set to 1 sample every one or two minutes. This allows, enough time to move the glass electrodes from one beaker to another before the next data point is sampled.

Target age groups

Ages 18+ (HNC / Degree)

National curriculum

The initial section of this experiment shows how to set up an ADC-16 to provide simultaneous four-channel pH monitoring with glass electrodes, at very high impedance, aided by FET input instrumentation amplifiers. The associated activity then looks at setting up and using such a system and discusses the benefits of it.