|Ages 18 +|
Potentiometric methods of analysis are based on measuring the potential difference between two electrodes (a cell) in such a way that the current passing between these two electrodes is practically zero. One of these electrodes is called the working electrode—this has a potential dependent on the concentration of a given ion present in the test solution, for example the glass electrode depends on hydrogen ions. The reference electrode forms the other half of the cell, this is of a fixed potential and independent of the sample composition, eg calomel electrode or Ag/AgCl (silver/silver chloride) electrode.
For accurate measurements of the cell’s potential, the resistance of the measuring device must be large with respect to the cell resistance (about 1000 times greater.) Since the electrical resistance of the cells electrodes may be 100 MΩ or more, the regular potential measuring devices used generally have an internal resistance of 1012 Ω or more.
Since most data loggers and typical voltmeters have an input impedance of only about 1 MΩ, they are not suited for potentiometric measurements, eg the determination of pH aqueous solutions which is the most common example of potentiometric measurements. Consequently, a higher input impedance amplifier must be used. The DrDAQ data logger includes such an amplifier to allow direct single-channel pH measurements.
This experiment describes the construction and use of a low–cost four–channel high input impedance module to be used with an ADC-16 high resolution data logger to allow simultaneous four–channel pH monitoring.
- ADC-16 data logger and terminal block
- 4 * INA116P Ultra low input bias current instrumentation amplifier
- 5 * BNC sockets
- 2 * D25 connector (1 male + 1 female)
- Glass combination ph electrodes (up to 4 electrodes)
- 4 * Beakers (250 mL)
- Series of buffer solutions
Experiment Set up
The INA116P has very high input impedance (1015 Ω) which suggests its use in measurements of cell potential, especially cells of high resistance similar to those employing glass electrodes. Conveniently the INA116P can be powered by the ADC-16 as shown in Figure 1, eliminating the need for an additional power supply.
Figure 1: INA116P Amplifier
Construct a four channel module using 4 INA116P amplifiers, as shown in the circuit diagram (Figure 2.) The circuit has the following features:
- Separate reference and working electrode input to every channel. This is suitable, for example, in the case of glass–reference combination electrodes where a BNC socket connects both the glass and the reference electrode simultaneously.
- Common reference electrode input. This is important in other potentiometric measurements using multiple working electrodes against a single reference electrode.
- The reference electrode input is connected to the ground. This provides stable potential recording.
- The Output (Ch1–4), ground, +5 V, and -5 V lines are connected to a built in D25 female connector fixed on the top of the project box as shown in Figure 3.
- Corresponding connection to the ADC-16 terminal block fixed to the ADC-16 are made using 7 wires. These correspond to Ch1-4, ground, +5 V and -5 V.
- BNC sockets are used in Ch1–4 and the common reference input to match the usual connectors for glass combination electrodes.
- The four–channel module is easily accommodated in a 4x6x11 cm project box as shown in Figure 3.
Figure 2: instrumentation amplifier circuit diagram
Figure 3: ADC-16 with 4–channel
high input impedance module
- Plug in the ADC-16 data logger to a PC installed with PicoLog software.
- Connect the 4–Ch module to the ADC-16.
- Connect up to 4 combination glass electrodes to the inputs labeled Ch 1–4.
- Using the scaling feature of Picolog calibrate each channel independently using standard buffer solutions. For combination pH glass electrodes provided by Pico, the potential difference is approximately zero at pH7. Consequently, a possible scaling equation could be: pH = 7.00 - ( X / 59.1 ) - where X is the potential reading in mV.
- For more accurate measurements, the “Table look up” scaling option is recommended to make calibrations using two or three buffers.
- Record simultaneously the pH values of 4 different buffer solutions.
- Switch the glass electrodes among different buffers of different pH values—this should produce results similar to those shown in figure 4.
Questions and discussion of results
Figure 4 shows simultaneous 4-ch pH monitoring. Each electrode is subject to 2 or 3 pH step changes. Then, each electrode was returned to its original buffer solution where the original pH values were almost recovered. This proves the reliability of the described system in accurate and reproducible pH monitoring. Of course, the quality of the glass electrode itself is essential in these measurements.
Figure 4: independent 4-channel pH monitoring in different buffer solutions
- Q1: Explain why pH measurements based on glass electrodes are exceptionally more challenging in comparison to measurements with other electrode types such as metallic electrodes.
- Q2: 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.
- Q3: Explain why the high input impedance voltmeter is commonly referred to as a pH-meter.
- Q4: 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?
- Q5: Can you think of another advantage of using such multi–channel recordings?
Modify the acid base titration curves experiment to use a single titrant (NaOH) added to four different beakers containing boric acid, acetic acid, hydrochloric acid, and phosphoric acid, respectively and monitor the pH during the progress of these four titrations simultaneously.
Modify the measuring the ph of milk experiment to simultaneously record the souring rate of four different milk samples kept at different temperatures.
Apply the four–channel module in other types of potentiometric measurements such as potentiometric titration, direct potentiometry using other electrodes such as platinum, silver wires or membrane ion selective electrodes. You have the capability of four pH meters.
Credits, comments and further info
INA116P data sheet- Texas Instruments
This experiment was written by Dr Sayed A. Marzouk - Assist. Prof. of Analytical Chemistry, United Arab Emirates University.