What is pH?
pH stands for the potential of hydrogen and is detected using an instrument- pH meter.
What is a pH meter?
It is an instrument used to measure the pH of an unknown solution. By identifying the hydrogen ion concentration in the unknown solution, a pH meter can help us determine whether our solution is alkaline or acidic.
How was pH measured before the existence of the pH meter?
Colorimetric assays – a color change can be observed upon the addition of a pH indicator solution. The intensity of color change can give us an idea of the pH change.
Litmus paper- In this classic method of pH measurement, a drop of sample solution is poured on a specialized litmus paper. The change in color of litmus paper indicates a pH change. The approximate pH can be identified by comparing the color to a standard color chart.
While these techniques may be simple to perform and portable, it is not accurate. Even a slight change in pH values (0.1 degrees change) can result in serious physiological changes. This is where a standard pH meter comes into use.
Components of a pH meter
It has three parts- an internal electrode (glass electrode), a reference electrode, and a voltmeter.
Both the electrodes are made of glass and have a silver wire coated with AgCl.
2 reactions mainly happen in a pH meter. They are:
- Silver metal (Ag) can lose an electron——— silver ion Ag+
- Silver ion can combine with chloride ion ——- AgCl
Both these reactions are reversible.
The reference electrode is filled with saturated KCl
The glass electrode is filled with 0.1 M HCl
The glass electrode has a bulb-like structure. Oxygen (O-) binds to H+ ions of the silica present in glass. This results in the formation of a hydrated gel layer. The hydrogel layer is formed on the region outside the glass bulb as well as inside. This makes the glass electrode act as a half-cell. This half-cell is connected with the reference electrode through the voltmeter. The prorated ceramic of the reference electrode allows the diffusion of ions thereby completing the circuit.
However, modern-day pH meters have both electrodes combined into one.
Working
The pH meter measures the acidity or alkalinity of a solution. In other words, it measures the hydrogen ion or hydroxyl ion concentration. The measured pH is the difference in hydrogen ion concentrations between the buffer solution and the unknown solution. Let us understand how it works by comparing its action under two conditions- 1. when the solution is acidic and 2. when the solution is basic.
Sample solution is Acidic | Sample solution is Alkaline |
| Sample has more H+ ions | Sample has less H+ ions |
| So, the outer hydrogel layer is more positive than the inner layer | So, the inner hydrogel layer is more positive than the outer layer |
| To balance the net charge, the H+ ion present in the HCl bind to the inner layer | To balance the net charge, the H+ ion bound to the inner layer gets released into the HCl solution |
| So, the concentration of H+ in the HCl solution decreases slightly | So, the concentration of H+ in the HCl solution increases slightly |
| The decrease in H+ ions results in an increase in Cl- ions | The increase in H+ results in a decrease in Cl- ions |
| To balance out the Cl- ions, the silver in the silver wire will lose an electron to form Ag+ (silver ion) | To balance out, the AgCl (coated on the wire) disassociates into Ag+ and Cl- ions. The Cl- ions will balance out the extra H+ ions |
| Since the silver wire is conducting, these electrons will give rise to an electric current and will change the potential of the glass electrode | Since Ag+ is now in excess, it moves to the silver wire, gains an electron and becomes metallic silver |
| This change in potential is measured by the voltmeter with reference to the reference electrode | This change in potential is measured by the voltmeter with reference to the reference electrode |
If the [H+] > [OH-], the solution is acidic
If the [H+] < [OH-], the solution is alkaline
If the [H+] = [OH-], the solution is neutral
The range of hydrogen ions concentration- 1 to 14
Any value less than 7 is acidic, equal to 7 is neutral, and more than 7 is alkaline.
Calibration
To calibrate the instrument means to “teach” the instrument what the standard pH would look like. We need solutions with known pH i.e. Buffers to calibrate our instrument. We insert our pH electrode into 3 different buffers- pH4, 7, and 10. we measure the potential difference between the two electrodes when dipped in each of the buffers and the instrument now registers it as the corresponding pH.
Factors affecting pH measurement
- Hydrogen ion concentration
- Temperature- The pH of a solution decreases with an increase in temperature.
The pH electrode, being highly sensitive is stored in a solution of potassium chloride (KCl). Why exactly do we store the electrode in KCl?
- It maintains the stability of the electrolyte
- It results in accurate measurement
- It maintains a stable ionic environment
- It prevents contamination of the electrode
- It extends the lifespan of the electrode
Now that you have understood the basic concepts, see if you can answer this question-
If a pH meter is giving unstable or fluctuating readings, what could be the possible reasons, and how would you troubleshoot them?
Thank you for sharing.