pH meter

What is pH? pH is a measure of the activity of the hydrogen ion in a solution. The pH value states the degree of the hydrogen ion activity in a solution therefore determining if the solution is an acid or base. It can also be expressed as 'the degree of acidity of a solution'. The letters 'pH' are used to represent the value for the hydrogen ion activity. p - is from the mathematical symbol of the negative logarithm, power. H - is the chemical symbol for hydrogen. Therefore pH can be described as the 'Power of hydrogen ions'.

pH Scale pH is measured on a scale from A pH value between indicates the solution is acidic A value of 7 states the solution is neutral A value between 8-14 states the solution is basic/alkaline. The pH value of a solution is directly related to the ratio between the hydrogen ion concentration and the hydroxyl ion concentration in the solution. [H+] : [OH-]. pH valueStateExample 0 - 6AcidicHCL, vinegar 7NeutralWater 8-14Basic (alkaline)NaOH, household ammonia (cleaning agents)

If the concentration of the hydrogen ion is > than the hydroxyl ion, the solution is acidic and therefore has a pH value <7. [H+] > [OH-] If the concentration of the hydroxyl ion > than the hydrogen ion, the solution is basic/alkaline and therefore has a pH value of >7. [OH-] > [H+] If the concentration of the hydrogen ion and the hydroxyl ion are equal, the solution is neutral and has a pH of 7. [H+] = [OH-]

How can pH be measured? The pH of a solution can be measured using a pH meter or pH indicator strips.

pH meter A pH meter is the instrument used to measure the pH of a solution. It consists of a digital voltmeter which connects to a glass electrode. The measuring/glass electrode and reference electrode are used in conjunction of each other to find the pH of a solution. A reference electrode that is built into the glass electrode is called a combination electrode. These are the most common types of glass electrodes used.

Combination pH electrode Thermometer Ag/AgCl Measuring electrode Reference electrode Glass bulb

Glass electrode and reference electrode A glass electrode consists of a glass bulb membrane which is sealed to a thicker glass tube. This tubular body acts as an insulator as it separates an internal solution and a Ag/AgCl electrode from the exterior solution. The glass bulb membrane is formed by glass blowers, who ‘blow’ molten glass into the shape of a thin-walled bulb approx. 0.1mm thick. This is a very simple but important step in the manufacturing of the electrode, as the thickness of the glass determines its resistance to the H+ ions and therefore affects the pH reading. The glass is most commonly composed of alkali metal ions which are used to sense the hydrogen ion concentration of the solution. The pH electrode must be kept moist at all times i.e. in a buffer, as hydration is required for the ion exchange process to occur.

How is the potential difference (the pH) of a solution measured? The alkali metal ions of the glass electrode and hydrogen ions of the solution undergo an ion exchange reaction generating a potential difference. There must be a potential difference measured at 2 points in order to obtain the pH value. Point 1 is the potential of the solution using the glass electrode. The potential that is generated at this site is due to the free hydrogen ions present in the solution. Point 2 is the potential of the reference solution within the electrode. The potential is generated when the internal element comes in contact with the reference solution. (Always constant as solution is within electrode)

The measuring/glass electrode obtains different voltages for each solution, which then ‘sends’ that voltage to the voltmeter and the reference electrode obtains and delivers a constant voltage to the voltmeter. The electrode is connected to a lead cable which is then connected to a voltmeter, i.e. the pH meter. This pH meter is then used to measure the potential difference of the solution and its changes as it crosses the glass membrane. It is the difference between the potential reference point and the measuring point that gives the ‘potential difference’, i.e. the pH value.

pH indicator strips pH indicator strips also known as climax strips are used for measuring the pH of a liquid. This is a fast, easy and cheap way of determining pH. The pH indicator strips consist of thin strips of absorbent paper saturated with coloured organic indicators that change colour when subjected to certain solutions. This colour change indicators the pH of the solution. The pH strip is dipped into the liquid for approx. 10 seconds and removed. Before reading the strip, wait approx. 10 seconds to ensure complete colour change/the colour develops. The colour on the paper will correspond to the colour sequence on the pH colour chart. This pH chart is used as a reference, by matching the pH indicator strip against the colour chart to determine the correct colour sequence and therefore the pH of the solution. The pH indicators can measure to approx. 0.3 of a pH unit.

This method of pH determination can be used to help in identifying samples/solutions and can also be used to determine the pH of bodily fluids for medical analysis. An example of how pH analysis can be useful for medical analysis – A specific pH range is required for the analysis of urine for certain medical tests e.g. to test for protein, creatinine etc. The urine must have a pH value of ≤3 (range 1 – 3), in order for these tests to be carried out. So the pH of the urine is determined using the pH indicator strips and if the value is not ≤3 then the pH of the urine must be altered in order to acquire the appropriate/required pH. The pH of the urine is altered to a pH of ≤3 by adding concentrated HCL until the pH reaches ≤3.

pH indicator strips

What have I learned? pH is a measure of the activity of the hydrogen ion in a solution pH Scale 0 – 6 = acid 7 = neutral 8 – 14 = basic pH Scale 0 – 6 = acid 7 = neutral 8 – 14 = basic A glass combination electrode consists of a measuring and a reference electrode, both required to measure potential difference i.e. pH The glass bulb of the glass electrode is made of alkali metal ions as they can sense/detect H+ ions in a solution. How pH indicator strips determine the pH of solution A voltmeter (pH meter) is connected to a glass electrode via. a lead cable which is used to measure pH.