pH Measurement

Draw a picture and label a pH ion-selective glass electrode.

1. H⁺-selective glass bulb. 2. Ag/Cl measuring electrode 3. Buffering solution. 4. sediment 5. Reference Ag/Cl electrode 6. KCl solution with known 'reference' pH 7. ceramic plug, porous acts as a salt bridge 8. plastic outer tube

What is the piece of equipment used to measure pH in solution?

An ion-selective glass electrode. Consisting of two separate electrodes; a reference electrode and a measuring electrode.

Outline limitations of the electrode.

H⁺ solubility changes with temperature, so incorrect readings can be given if the temperature of the machinery is altered or if the patient is hypothermic and the machine assumes 37 degrees (most machines allow for temperature correction)

What is pH

The activity of H⁺ in solution. Or less accurately, the -Log of the concentration H⁺ in solution.

How does the electrode work?

The blood sample is passed over the glass bulb. A constant electrical potential is maintained by the reference electrode. The measuring electrode ion-selective glass absorbs H⁺ from the blood. The gradient between the H⁺ concentration at the measuring electrode versus the reference electrode creates an electrical potential difference which is measured by the probe and converted into a H⁺ ion difference.

Outline some basic maintenance steps for the probe.

The probe glass needs to be cleaned after each use and should be calibrated against two solutions of known pH, one being the same as the reference buffer (as a zero) and then against a solution with a known pH.

Give values for pH of 6, 7, 7.4 and 8

pH of 6 = 1000 nmol/L H⁺ pH of 7 = 100 nmol/L H⁺ pH of 7.4 = 40 nmol/L H⁺ pH of 8 = 10 nmol/L H⁺