Introduction
The pH of aqueous solutions can be taken as negative logarithm of hydrogen ion activity (pH = -log (H+) pH value from 0 to 7 denote diminishing acidity while 7 to 14 denote increasing alkalinity and 7 is neutral.
Increase or decrease in pH units is equivalent to tenfold decrease or increase in hydrogen ion concentration.
Environmental Significance
The pH of nature water lies in the range between 6.8 to 8.2. pH is governed by the carbon dioxide/bicarbonate/carbonate equilibrium.
Changes in equilibrium are brought out by biochemical reaction induced by microorganisms. Since biological activities are pH specific, determination of pH is very important.
pH has to be determined immediately after collecting the sample form water sources or aerobic/anaerobic reactor.
Procedure
The pH is determined by measurement of the emf of a cell comprising an indicator electrode (an electrode responsive to hydrogen ions such as glass electrode) immersed in this cell is measured with the pH meter.
Calibrate electrode system against standard buffer solution of known pH. Since, buffer solutions may deteriorate as a result of mould growth or contamination, preparation of fresh buffer is needed. pH 4 buffer is best for the single glass electrode. Saturated KCl is required for calomel and Ag/AgCl reference electrode.
Before use, remove electrode form storage solutions, rinse, blot dry with a soft tissue paper and place in ‘4 pH ‘ buffer solution and set the isopotential point.
Select the second buffer of ‘7 or 9 pH‘ and calibrate. Record temperature of measurement and adjust it accordingly by setting temperature dial on meter so that meter indicates pH value of buffer at test temperature.
Remove electrodes from buffer, rinse thoroughly with distilled water and dry electrodes as described above.
The purpose of standardization is to adjust response of the glass electrode to the instrument. When only occasional pH measurements are made, standardization of the instrument before each measurement is desirable.
After standardization immerse electrode(s) in the test sample and read the pH. pH meter can be used for titration in case the sample which are highly coloured or the point of inflection is most critical.