pH is a measure of how acidic/basic water is. The range goes from 0-14, with 7 being neutral. pHs of less than 7 indicate acidity, whereas a pH of greater than 7 indicates a base. pH is really a measure of the relative amount of free hydrogen and hydroxyl ions in the water. Water that has more free hydrogen ions is acidic, whereas water that has more free hydroxyl ions is basic. Since pH can be affected by chemicals in the water, pH is an important indicator of water that is changing chemically. pH is reported in "logarithmic units".
Fig. 1 pH scale
pH measurements have been, and continue to be, widely used as a rapid, accurate measure of the acidity of fluids of all sorts. Common methods of accurate measurement are electrochemical methods via electrodes and millivoltmeter (pH meter).
A typical pH meter has two basic components: the meter itself, which can be a moving-coil meter or a digital meter, and either one or two probes. If the pH meter has two probes (measuring electrode and reference electrode), each one is a separate electrode; if it has only one probe, both of the two electrodes are built inside of it. The electrodes aren't like normal electrodes (simple pieces of metal wire); each one is a mini chemical set.
When we dip the two electrodes into the blue test solution, some of the hydrogen ions move toward the outer surface of the electrode and replace some of the metal ions inside it, while some of the metal ions move from the electrode into the blue solution (see Fig.2). This ion-swapping process is called ion exchange, and it's the key to how a electrode works. Ion-swapping also takes place on the inside surface of the electrode from the orange solution. The two solutions on either side of the electrode have different acidity, so a different amount of ion-swapping takes place on the two sides of the electrode. This creates a different degree of hydrogen-ion activity on the two surfaces of the electrode, which means a different amount of electrical charge builds up on them. This charge difference means a tiny voltage which shows us the pH.
Fig. 2 Schematic diagram of pH meter working principle
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