If you're using litmus paper, none of this matters. The basic idea is that the paper turns a slightly different color in solutions between pH 1 and 14 and, by comparing your paper to a color chart, you can simply read off the acidity or alkalinity without worrying how many hydrogen ions there are. But a pH meter somehow has to measure the concentration of hydrogen ions. How does it do it?

An acidic solution has far more positively charged hydrogen ions in it than an alkaline one, so it has greater potential to produce an electric current in a certain situation—in other words, it's a bit like a battery that can produce a greater voltage. A pH meter takes advantage of this and works like a voltmeter: it measures the voltage (electrical potential) produced by the solution whose acidity we're interested in, compares it with the voltage of a known solution, and uses the difference in voltage (the "potential difference") between them to deduce the difference in pH.

Attention: use of pH Sensor

1)       Touch the pH sensor with responsibility- it is breakable!

2)       Store the pH sensor always in immersed condition within the solution approved by the company or neutral solution of KCl (3M-4M).

3)       Always maintain the inner level of filling liquid beyond the level of measured solution.

4)       Load pH sensor (the flowing type) by exact filling solution (as suggested by the manufacturer – normally KCl solution, 3M to saturated) to not let it dry inside.

5)       You may store the sensors as dry if they are not used for a long period of time to prevent aging. Aging only happens when the sensor is moist. Don’t do it with gel sensors – certain must be put in a strong solution of KCl only.

6)       If the pH sensors are dried soak them at least 24 hours before using.

7)       Clean the sensors immediately when you are using a solution that contains substances that are able to plug the junction or stick to the glass bubble.

8)       Avoid immersing pH sensors in solvents that can dissolve glass such as hydrofluoric acid (or acidified fluoride solution), concentrated alkalies.

9)       Avoid immersing sensors within a dehydrating solution such as ethanol, sulfuric acid, etc.

10)    Avoid rub or wipe sensor bulb.

11)    Don’t clean the pH sensors with organic solvents.