pH & pH Meter

pH is a unit of measure which describes the degree of acidity or alkalinity (basic) of a solution.

It is measured on a scale of 0 to 14. The formal definition of pH is the negative

logarithm of the hydrogen ion activity. pH = -log[H+]

The pH value of a substance is directly related to the ratio of the hydrogen ion and hydroxyl ion concentrations.

If the H+ concentration is higher than OH- the material is acidic.

If the OH- concentration is higher than H+ the material is basic.

7 is neutral, < is acidic, >7 is basic

The pH scale corresponds to the concentration of hydrogen ions.

The addition of acid to water increases the concentration of hydrogen ions and reduces the concentration of hydroxyl ions

The addition of a base would increase the concentration of hydroxyl ions and decrease the concentration of hydrogen ions

How a pH meter works When one metal is brought in contact with another, a voltage difference occurs due to their differences in electron mobility.

When a metal is brought in contact with a solution of salts or acids, a similar electric potential is caused.

Similarly, an electric potential develops when one liquid is brought in contact with another one, but a membrane is needed to keep such liquids apart. 

A pH measurement system consists of three parts: a pH measuring electrode, a reference electrode, and a Transmitter

The pH measuring electrode is a hydrogen ion sensitive glass bulb.

The reference electrode output does not vary with the activity of the hydrogen ion.

There are two electrodes inside the probe that measure voltage.

One is contained in liquid with fixed pH.

The other measures the acidity of the sample through the amount of H+ ions.

A voltmeter in the probe measures the difference between the voltages of the two electrodes.

The meter then translates the voltage difference into pH and displays it on the screen.

Temperature compensation is contained within the instrument because pH electrodes are temperature sensitive.

Temperature compensation only corrects for the change in the output of the electrode, not for the change in the actual solution.

Buffers are solutions that have constant pH values and the ability to resist changes in pH.

They are used to calibrate the pH meter.

Are available in

o 4.01 pHo 7.00 pHo 9.18 pHo 10.01 pH

To perform pH measurement, the pH meter must always be calibrated using the standard solution. The general calibration method is "two-point calibration" which is performed using two types of pH standard solution, but the simple "one-point calibration" is conducted using one type of pH standard solution. As it is also called a "buffer solution," a pH standard solution has the property (buffer action) of protecting against the addition of an acid or base and minimizing pH changes. The types of pH standard solutions and pH values at each temperature are specified.

Electrodes cannot be produced with exactly identical characteristics.

Zero pH and sensitivity will vary with time and different manufacturers

produce electrodes with different nominal values. The calibration

matches the pH meter to the current characteristics of the electrodes.

The calibration process is generally performed by measuring in two different

buffer solutions. This enables both pH° (zero pH) and the slope

(sensitivity) to be determined.

0 5% Sulphuric acid, H2SO4, battery acid.

1 0.1 N HCl, hydrochloric acid (1.1)

2 Lemon juice. Vinegar (2.4-3.4)

3 wine (3.5-3.7)

4 Orange juice. Apple juice (3.8). Beer. Tomatoes.

5 Cottage cheese. Black coffee. Rain water 5.6.

6 Milk. Fish (6.7-7). chicken (6.4-6.6). 

7Neutral: equal numbers of hydrogen and hydroxyl ions. Blood (7.1-7.4). Distilled water without CO2, after boiling.

8 Sea water (8.1). Egg white.

9 Borax. baking soda.

10 Milk of magnesia

11 Household ammonia

12 Photographic developer

13 Oven cleaner

14 Sodium lye NaOH, 1 mol/litre.

Damage in probe Sensor cable faulty Sensor weak Deposition on sensor KCL level down KCL Characteristics not good Power failure or Less power Termination looseness Moisture in Transmitter's housing Turbulences in Process Flow