pH - Eutech Instruments - #2

About pH/ORP Measurement

In pure water, the H₃O+ ion concentration is 1 x 10^-7 Moles/litre. Using the formula definition above, pH value of pure water is derived as follows:

There is an equal concentration of H₃O+ ions and OH^- ions in pure water at 25 °C. The equilibrium constant, Kw of water, is the product of the H₃O+ ion and OH^- ion concentrations:

Hence,

The Kw of water must remain constant. Therefore, an increase in either the H₃O⁺ ions or OH^- ions will result in a decrease of the other ions, resulting in acidity (increased H₃O+) or alkalinity (increased OH^-).

pH is always measured across a medium.

Although the litmus paper is one of the most common methods of pH measurement, it can only provide a rough indication which might be insufficient in most applications.

The more accurate method involves the use of a measurement system that consists of a pH meter and a pH electrode that has a hydrogen ion sensitive glass bulb. The most common sensing element used in the electrode is the glass membrane as it is selective for H⁺ ions i.e. H⁺ ions can permeate through the hydrated layer of glass membrane. However the electrode body may not necessarily be glass.

The movement of ions into the hydrated membrane changes the electrochemical effect inside the glass which is measured in mV and then converted via the pH meter to be reflected as a pH value.

Therefore depending on the concentration of ions in the solution, the mV and hence pH varies.

The performance of an electrode is dependent on two parameters - Offset and Slope.

Theoretically, when placed in pH 7.00 buffer at 25 °C, a pH electrode produces 0 mV which the pH meter reads as 7.00 pH. The difference between 0 mV and the electrode's actual reading is called the offset error which can be as high as ±25 mV.

In other words, when the electrode is not in measurement or in pH 7 buffer solution, the output (or reading) will be known as the offset.

While in theory, the mV value should be zero, however in practice this is rarely the case because of the following reasons:

• Liquid difference

• Bulb composition

• Wire geometry difference and other factors

About pH Measurement

pH is one of the most common parameters measured in a wide variety of industries such as water and wastewater treatment, agriculture research and production, environmental monitoring, chemical and life sciences research, electronics production as well as other industrial applications.

Here are examples of pH in a few common industrial and household products:

Household dye [13.6] Copper plating [12.8]

Ammonia [11.4]

Milk of magnesia [10.3] Borax [9.3]

[13.1] Bottle washing [12.6] Bleach

[11.3] Brass plating

[9.4] Lime-soda softening

[8.4] Baking soda [8.0] Seawater

[7.4] Swimming pool water [7.0] Distilled water

[6.3] Brewing process [5.8] Nickel plating

[5.0] Food processing [4.3] Orange juice

[3.2] Photo engraving [2.3] Lemon juice

Saltwater aquarium [8.0] Blood [7.5] Fresh water aquarium [7.1] Milk [6.7]

Corn [6.2]

Boric acid [5.0]

Pickle processing [3.5] Vinegar [2.9]

Battery acid [0.3]

pH is the measure of how acidic or alkaline a substance is on a scale of 0 to 14. pH values below 7 represent the acidic half of the scale; pH values above 7 represent the alkaline or basic half of the scale. The pH value 7 is neutral, i.e. neither acidic nor alkaline.

pH can be defined by the following equation:

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