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Basics Of pH Control

Source: Emerson Process Management, Rosemount

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Application Note: Basics Of pH Control

By definition, pH is the negative logarithm of the hydrogen-ion concentration in aqueous solution. This means that a solution having a pH value of 4 has ten times more hydrogen ions than a solution whose pH is 5. For control systems neutralizing spent acids or bases, pH value per se is of little consequence except as a control point for the neutralization.

Acids are either "strong" or "weak." This relates to the amount of free hydrogen ions in the solution of a given concentration. Thus, nitric acid is a strong acid, since all of the nitric-acid molecules are dissociated into active hydrogen ions and nitrate ions. However, acetic acid is a weak acid. A solution of acetic acid of the same molar concentration as nitric acid would have a very different pH value, since most of the acetic acid molecules do not dissociate into hydrogen ions and acetate ions. Yet both the nitric and acetic acids have the same total acidity and, therefore, each require the same amount of neutralizing base.

Similarly, there are strong and weak bases. Sodium hydroxide is a strong base, while ammonia or soda ash are weak bases. Equal molar concentrations of these bases will all have the same capacity to neutralize a given quantity of acid.

Titration is the popular method for determining total acidity or basicity of a solution. It is necessary in the design of the pH control system to determine the size of the final control elements — particularly the element that determines the flow of reagent — and titration is the technique used to collect data for sizing.

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Application Note: Basics Of pH Control

Emerson Process Management, Rosemount