The pH of an aqueous (water-based) solution is the measure of how acid or basic the solution is. Solutions with a pH less than 7 are said to be acid and solutions with a pH greater than 7 are basic or alkaline. Pure water has a pH very close to 7.
pH levels are very important in industries like the Food & Beverage industry. Juice and lemonade for instance are very acid and aggressive to metal surfaces and speed up corrosion of mild steel. But also cleaning solutions often have an extremely high level of acidity or basicity.
Calculating pH and pOH
It is the presence of hydronium ions relative to the hydroxide ions that determines a solution’s acidity or basicity. In pure water, there is an equal number of hydroxide and hydronium ions. Making pure water a neutral solution. The acidity or basicity of an aqueous solution can be determined by either measuring the pH or the pOH.
pH is calculated when an acid is dissolved in water. pOH is calculated when a base is dissolved in water. In both cases, the value is determined by the decimal logarithm of the reciprocal of the hydrogen ion (pH) or the hydroxide ion (pOH) activity in a solution. The hydrogen or hydroxide ion activity can easily be determined by first calculating the Molarity or the molar concentration in the solution. Molarity is the number of moles of solute in exactly one liter of a solution.
Because – at 25 degrees Celcius – the sum of pH and pOH is always 14, you only need to measure one to also know the other.
Example – Hydrochloric Acid
Using a 0,01% hydrochloric acid (HCl) solution to clean some components.
A 0,01% HCl solution is 0.1 g HCl in 1000 g water. Because HCl has about 36.5 g/mol, this gives a Molarity of 0.1 g/l / 36.5 g/mol = 0.0027 mol/l.
Because Hydrocholoric Acid is an acid, we calculate the pH:
pH = -log[0.0027] = 2.6
and therefore pOH = 14 – 2.6 = 11.4
Example – Caustic Soda
Using a 0.3% caustic soda (NaOH) solution to clean production equipment.
A 0.3% NaOH solution corresponds to 3 g NaOH in 1000 g water. Because NaOH has about 40 g/mol, the Molarity is 3 g/l / 40 g/mol = 0.075 mol/l.
Because caustic soda is a base, we calculate the pOH:
pOH = -log[0.075] = 1.12
and therefore pH = 14 – 1.12 = 12.9
Keep in mind these calculations are only valid under the following conditions:
- A strong acid or base is being used for the solution which is dissociating completely (like hydrochloric acid or caustic soda)
- There is no other additive in the solution
- The temperature is 25°C
Using an acid or basic cleaning solution or detergent can have adverse effects on (metal) materials used in an installation. It is therefore important to know the acidity or basicity of these solutions and to know whether the materials used in the installation are able to withstand them.
Article by Frank Sebold – Product Manager at Danfoss Power Electronics