How to determine free mineral acidity using titrimetric method

Acidity (Free Mineral) Titrimetric Method (0-200 ppm)

Acidity (Free Mineral) Free mineral acidity (FMA) is present in a sample whenever its pH is less than 4.3. Although it is unusual to encounter a raw water containing free acidity when acidity is present, it is very significant since acids contribute to corrosiveness and influence certain chemical and biological processes. Free acidity is due to the presence of acids such as sulfuric, nitric and hydrochloric and does not include carbonic acid formed by the combination of carbon dioxide and water. The effluent from a hydrogen zeolite exchange unit will also contain free acid in proportion to the cations present in the raw water. Determination of the free acidity content of the hydrogen zeolite effluent controls the operating run of these units as well as indicates when regeneration is required.

Theory of Test-

To determine free mineral acidity, the sample is titrated with a standard solution of sodium carbonate to a methyl orange end point (approximately pH 4.3). [Total acidity may be determined by titrant with the same titration to a phenolphthalein end point. (pH 8.3)]. 

Apparatus Required

Buret, 25 ml - 1

Casserole, 210 ml - 1

Cylinder, graduated, 50 ml - 1

Stirring glass rod - 1

Chemicals Required

Methyl Orange Indicator

Sodium Carbonate, 0.02N

Procedure For Test

Measure a clear 50-ml sample of water in the graduated cylinder and transfer to the casserole. Add 4 drops of methyl orange indicator. Titrate from the buret with standard 0.02N sodium carbonate until, the color changes from pink to yellow. Record the volume (ml) of sodium carbonate required.

Calculation of Results

FORMULA :

ppm FMA as CaCO3 =Ml 0.02N sodium carbonate x 1000/ ml sample

the free mineral acidity in ppm CaCO3 is equal to ml of 0.02N sodium carbonate multiplied by 20.

Limitations of Test

Colored or turbid samples may obscure color change at the end point. A pH meter may be used as an indicator in these situations with titration to pH 4.3. The pH meter should be calibrated prior to use by an appropriate standard buffer. Free available chlorine in the sample may bleach the indicator. This source of interference may be eliminated by treating the sample with one drop of 0.1N sodium thiosulfate.