Chemical parameters- Alkalinity

ALKALINITY

Principle

  • The amount of acid required to titrate the bases of the given water sample is a measure of its alkalinity. Bicarbonates, carbonates and hydroxides are considered to be the chief bases in natural water. Water samples containing bases turn yellow by the addition of methyl orange indicator. The water samples containing measurable carbonate ions turn pink by the addition of phenolphthalein indicator (pH about 8.4) and the measurement of alkalinity is done by titrating the samples in two stages. In the first stage, when the water is titrated with phenolphthalein indicator, its carbonate content is converted into bicarbonates.
  • In the second stage of titration, when the methyl orange indicator is added to the above sample and the titration is continued to the methyl orange end point and during this titration, all the bicarbonates (HCO3) are converted into carbon dioxide and water.

HCO3- + H → H2O + CO2

Reagents

  • Phenolphthalein indicator solution: 0.50 g of phenolphthalein is dissolved in 50 ml of 95% ethyl alcohol and to which 40 ml of distilled water is added.
  • Methyl orange indicator solution: 0.05 g of methyl orange is dissolved in 100 ml of distilled water.
  • 0.02 N standard sodium carbonate: 0.06 g of anhydrous sodium carbonate is dissolved and the solution is made up to 1 litre using CO2 free distilled water which is prepared by boiling distilled water for 10-15 min, before cooling. The Na2CO3 solution should be used within a few hours of preparation.
  • 0.1 N and 0.02 N standard sulphuric acid titrant: 2.8 ml of conc. Sulphuric acid is diluted to 1 litre with CO2 free distilled water, in the preparation 0.02 N sulphuric acid, 200 ml of 0.1 N H2SO4 solution, 10 ml of 0.02 N. Na2CO3 is pipetted into a 250 ml beaker and to which 90 ml of CO2 free distilled water and 4 - 8 drops of methyl orange indicator solution are added. A number of drops of indicator which allows easy end point detection are selected and this number of drops are used in all subsequent titrations. When it is titrated with standard sulphuric acid, one drop of acid will change the colour of methyl orange from yellow to faint orange, which is the end point. The normality of the sulphuric acid solution is calculated by the following equation,

NV = nv

  • Where N = Normality of the standard solution

V = Volume (ml) of the standard solution used in the titration

n = Normality of the solution being standardized

v = Volume (ml) of the solution being standardized

Standard sodium hydroxide titrant (0.02 N)

  • For the preparation of a 1 N sodium hydroxide, 40 g of NaOH is dissolved in 1 litre of distilled water. To get 0.02 N NaOH, 20 ml of the solution is diluted to 1 litre with freshly boiled and cooled distilled water. This solution should be standardized every day before use. To standardize the 0.02 N NaOH , 20 ml of the standard H2SO4 solution is pipetted into a beaker and to which 80 ml of distilled water is added. Drops of solution is added until a faint pink end point appears. The normality of the base is calculated from the relationship NV = N’V’

Procedure - Phenolphthalein alkalinity

  • 1000 ml of water sample is taken in a 250ml beaker and to it. 2 drops of phenolphthalein indicator solution are added. If the solution turns pink, it is titrated with standard sulphuric acid solution until the pink, colour of the sample disappears.
  • For the determination of total alkalinity, 4 to 8 drops of methyl orange indicator solution is added to a 100 ml sample which is titrated with standard sulphuric acid until the colour of the solution changes from yellow to faint orange.
  • The sample used for the determination of phenolphthalein alkalinity may also be titrated with the methyl orange indicator until the end point (faint orange) appears. The titre values of phenolphthalein alkalinity should be added to titre values obtained in the methyl orange end point for the determination of total alkalinity.

Total alkalinity (mg/l) = ml of titrant x N x 50 x 1000  / Sample volume (ml) 


Last modified: Saturday, 17 September 2011, 7:39 AM