Determination of Alkalinity
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manunandananDetermination of Alkalinity
Uploaded by
manunandananDETERMINATION OF ALKALINITY
Aim: Determination of Alkalinity in given water sample by volumetric method
Apparatus
Burette, Pipette, Conical flask, Dropper, measuring cylinder, Beaker etc.
Chemicals
0.02 N HCl, phenolphthalein indicator & methyl orange indicator.
Theory
Due to the presence of those minerals, which increase the concentration of OH¯ ions in
water, the pH of water increases & it becomes alkaline. Alkalinity is divisible into
bicarbonate alkalinity, carbonate alkalinity & hydroxide alkalinity. In most waters
bicarbonates (HCO3 -) and carbonates (CO3 -2) are the major bases. Alkalinity refers to the
capability of water to neutralize acid. Alkalinity is often related to hardness because the
main source of alkalinity is usually from carbonate rocks (limestone), which are mostly
CaCO3. If CaCO3 actually accounts for most of the alkalinity, hardness in CaCO 3 is equal to
alkalinity. The above alkalinities can be determined volumetrically by titrating water sample
against standard acid using methyl orange and phenolphthalein indicators.
Procedure:
Part A: Preparation of solutions
1) Standard 0.02 N HCl solution: Dilute 1.72 ml of concentrated AR grade hydrochloric acid
(11.6N) to 1 litre with distilled water in a volumetric flask.
2) Phenolphthalein indicator: Dissolve 500 mg of phenolphthalein in 50 ml ethanol & 50 ml
distilled water (100 ml 50%ethanol).
3) Methyl orange indicator: Dissolve 50 mg of methyl orange in 100 ml distilled water.
Part B: Determination of alkalinity
If the pH of water sample is above 8.00 then follow the procedure given below. Pipette out
25 ml of filtered water sample in a conical flask & add 4 drops of alcoholic phenolphthalein
indicator to it. If the solution turns pink then titrate it with 0.02N HCl until colourless.
Let this burette reading be V1. To the same solution or to the solution, which remains
colourless even after adding phenolphthalein indicator, add 4 drops of methyl orange
indicator. If the solution becomes yellow then continue titration till orange pink colour is
obtained at the end point. Let this burette reading be V2 from the beginning and is called
methyl orange end point. Repeat the titration for two more times & find the constant
burette reading.