Determination of alkalinity

ALKALINITY

AIM:- To determine the alkalinity of the given sample water.

APPARATUS:- PH meter and Glassware.

THEORY:- The alkalinity of a water sample is its capacity to neutralize acids. Alkalinity is mainly due to presence of carbonate, bicarbonate, hydroxides and less frequently, borate’s, silicates and phosphates. It is expressed in mg/l of calcium carbonates. Two types of alkalinities are generally calculated, namely, phenolphthalein alkalinity and methyorange alkalinity. Based on these two different forms of alkalinities can be estimated. Alkalinity measurements are made by titrating the sample water with an acid and determining the hydrogen equivalent. Hydrogen ions from the acid reacts with the alkalinity according to the following equations.

                        H⁺ +  OH^- ó H2O

                        Co3^- + H⁺ ó H Co3^-

                        HCo3^- + H⁺ ó H2Co3^-

            If acid is added slowly to the water and the PH it recorded for each addition, a titration curve is obtained as shown below.

Graph

 

The significant aspects of the curve are the inflection points that occur at approximately PH8.3 and PH4.5. At PH8.3, all hydroxides and half of the carbonates are neutralized. At PH4.5, remaining half of the carbonates and bicarbonates are neutralized. Thus the amount of acid required to titrate a sample to PH4.5 is equivalent to the total alkalinity of the sample water.

RELEVANCE:- Alkalinity plays a very important role in chemical coagulation and in biological waste treatment processes, especially in aerobic digestion as this provides buffering systems. Excess alkalinity imparts a bitter taste to waste. Alkalinity is an important parameter in evaluating the optimum coagulant dosage. The permissible limit of carbonate alkalinity in domestic water supplies is 120ppm.

CHEMICAL REAGENTS:-

i)                    Standard sulphuric acid (0.02N)

ii)                  Phenolphthale in indicator

iii)                Methyl orange indicator

iv)                Sodium carbonate solution.

PROCEDURE 1:-

1. Standardization of H₂So₄ (titrant) by potentremetric titration:- Take 40ml Naco₃ solution and add 100ml distilled water. Titrate with H₂So₄ till PH of 1-2 is reached. Add acid in amounts of 0.5ml and note down the corresponding PH. Plot a graph between volume of H₂So₄ and the corresponding PH. From the graph, determine the volume off H₂So₄ at a PH of 4.5. calculate the normality of the acid using the formula.

                 

                                       A*B___

N =       53*C

Where

A = Volume of Na₂co₃ (40ml)

B = Grams of Na₂co₃ in 1 lit. = 2.5 grams

C = Volume of H₂So₄ for a PH of 4.5

Adjust the normality of H₂So₄ to 0.02N using the relation

                        N₁V₁ = N₂V₂

2. Determination of ‘P’ and ‘T’ alkalinity:- Take 25ml sample water and make it to 50ml using distilled water. Add 2 drops of phenolphthalein indicator and titrate with 0.02N H₂S0₄ till pink colour disappear. Note down the volume of H₂So₄ as V₁. To the same sample add 2 drops of methylorance indicator and titrate with H₂So₄ till colour changes to pinkish yellow. Note down the total volume of acid consumed as V.

OBSERVATIONS:-

S.No	Burette Readings		Vol. of Titrant used
	Initial	Final

Volume of acid	PH

PROCEDURE 2 :-

.Take 50 ml of sample water in a conical flask and add 3 to 4 drops of phenolphthalein indicator, color changes to pink.

.Titrate against 0.02 N HCL till colorless as end point.

. Note down the volume consumed from burette to measure phenolphthalein alkalinity. .To the same sample add 3-4 drops of methyl orange indicator and color changes to   orange.

. Titrate it against 0.02 N HCL till solution changes to pink as end point.

. Note down the volume consumed from burette to measure methyl orange alkalinity.

. The sum of phenolphthalein alkalinity and methyl orange alkalinity gives total alkalinity.

Calculations:- Total alkalinity =        V1 x 1000         +               V2  x 1000 

                                                       ml of water sample            ml of water sample

 where V1 and V2 are volume consumed from burette for phenolphthalein alkalinity and  methyl orange alkalinity.

CALCULATIONS for First Procedure:-

Phenolpthalein Alkalinity “P”  =     V1*N*50000

as mg/l caco₃                                  ml of sample

Total alkalinity “T” as mg/lit as caco₃ = V*N*50000

                                         ml of sample

Calculate the alkalinity due to various constituents using the table given below.

	Hydroxide Alkalinity	Carbonate Alkalinity	Bicarbonate Alkalinity
If P=0 If P=T/2 If P<T/2 If P>T/2 If P=T	0 0 0 (2P – T) T	0 2P 2P 2(T – P) 0	Total Alkalinity T T – 2P T – 2P 0 0

RESULT:-

GRAPH:- Plot a graph taking PH value on Y-axis and corresponding volume of acid on X-axis. From the graph obtain the volume of acid corresponding to a PH of 4.5. Also mark the inflection points (i.e. point on the graph corresponding to PH 8.3 and 4.5)

INTERPRETATION OF RESULTS:-