Determination of calcium and magnesium in water

Determination of calcium and magnesium in water

AIM:

To determine the amount of calcium present in the whole of the given sample.

PRINCIPLE:

When calconcarboxylic acid (patton and reeder's reagent) is added to the water,it only combines with calcium ions at the PH 12 and forms a wine red colour.when this solution is titrate against EDTA solution,the calcium ions are detach from calconcarboxylic acid and attach to the EDTA molecules and forms a complex called complexometric titration.the solution remains as lavender blue colour.

STEP-1: Ca(sample) + Patton and reeder's(lavender blue colour) -------> [Ca-Patton and reeder's] wine red.

STEP-2:  [Ca-Patton and reeder's] wine red + EDTA(colourless) ------->[Ca-EDTA]lavender blue colour. 

APPARATUS REQUIRED:

1)Conicle flasks.

2)Pipette.

3)Burette.

4)Standard volumetric flasks.

5)Glass rod.

6)Beakers.

CHEMICALS REQUIRED:

1)Reagent grade water (Distilled water or millipore water).

2)Disodium salt of EDTA.

3)Patton and Reeder's reagent.

4)Sodium sulphate or potassium sulphate

5)Sodium hydroxide

6)Calcium carbonate.

7)Hydrochloric acid.

REAGENTS AND PREPARATIONS:

1)Standard 0.01M EDTA solution:

         

The EDTA is not directly soluble in water so we use  Disodium salt of EDTA.

Dissolve 3.723g of Disodium salt of EDTA in reagent grade water containing beaker and make up to 1000ml mark in volumetric standard flask.shake well and lable it.

2)1N Sodium hydroxide:

Dissolve 40g of sodium hydroxide in 1000ml distilled water.

3)Patton and reeder's indicator:

It is a crystalline indicator.mix 1g of Patton and reeder's reagent with 100g of sodium sulphate or potassium sulphate and crush well.

4)Standard Calcium carbonate solution for standarization:

Dry little amount of Calcium carbonate in oven at 180 degrees for an hour.weight 0.1g and dissolve it with small amount of distilled water.add a drop of 1:1 HCL.then make up  to 100 ml using reagent grade water.

PROCEDURE:

 

Wash Conicle Flasks thoroughly with distilled water. Pipette out and add the 2 ml of 1N NaOH solution to buffer the sample PH to 12. add a pinch of patton and reeder's indicator.it become blue colour.then add the sample aliquot.now the solution become wine red if calcium ions are present or the Colour wine red that is directly proportional to the concentration of calcium ions.immediately start titration against 0.01M disodium salt of EDTA solution taken in the Burette.add EDTA slowly and shake constantly.the end point is lavender blue colour or no further Colour change occure.note the volume of EDTA consumed.

STANDARDIZATION OF 0.01 EDTA:

We preparing  the EDTA solution that is not the correct molarity for sometimes.molarity may differ by various factors like temperature,time,etc.to know the exact molarity of that EDTA solution.we have to do standardization.the procedure of standarization is same as told earlier.but we using here standard calcium carbonate solution instead of sample.take two clean conicle flasks,add 2 ml of 1N NaOH solution and a pinch of patton and reeder's indicator.add 10 ml of standard calcium carbonate solution and titrate against 0.01M EDTA solution and note the volume.

Calculation of standardization:

                V1×M1/V2 = M2

V1 = Volume of calcium carbonate solution (10ml)

V2 =  Volume of 0.01M EDTA consumed.

M1 = Standard Molarity of EDTA(0.01)

M2 = The exact Molarity of present time.

Example:

 

Burette reading of 0.01M EDTA is 9.7 ml for 10ml calcium carbonate solution taken.so the Molarity will be

           10.0 × 0.01 / 9.7 = 0.0103M

SAMPLE CALCULATION:

Ca = (V2-V1) × M × 40.08 × 1000 / Sv 

V1 = Initial reading of burette.

V2 = Final reading of burette.

M = Molarity of EDTA solution from standardization. 

40.08 = Atomic weight of calcium.

1000 = For milligram conversion

Sv = Sample volume.

Model calculation:

Ca = (4.4-0) × 0.0103 × 40.08 × 1000 / 10 = 181.6mg/l.

Determination  of magnesium in water by calculation method.

The two ions calcium and magnesium are major ions that present in water.so magnesium can be calculated by subtracting calcium from total harness.

there are two types of calculations for calculating magnesium from calcium and total hardness.

Type 1:

Substract the burette reading of calcium from the burette reading of total hardness of the same sample.then the value insert in the same model calculation of total hardness or calcium as given below.

CALCULATION:

Mg = (TH burette reading - Ca burette reading) × Molarity of EDTA × 24.32 × 1000 / Sv 

Where,

Mg - Magnesium.

24.32 - Atomic weight of magnesium.

1000 - milligram per litre.

Sv -sample volume same as TH and Ca.

EXAMPLE:

Burette reading of total harness is 6.4ml and burette reading of calcium is 3.0ml for 10ml sample.

So 6.4 - 3.0 = 3.4ml of magnesium.

Mg = 3.4 ×  0.01  × 24.32 × 1000 / 10 = 82.68mg/l.

Type 2:

Substract the mg/l of calcium from mg/l of total hardness.

Mg = (mg/l of TH - mg/l of ca expressed in caco3) × 24.32 × 0.01

Mg/l of TH - the value got from total harness calculation.

Mg/l of ca expressed in caco3 - value got from calcium in which the calcium atomic weight replaced by caco3 molecular weight called calcium hardness. 

24.32 - Atomic weight of magnesium

0.01 - Molarity of EDTA

EXAMPLE:

Mg = (20-10) × 24.32 × 0.01 = 2.43mg/l.

Conditions for this two types of calculations:

*Sample volume of total hardness and calcium must be same.

*the Molarity of EDTA must be same.