ENV 463:Determination of CaCO3 in water samples

"We will go through some basic concepts in the Lab , in the meantime please read through this weeks lab".. O.G.S.O

Introduction

Total hardness is defined as the sum total of the calcium and magnesium concentrations, both expressed as calcium carbonate in milligrams per litre. Originally, water hardness was understood to be a measure of the capacity of water to precipitate soap. Soap is precipitated chiefly by the calcium and magnesium ions present. Other polyvalent cations also may precipitate soap, but they often are in complex forms, frequently with organic constituents, and their role in water hardness may be minimal and difficult to define. When hardness numerically is greater than the sum of carbonate and bicarbonate alkalinity, that amount of hardness is equivalent to the total alkalinity called carbonate hardness. The amount of hardness in excess of this is called noncarbonate hardness. Hardness in water may range from zero to hundreds of milligrams per litre, depending on the source and treatment to which the water has been subjected.

Two methods could be used to determine hardness in water; hardness by calculation and the ethyenediaminotetraacetic acid disodium salt (EDTA) titration technique. We will be using the EDTA titrimetric method. This method is applicable to drinking, surface and saline waters, domestic and industrial waste waters. It is also suitable for all concentration ranges of hardness; however, in order to avoid large titration volumes, use a sample aliquot containing not more than 25 mg CaCO3. in this method, heavy metals are the main interference, causing fading of indistinct endpoints or stoichiometric consumption of EDTA. Suspended or colloidal organic matter may also interfere with the endpoint (in your report, explain how this inmterference could be overcome). In samples with low temperatures, colour changes become very slow while in hot samples; indicator decomposition could be a problem. A limit of 5 minutes set foe the duration of titration to minimize the tendency toward CaCO3 precipitation is advised. Titration should also be conducted at or near normal room temperature.

Apparatus and materials

Glassware – 25ml or 50 ml burettes
Volumetric pipettes
Graduated cylinders 100 ml Erlenmeyer flasks 250 ml
Volumetric flasks

Reagents

1. Buffer solutions; disodium salt of EDTA, magnesium sulphate heptahydrate, ammonium chloride, and ammonium hydroxide. (Na2EDTA + MgSO4.7H2O + NH4Cl + NH4OH)
2. Erichrome black T indicator
3. EDTA Titrant 0.02 N
4. Calcium carbonate standard solution 1 ml = 1 mg CaCO3
5. Hydrochloric acid solution 1 + 1
6. Methyl red indicator
7. Ammonium hydroxide (NH4OH ) 3 N

Procedure

Standardization of EDTA titrant with CaCO3 standard solution

1. Pipette 10 ml CaCO3 standard solution into 100 ml Erlenmeyer flask
2. add 50 ml of distilled water
3. Using a pipette, add 1 ml buffer solution and a small scoop of Eriochrome black T indicator. Mix well. Solution should be wine red.
4. Rinse the burette three times with the EDTA titrant
5. Fill burette with the EDTA titrant
6. Remove any air bubbles from the burette and bring level of titrant to 0.00 ml
7. Titrate the content of the Erlenmeyer flask with EDTA solution until the red tint disappears. The colour will turn purple. A few drops of EDTA at 3 – 5 seconds intervals will turn the mixture into a blue colour. This is the endpoint. Record the volume of EDTA used.
8. Perform this titrant check two more times.
9. Calculate the normality of the EDTA as follows

M1 V1 = M2 V2
Where M1 = Concentration of CaCO3
M2 = Concentration of EDTA
V1 = volume of CaCO3 used
V2 = volume of EDTA used
Titrimetric procedure

1. Measure 25 ml sample into a 250 ml Erlenmeyer flask and add about 50 ml of distilled water
2. Measure about 60 mL of distilled water into another 250 mL Erlenmeyer flask (Blank)
3. Using a pipette, add 1 ml buffer solution and a small scoop of Eriochrome black T indicator. Mix well. Solution becomes red.
4. Remove any air bubbles from the burette and check 0.00 level.
5. Titrate sample with the standardized EDTA until the red tint disappears. Continue titration slowly until the solution turns blue. This is the endpoint. Record the volume of EDTA used.

6. Calculation

Mg/L Hardness as CaCO3 = (V –B) x N x 50 x 1000/SV

Where V = volume of titrant used for sample (mL)
B = volume of titrant used for blank (mL)
N = the determined normality of EDTA titrant
50 = equivalent weight of CaCO3
SV = sample volume used use appropriate dilution factor if necessary