Chemistry 1.6: Total Hardness Procedure

Total Hardness

• Take 25 or 50 ml of a well-mixed sample in a conical flask
• Add 1-2 ml of buffer solution and 1 ml of inhibitor
• Add a pinch of Eriochrome black-T and titrate with standard EDTA (0.01M) until the wine red color changes to blue
• Note the volume of EDTA required (A)
• Run a reagent blank and note the volume of EDTA (B)
• Calculate the volume of EDTA required by the sample, C = A – B

Calcium Hardness

• Take 25 or 50 ml of sample in a conical flask
• Add 1 ml of NaOH to raise the pH to 12.0 and add a pinch of muroxide indicator
• Titrate immediately with EDTA until the pink color changes to purple
• Note the volume of EDTA used (A1)
• Run a reagent blank and note the volume of EDTA (B1)
• Calculate the volume of EDTA required by the sample, C1 = A1 – B1

Problems Based on Calorific Value

• Problem 1: If a sample of water contains 50 mgs of Ca2+ ions per litre, calculate its hardness in terms of CaCO3 equivalent
• Solution: Amount equivalent to CaCO3 = 50 × 100/40 = 125 mgs/lit
• Problem 2: A water sample contains 204 mgs of CaSO4 and 73 mgs of Mg(HCO3)2 per litre. What is the total hardness in terms of CaCO3 equivalent?

EDTA Titration

• EDTA forms colorless stable complexes with Ca2+ and Mg2+ ions present in water at pH 9-10
• Buffer solution (NH4Cl + NH4OH) is used to maintain the pH of the solution
• Eriochrome Black-T (EBT) is used as an indicator
• EDTA titration with EBT indicator involves a sharp color change from wine red to blue

Effects of Hard Water

• In sugar industry: hardness creates difficulties in sugar refining and crystallization of sugar, and the sugar becomes deliquescent
• In paper industry: calcium and magnesium salts interfere with the smooth and glossy finish of the paper, and iron salts interfere with the coloration
• In laundry: hard water causes wastage of costly soap and staining of iron salts
• In pharmaceutical industry: hard water is not suitable for preparing drug solutions
• In concrete making: hard water affects the hydration of cement and final hardening of cement
• In steam generation in boilers: hard water creates problems like scale and sludge formation, corrosion, priming and foaming, and caustic embrittlement