Determination of hardness of water

Determination of hardness of water

Principal:

The hardness of water is a result of a high concentration of calcium, magnesium, and iron. The most abundant ions in hard water are calcium and magnesium. Calcium carbonate from limestone and magnesium carbonate from dolomite present in the soil, dissolve in water increase the hardness of the water. Calcium and magnesium ions present in water cause soap precipitation and scaling. Hard water can be treated by using an ion exchanger mechanism to convert it into soft water. Total hardness is total calcium and magnesium ionic concentration in the water which may be present as temporary or permanent hardness. Temporary hardness is the concentrations of calcium and magnesium ions that can be removed as insoluble carbonate by simple heating but permanent hardness is removed by using chemical treatment. The collective result of temporary hardness and permanent hardness is total hardness.

Equation:

Ca2+ (aq) + 2HCO3(aq) → CaCO3(s) +H2O (l) + CO2 (g)

Mg2+ (aq) +2HCO3 (aq) → MgCO3 (s) + H2O (l) + CO2 (g)

The concentration of calcium and magnesium ions in water can be determined by complex-metric titration. The water sample is treated with a standard EDTA solution, in the presence of Eriochrome black T indicator.

Apparatus:

  • Conical flask
  • Burette
  • An iron stand
  • Awash bottle
  • Beakers

Reagent:

  • Eriochrome Black T( 0.5%w/v in 95% ethanol v/v)
  • An NH3-NH4Cl pH buffer solution ( prepared by dissolving 3.2g of NH4Cl in the water then add 29mL concentrated NH3 and dilute to about 50 mL)
  • EDTA (C10H16N2O8 M/W 292 27g) ,(0.01M 2.92g/L or 0.70 g /250 mL)

Also, Check CLEANSING OF GLASSWARE(Analytical assignment)

Procedure:

  • Pipette out 10 ml of the water sample into a 250mL Erlenmeyer flask and add 20 ml of distilled water
  • Add a 1mL of pH 10 buffer and at least 5 drops of indicator Eriochrome black T.
  • Now titrate the water sample taken in the conical flask with 0.01M EDTA until the wine red color of solution changes through purple to pure blue
  • At the reach of an endpoint, EDTA should be added dropwise
  • Note the volume and repeat the same procedure three times
  • Prepare the blank solution and also note three readings.

Hardness of water/Types of Hardness of water/unit of hardness of water

Supposed Observations and Calculations:

Sr.no Initial reading(mL) Final reading(mL) Volume used(mL)
1 0.0 9.6 9.6
2 9.6 19.3 9.7
3 19.3 28.8 9.5

 

Mean volume = V1 +V2 +V3/3

= 9.6 +9.7+9.5/3

= 9.6 mL

Volume of EDTA used = 9.6 mL

Molarity of EDTA = 0.01M

Molecular weight of CaCO3 = 100 g/mol

Volume of the sample used = 10 mL

Also, Check  Standardization of Potassium per Magnate

Water hardness is expressed in terms of CaCO3 (ppm) and can be calculated as:

Water hardness =

Water hardness =

= 0.96 ppm

Determination of hardness of water

Result:

The hardness of water is 0.96 ppm

Precautions:

  • The fumes emitting from the NH3/ NH4Cl buffer are irritating, toxic, and corrosive
  • The buffer should be added before the addition of the indicator because the indicator can react with a small amount of iron present in the water which could change the wine red color to violet color near the endpoint. So it makes it difficult to note an endpoint.

Also, Check 

Determination of hardness of water

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