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  Determination of hardness of water, Chemistry tutorial

Introduction:

Hard water is mainly due to metal ions (or minerals) which are dissolved in the ground water. Such minerals comprise Ca2+, Mg2+, Fe3+, SO42- and HCO3-. Whenever this water evaporates or boils, the complex to dissolve metal salts remain as a scaly residue. Hard water reduces the efficiency of soap and detergents. Calcium ions generally make the most important contribution to water hardness. This is the reason why hardness is computed in terms of mg CaCO3 /L of solution. Hardness is as well reported in units of parts per million (ppm). Water having a hardness value of < 60 ppm is considered 'soft' and water having >200 ppm is considered 'hard'. The disodium salt of EDTA was employed to find out the concentration of M2+ metal ion impurities in hard via by a Complexometric titration. EDTA is a versatile chelating agent. A chelating agent is a substance whose molecules can form quite a few bonds to a single metal ion. Chelating agents are multi-dentate ligands. EDTA acts as the chelating agent as each nitrogen and one oxygen from each of the four carboxylic acid groups has an electron pair to, donate (ligand- a substance which binds by a metal ion to form a complex ion) to a metal ion center, making EDTA a hexadentate  ligand that forms an octahedral complex. The indicator, Eriochrome Black T lets the detection of whenever the EDTA has totally chelated the metal impurities. In the presence of a metal cation, Erichrome Black T makes a pink complex.

Principle:

It is a Complexometric titration of calcium and magnesium by an aqueous solution of disodium salt of EDTA at pH value of 10 and Mordant black 11 as an indicator, which forms claret or a violet color in the presence of magnesium and calcium ions.

The sodium salt of EDTA dissociates to ions in the aqueous solution.

Na2Y2H2.2H2O(s) → 2Na+ (aq) + H2Y2- (aq) + 2H2O (l)

Throughout the titration EDTA reacts first by free calcium and magnesium ions in the solution

Me2+ (aq) + H2 Y2- (aq) ↔ MeY2- (aq) + 2 H+ (aq)

Me2+ (Ca2+, Mg2+) 

and at the end/equivalent point EDTA reacts by the calcium and magnesium ions superseded from their complexes, which were made by the indicator molecules, discharging indicator and causing the color to change from claret or violet to blue.

At the equivalence point, the amount is:

n(Ca2+ + Mg2+) = n(EDTA)

The outcomes are given in the amount of substance concentration (that is, molarity).

Apparatus:

Ordinary laboratory apparatus and Instruments:

  • Weighing vessel
  • Calibrated analytical balance 
  • Electric oven for drying
  • Burette, 25 ml capacity, graduated in divisions of 0.05 mL, class A or equivalent
  • 2 x Analytical flasks 1000 ml
  • Polyethylene bottle 1 L
  • 4 x Erlenmeyer flasks 500 ml, 250 ml

Reagents and materials:

  • Ammonia buffer solution 
  • EDTA standard solution 

Materials:

  • Ammonium chloride 
  • Ammonia solution w = 25%, ρ (20oC) = 0.91 g/mL
  • Disodium magnesium salt of EDTA (C10H12N2O8 Na2Mg) 
  • Disodium salt of EDTA dehydrate (C10H14N2O8Na2.H2O)
  • Triethanolamine (OHCH2CH2)3N)
  • Mordant black 11 (C20H12N3O7SNa)
  • Hydrochloric acid 4 mol/L
  • Eriochrome black T
  • Methyl red indicator 
  • Ethanol 

Reagent solutions:

  • Ammonia buffer solution.
  • Dissolve around 67.5 g of ammonium chloride in 570 mL of ammonia solution. Then add 5.0 g sodium magnesium salt of EDTA and dilute to 1000 mL by distilled water.
  • EDTA Standard volumetric solution c = 10 mmol/L.
  • Dry a part of EDTA dihydrate salt for 2 hour at 105oC, dissolve 3.725 g of the dry salt in water and dilute to 1000 mL in the volumetric flask.
  • Store the EDTA solution in a polyethylene bottle and check the concentration at normal intervals.

Standardization:

  • Standardize the EDTA solution against the calcium reference standard solution via the method illustrated in clause below.
  • Make use of 20.0 mL of the calcium standard reference solution and dilute to 50 ml.
  • The concentration of EDTA solution c(EDTA) deduced in millimoles per litre is represented by relationship:

C (EDTA). V (EDTA) = c (Ca2+). V (Ca2+)

Here,

c(Ca2+) = The concentration deduced in millimoles per litre of the standard reference solution.

V(Ca2+) = The volume in milliliters of calcium standard reference solution

V(EDTA) = Volume in milliliters of EDTA solution utilized for standardization

c(EDTA) = [c(Ca2+), V(Ca2+)]/V(EDTA)

Calcium standard reference solution c(CaCO3) = 10 mmol/L

Dry a sample of pure calcium carbonate for around 2 hours at 150oC, and allow cooling to room temperature in the desicator. Put 1.001 g of it in a 500 mL conical flask and moisten it by distilled water. Add 4 mol/L hydrochloric acid in drops till all the carbonate is dissolved. Avoid adding surplus acid. Add 200 mL of water and boil for some minutes to strip out carbon-dioxide. Cool to room temperature and add some drops of methyl red indicator solution. Add 3 mol/L ammonia solutions till the solution turns orange.

Transfer the solution quantitatively to the 1000 mL volumetric flask and make up to the mark by distilled water.

1 mL of the solution includes 0.4008 mg (0.01 mmol) of calcium.

Note: Commercially available solution might be utilized.

Mordant black 11, indicator:

Dissolve around 0.5 g of mordant black 11, the sodium salt of (11-hydroxi-2naphtylazo-6-nitro-2-naphtol-4-sulfonic acid) in 100 mL Triethanolamine. Up to 25 mL ethanol might be added rather than Triethanolamine to decrease the viscosity of the solution.

NOTE: To facilitate the detection of end point, the indicator might be modified via the solution of 0.17 g methanol salt (4-anilido azobenzene sulphonic acid sodium salt). The color will now transform from red to pale grey or green.

Procedure:

Bu using a pipette transfer 50.0 mL of the test solution to a 250 mL conical flask. Add 4 mL of buffer solution and 3 drops of Mordant black 11 indicator. The color of solution must now turn to claret or violet and the pH value must be 10.0 ± 0.1.

Titrate without delay. Add the EDTA solution whenever the color of the solution begins to change from claret or violet to blue. The end point is reached whenever the last shade has disappeared.

Titrate additional test part in the given manner.

1) Transfer 50.0 ml of the test solution to the flask. Add EDTA solution to the sample amounted 0.5 ml less, than it was consumed for the first sample. Add 4 ml of the buffer solution and 3 drops of mordant black 11 indicators. Add EDTA till the end point is reached.

2) Whenever the consumption of EDTA is less than 4.5 mL, titrate a smaller sample volume. Add distillated water till reaching a net titrated volume of 50 ml. 

3) Whenever the consumption of EDTA solution exceeds 20 ml, titrate by employing a small test portion. Add distillated water in such a way that the starting volume is 50 ml.

Expression of the results:

The sum of calcium and magnesium contents c (Ca2++ Mg2+) deduced in millimoles per litre is represented by the relationships

V(sample).c (Ca2+ + Mg2+) = V(EDTA).c(EDTA)

c(Ca2+ + Mg2+) = V(EDTA).c(EDTA)/V(sample)

V(EDTA) = The volume in milliliters of EDTA utilized in the titration 

c(EDTA) = The concentration deduced in millimoles per litre of EDTA solution.

V(sample) is the volume in milliliters of the test solution (generally 50 mL)

Precision:

The repeatability of the process is ± 0.04 mmol/L, corresponding to around 2 drops of the EDTA solution.

Result:

Average

c(Ca2+ + Mg2+)

(mmol/L)

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