UNIT 5 Determination of hardness of water using complexometric titration – INORGANIC CHEMISTRY PRACTICALS

UNIT 5 Determination of hardness of water using complexometric titration

1.0  Introduction

2.0  Objectives

1. Main Content
2. Principle  
3. Apparatus
4. Procedure

4.0  Conclusion

5.0  Summary

6.0  Tutor Marked Assignments

7.0  References/Further Reading

1.0
INTRODUCTION

Hard water is due to metal ions (minerals) that are dissolved in the ground water. These minerals include Ca²⁺, Mg²⁺, Fe³⁺, SO₄^2- and HCO^3- . When this water evaporates or boils, the difficult to dissolve metal salts remain as a scaly residue. Hard water inhibits the effectiveness of soap and detergents. Calcium ions typically make the most significant contribution to water hardness. This is why hardness is measured in terms of mg CaCO₃ /L of solution. Hardness is also reported in units of parts per million (ppm). Water with a hardness value of < 60 ppm is

considered  “soft”  and  water  with  >200  ppm  is  considered  “hard.”  

The disodium salt of EDTA was used to determine the concentration of M²⁺ metal ion impurities in hard water by a complexometric titration. EDTA is a versatile chelating agent. A chelating agent is a substance whose molecules can form several bonds to a single metal ion. Chelating agents are multi-dentate ligands. EDTA acts as a chelating agent because each nitrogen and one oxygen from each of the four carboxylic acid groups has an electron pair to , donate (ligand- a substance that binds with a metal ion to form a complex ion) to a metal ion center, making EDTA a hexadentate ligand which forms an octahedral complex. An indicator, Eriochrome Black T enables the detection of when the EDTA has completely chelated the metal impurities. In the presence of a metal cation, Eriochrome Black T forms a pink complex.

 2.0  OBJECTIVE

At the end of this unit you should be able

• Define hardness of water
• Standardize an EDTA solution
• Describe the function of EDTA in the experiment
• to perform an experiment to determine hardness of water using complexometric titration

1. MAIN CONTENT
2. Principle This is a complexometric titration of calcium and magnesium with 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 calcium and magnesium ions.

The sodium salt of EDTA dissociates into ions in aqueous solution

Na₂Y ₂H ₂.2H ₂O(s) → 2Na⁺ (aq) + H₂Y^2- (aq) + 2H₂O(l)

During the titration EDTA reacts first with free calcium and magnesium ions in the solution

Me²⁺ (aq) + H₂ Y^2- (aq) ↔ MeY^2- (aq) + 2 H⁺ (aq)

Me2+ (Ca 2+ , Mg2+)

and at the end/equivalent point EDTA reacts with the calcium and magnesium ions superseded from their complexes, which were created with the indicator molecules, liberating 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 results are given in amount of substance concentration (molarity).

3.2
Apparatus
Instruments

Calibrated analytical balance Electric oven for drying

Ordinary laboratory apparatus

Weighing vessel

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 (20 0C) = 0.91 g/mL

Disodium magnesium salt of EDTA (C₁₀H₁₂N₂O₈ Na₂Mg)

Disodium salt of EDTA dehydrate (C₁₀H₁₄N₂O₈Na₂.H₂O)

Mordant black 11 (C₂₀H₁₂N₃O₇SNa)

Eriochrome black T

Hydrochloric acid 4 mol/L

Methyl red indicator

Triethanolamine (OHCH₂CH₂)₃N)

Ethanol

Reagent solutions

Ammonia buffer solution

Dissolve 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 with distilled water

EDTA Standard volumetric solution c = 10 mmol/L.

Dry a portion of EDTA dihydrate salt for 2 h at 105 0C, dissolve 3.725 g of the dry salt in water and dilute to 1000 mL in the volumetric flask.

Store EDTA solution in a polyethylene bottle and check the concentration at regular intervals.

Standardisation

Standardise the EDTA solution against the calcium reference standard solution by the procedure described in clause below

Use 20.0 mL of the calcium standard reference solution and dilute to 50 mL.

The concentration of the EDTA solution c(EDTA) expressed in millimoles

per litre is given by relationship

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

 c(EDTA) =  

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

Dry a sample of pure calcium carbonate for 2 hours at 150 ^oC, and allow cooling to room temperature in a desiccator. Place 1.001 g of it in a 500 mL conical flask and moisten it with distilled water. Add 4 mol/L hydrochloric acid in drops until all carbonate is dissolved. Avoid adding excess acid. Add 200 mL of water and boil for few minutes to strip out carbondioxide. Cool to room temperature and add a few drops of methyl red indicator solution. Add 3 mol/L ammonia solution until the solution turns orange. Transfer the solution quantitatively to a 1000 mL volumetric flask and make up to the mark with distilled water.

1 mL of the solution contains 0.4008 mg (0.01 mmol) of calcium. Note – Commercially available solution may be used.

Mordant black 11, indicator

Dissolve 0.5 g of mordant black 11, the sodium salt of (1-(1-hydroxi-2naphtylazo-6-nitro-2-naphtol-4sulfonic acid) in 100 mL triethanolamine. Up to 25 mL ethanol may be added instead of triethanolamine to reduce the viscosity of the solution.

NOTE– To facilitate the detection of the end point, the indicator may be modified by the solution of 0.17 g methanol salt (4-anilido azobenzene sulfonic acid sodium salt.). The color will now change from red to pale grey or green.

3.3 Procedure

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 the solution should now turn to claret or violet and the pH value should be 10.0 ± 0.1.

Titrate immediately. Add the EDTA solution when the color of the solution starts to change from claret or violet to blue. The end point is reached when the last shade has disappeared.

Titrate additional test portion in the following manner.

Transfer 50.0 mL of the test solution in to a 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 indicator. Add EDTA until the end point is reached.

If the consumption of EDTA is less than 4.5 mL, titrate a smaller sample volume. Add distillated water until reaching a total titrated volume of 50 mL.

If the consumption of EDTA solution exceeds 20 mL, titrate using a small test portion. Add distillated water so that the starting volume is 50 mL.

Expression of the results

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

 V

the volume in millilitres of EDTA used in the titration

is the concentration expressed in millimoles per litre of the EDTA solution. V is the volume in millilitres of the test solution (normally 50 mL) For calculation of hardness see the annex.

Precision

The repeatability of the procedure is ± 0.04 mmol/L, corresponding to approximately 2 drops of the EDTA solution

Result

4.0  CONCLUSION

Hardness of water can be determined using laboratory complexometric titration.

5.0  SUMMARY

In this unit you have learnt the following:

• Ground water in particular and fresh water flowing in rivers and canals or waters in ponds and lakes are having different metal salts in different proportions.
• Sometimes these metal ions can be easily removed by boiling or simple precipitation when the quality of water is termed as temporary hardness.
• Some waters have these metal salts in such form that it cannot be removed by simple means.
• This quality of water is known as permanent hardness. It is easy to determine simple metal ions by using simple titration techniques.
• But to establish the exact amounts of various metal ions in a sample of hard water, a complexometric titration is required and these titrations are conducted by using EDTA.
• You have performed an experiment to determine water hardness using complexometric titration

6.0 TUTOR MARKED ASSIGNMENT

1. When determining the total calcium and magnesium ion content in water using EDTA, which of the following is established? a carbonate hardness c(Ca²⁺) b magnesium hardness c(Mg²⁺) c total hardness c(Ca²⁺ + Mg²⁺)
2. What is the function of EDTA in determination of water hardness ?
3. Mention two indicators used in the complexometric determination of water hardness of water.
4. You are using EDTA with a molarity of .0080 for the titration. You titrate 50.00 mL of water sample using 10.68 mL of EDTA.

   What is the CONCENTRATION of Ca²⁺ ion?

solution

What is the hardness?