Hardness of Water

Introduction

The overall purpose of this experiment is to identify the hardness of water in terms of calcium and magnesium concentrations. There are four components in this lab in part A, the total hardness of sample water can be identified through standard titration method with EDTA solution. This will give us the overall concentration of calcium and magnesium concentration together. In part B of this experiment, same titration procedures have been repeated for Brita Filtered Water as well for tap water for identifying the total hardness of them. In part C, standard calcium solutions have been prepared. This is necessary because i 23523y2416x n part D, we will need these standard concentrations for using ion-selective electrode in order to get the calcium concentration of the sample water. After all of these, magnesium concentration in water can be figured out by differences between overall hardness concentration and calcium concentration.

Procedures

A - Determination of Total Hardness

Obtain 300 ml of one water sample and 300 ml of EDTA solution. Record the number of water sample and concentration of the EDTA solution.

Pipette a 25 ml portion of water sample into each of three titration flasks. To each flask, add 5 ml of PH 10 buffer, 25 ml of distilled water and 12 drops of calmagite indicator.

Prepare a titration blank with 50 ml distilled water, 10 ml of PH 10 buffer and 10 drops of calmagite indicator. Add EDTA solution dropwise until a blue color is reached. This is the end point we are looking for using titration.

Titrate the sample solutions with EDTA until a purple color is obtained. Swirl the flask for a minute then continue titrating dropwise until the sample color matches the color of titrated blank solution.

B - Reduction of [Ca²⁺] by Brita Water Filter

Repeat the same titrations as in part A with tap water as well as water from the Brita pitcher provided.

C - Preparation of Standard Calcium Solutions

Use volumetric pipette and volumetric flasks, prepare standard calcium solutions of 10 ppm, 50 ppm, 100 ppm and 500 ppm by appropriate dilution of the 1000 ppm Ca²⁺ standard to 100 ml, with distilled water.

D - Determination of [Ca²⁺] by ion-selective electrode.

Obtain two 50 ml plastic beakers, calcium ion-selective electrode as well as the computer kit.

Mount the electrode on a burette stand according to the lab manual.

Using the 50ml beakers as well as plastic beakers, fill each of the beakers with at least 40 ml of one of the standard solutions then label them.

Attach the electrode to the serial box and start up the computer.

Lift the electrode gently from the beaker in which it soaked, rinse with distilled water and blot gently with a Kimwipe.

Place the electrode in the beaker which contains the 100 ppm standard solution. Gently swirl the beakers to ensure that all the working elements of the electrode are exposed to the standard solution.

Using the software in the computer, measure the calcium concentrations of the standard solutions each separately.

Measure the calcium concentration of the sample water using the calibration curve generated by the standard solutions.

Results Discussion

The overall concentration of calcium and magnesium for each water sample can be identified through the standard titration with EDTA solution and the results for each type of water samples are given as follows:

Part A - Water Sample B

The total amount of calcium and magnesium ions in moles are the same as the total number of moles which EDTA is consumed. The concentration of EDTA is given as 0.05 M.

Total number of moles which EDTA is consumed = EDTA Concentration * Volume of Consumption

= Total moles of calcium and magnesium ions

The volume of water sample used = 25 ml

Therefore the total hardness of sample water calculated in mol/L

= Total moles of calcium and magnesium ions (mol) / the volume of water sample used

Part B - Brita Filtered Water

Part B - Tap Water

Part D

In this part of experiment, calibration graphs are generated using the standard solutions and as we measured the calcium content of the sample water B, we can get the calcium concentration of the water in ppm using the graphs.

The calibration graphs for Trial 1 and Trial 2 are generated and shown as follows:

The measured calcium content and calculated calcium concentrations are listed in the following table.

The calcium concentrations x are calculated by substitute the ISE calcium content as y value into the equations correspondingly. The calcium concentration in mol/L is calculated by converting from mg/L (ppm) unit. The molar mass of calcium is 40.078 g/mol.

ppm = mg/L = (mg*10^-3 / calcium molar mass)/L = mol/L = M

In part A since we already get the overall concentration of calcium and magnesium of the sample water, therefore we can find the magnesium concentration by subtracting the calcium concentration from the overall concentration. The final results are listed in the following table.

Question Answers and Calculations

l          Mass of Lime required per day for softening the water to no more than 200 mg/L CaCO₃

The mass fraction of Calcium ion concentration in water sample

= (213.7 mg/L) / (1771.9 mg/L) = 0.12

The mass fraction of Magnesium ion concentration = 1 - 0.12 = 0.88

In order to reduce the total hardness to no more than 200 mg/L,

1571.9 mg/L hard elements in water have to be removed.

In these removed hard elements,

1571.9 mg/L * 0.12 = 188.628 mg/L = 0.0047065 mol/L will be Calcium

1571.9 mg/L * 0.88 = 1383.272 mg/L = 0.056913 mol / L will be Magnesium

Total moles of calcium per day = 0.0047065 mol/L * 750000000 L = 3529875 moles

Total moles of magnesium per day = 0.056913 mol/L * 750000000 L = 42684750 moles

Since one mole of magnesium ions react with 2 moles of lime and one mole of calcium reacts with 1 mole of lime (according to the stoichiometric coefficients in the reaction equation), therefore the total number of moles of lime required

= 3529875 + 42684750 * 2 = 88899375 moles

The total number of moles of lime * molar mass of lime

= 88899375 moles * 74.09268 g/mol = 6586792944 g 6.59 * 10⁶ kg / day

= total mass of lime required per day for softening the water.

l          Mass of soda ash required per day for reduce the residual hardness of water to no more than 100 mg/L CaCO₃

In order to further softening the water to no more than 100 mg/L CaCO₃, 100 mg/L of total hardness has to be removed again using soda ash

100 mg/L * 0.12= 12 mg/L = 0.0002994 mol/L will be Calcium

100 mg/L * 0.88 = 88 mg/L = 0.003621 mol / L will be Magnesium

Total moles of calcium per day = 0.0002994 mol/L * 750000000 L = 224550 moles

Total moles of magnesium per day = 0.003621 mol / L * 750000000 L = 2715750 moles

Since one mole of magnesium ions and one mole of calcium ions both react with 1 mole of soda ash correspondingly (according to the stoichiometric coefficients in the reaction equation), therefore the total number of moles of soda ash required

= 224550 + 2715750 = 2940300 moles

The total number of soda ash * molar mass of soda ash

= 2940300 moles * 105.98844 g/mol = 311637810.1 g 3.12 * 10⁵ kg / day

= total mass of soda ash required per day for further softening the water.

l          Total mass of solid produced by the water softening

For both lime and soda ash water softening, CaCO₃, MgCO₃, Mg(OH)₂ solids will be produced and their total masses are calculated as follows:

1^st step lime softening:

Moles of CaCO₃ solid produced = 3529875 * 2 + 42684750 * 2 = 92429250 moles

Moles of Mg(OH)₂ solid produced = 42684750 moles

2^nd step soda ash softening:

Moles of CaCO₃ solid produced = 224550 moles

Moles of MgCO₃ solid produced = 2715750 moles

Therefore total mass for each of these solids are

CaCO₃ solid = (92429250 moles + 224550 moles) * 100.0869 g/mol 9.27 * 10⁶ kg / day

MgCO₃ solid = 2715750 moles * 84.3139 g/mol 2.29 * 10⁵ kg / day

Mg(OH)₂ solid = 42684750 moles * 58.31968 g/mol 2.49 * 10⁶ kg / day

l          Discussion of Brita filtration

Comparing the results obtained in part A and par B, the water sample has a total hardness concentration of 0.06944 mol/L; however the overall water hardness concentration from Brita filtration has a total number of 0.00036 mol/L. Obviously, the filtered water is about 200 times less hardness comparing to the original water sample and the Brita filtration is very effective for reducing the hardness of water at this point. The Brita filtration can also help the municipality to reduce cost of buying the lime and soda ash for softening the water hardness and it is being an environmental friendly process.

Conclusion

The total hardness of water is 1771.9 mg/L. Concentrations in water sample of calcium is 213.7 mg/L and for magnesium is 1558.2 mg/L. The mass of lime required to reduce the total hardness to no more than 200 mg/L CaCO₃ is about 6.59 * 10⁶ kg per day. The mass of soda ash required to reduce the residual total hardness to no more than 100 mg/L CaCO₃ is about 3.12 * 10⁵ kg per day. The mass of CaCO₃, MgCO₃ and Mg(OH)₂ solids generated as a result of the water treatments are about 9.27 * 10⁶ kg, 2.29 * 10⁵ kg, 2.49 * 10⁶ kg per day correspondingly.