Determination of total hardness of water
Experimental purpose
1. Understand the basic principles and methods of coordination titration.
2. Understand the concept of water hardness and its representation.
Experimental principle
Water containing calcium and magnesium ions is called hard water. Determining the total hardness of water is to determine the total content of calcium and magnesium ions in water, which can be determined by EDTA coordination titration:
Before titration: M+EBT M-EBT
(red)
Main reaction: M+Y MY
End point: M-EBT+Y MY+EBT
(red) (blue)
When the solution changes from red to blue, titration is the end point.
During titration, interfering ions such as Fe3+ and Al3+ can be masked by triethanolamine. Heavy metal ions such as Cu2+, Pb2+ and Zn2+ can be masked by KCN, Na2S or mercaptoacetic acid.
There are many expressions of water hardness. This experiment requires the total amount of Ca2+ and Mg2+ in each liter of water (converted into the mass of CaO), and the unit is mg L- 1.
Equipment and drugs
1. Equipment balance (0. 1g, 0. 1mg), volumetric flask (100mL), pipette (20mL), acid burette (50mL), conical flask (250mL), etc.
2. Drugs HC 1 (1: 1), disodium ethylenediamine tetraacetate (Na2H2Y 2H2O, A.R.), basic magnesium carbonate [mg ([mg (OH) 24mgco3.6H2O, reference reagent), NH3-NH4Cl buffer.
experimental method
I. Preparation of Mg2+ standard solution (about 0.02 mol L- 1)
Accurately weigh 0.2~0.25g of basic magnesium carbonate reference reagent, put it in a beaker of 100mL, moisten it with a little water, cover the watch glass, and slowly add 1: 1HC 1 to dissolve it (about 3~4mL). Add a small amount of water to dilute, quantitatively transfer to 100mL volumetric flask, dilute with water to scale, and shake well.
Its concentration calculation:
Second, the preparation and calibration of EDTA standard solution
Preparation of 1.EDTA standard solution (about 0.02 mol L- 1)
Weigh 2.0g disodium ethylenediamine tetraacetate (Na2H2Y 2H2O) and dissolve it in 250mL distilled water, then transfer it to a polyethylene plastic bottle for preservation.
2.2 calibration. EDTA standard solution concentration
Transfer the Mg2+ standard solution to a 250mL conical flask with a 20mL pipette, add 10mL ammonia buffer solution and 3 ~ 4 drops of EBT indicator, and titrate with 0.02 mol L- 1 EDTA standard solution until the solution turns from purple to blue. Parallel calibration for 3 times.
Calculation of EDTA concentration: Take the average of three determinations.
Three. Determination of total hardness of water
Use a 20 ml pipette to suck the water sample into a 250 ml conical flask, add 6 ml ammonia buffer solution, 3 ml triethanolamine solution 1: 1 and 3~4 drops of EBT indicator, and titrate with EDTA standard solution until the solution turns from purple to blue. Parallel determination for 3 times.
Calculation of total hardness of water: take the average of three determinations.
Think about a problem
1. Why should I add ammonia buffer solution when titrating Ca2+ and Mg2+ with EDTA?
2. What is the purpose of adding triethanolamine when determining the total hardness of water?
3. When the reference reagent of basic magnesium carbonate is decomposed with HCl, what effect will it have on the calibration results if it is not wetted with water first or the HCl is too concentrated?
4. When the calcium content in the water sample is high and the magnesium content is low, a small amount of MgY is often added to the solution first. What is this for? Why?