Experimental purposes:
1. Understand the concept of water hardness, the significance of measuring water hardness, and the expression method of water hardness;
2. Understand the principle and method of EDTA determination of calcium and magnesium in water, including acidity control and indicator selection;
3. The effect of adding Mg-EDTA in the determination process and its influence on the determination results.
Experimental principle:
The hardness of water originally refers to the ability of calcium and magnesium ions to precipitate soap. The total hardness of water refers to the total concentration of calcium and magnesium ions in water, including carbonate hardness (that is, calcium and magnesium ions can precipitate in carbonate form after heating, so it is also called temporary hardness) and non-carbonate hardness (that is, the part that calcium and magnesium ions cannot precipitate after heating, also called permanent hardness).
The expression method of hardness has not been unified. At present, there are two commonly used expressions in China: one is to convert the measured calcium and magnesium into the mass of CaO, that is, the number of milligrams of CaO in each liter of water, in mg L-1; The other is calculated in degrees (0): 1 hardness unit means that 1 part of water contains 1 part of CaO (that is, every liter of water contains 10 mg of Cao),1=10 ppm of Cao. This representation of hardness is called German Degrees.
Requirements of industrial water and drinking water for water hardness: The hygienic standard for drinking water in China stipulates that the hardness calculated by CaCO3 shall not exceed 450 mg L-1.
Ca2++EDTA = calcium -EDTA
Mg2+ + EDTA = Mg-EDTA
Experiment preview:
Understand the preparation and calibration methods of EDTA standard solution;
Understand the determination principle and method of total hardness of water.
Basic operation:
Preparation and calibration of EDTA standard solution;
Basic operation of acid burette.
Experimental steps:
Adding ammonia buffer solution with pH= 10.0 and a small amount of chrome black T indicator to the water sample, the solution is red; When titrating with EDTA standard solution, EDTA first coordinates with free Ca2+ and then with Mg2+. At the metering point, EDTA takes away Mg2+ from MgIn-, thus making the indicator free, and the color of the solution changes from red to pure blue, which is the end point.
When the water sample contains a small amount of Mg2+, because the color sensitivity of CaIn- is much worse than that of MgIn-, a sharp end point is often not obtained. In order to improve the sensitivity of endpoint discoloration, an appropriate amount of Mg2+ can be added to EDTA standard solution (before EDTA calibration, so as not to affect the titration quantitative relationship between EDTA and measured ions), or a certain amount of Mg-EDTA salt can be added to buffer solution.
The total hardness of water can be calculated from the concentration and consumption volume V 1 (ml) of EDTA standard solution cEDTA. In terms of CaO, the unit is mg/L.
Precautions:
When measuring the total hardness, adjust the pH value with ammonia buffer solution;
Pay attention to adding masking agent to mask interfering ions, and the masking agent should be added before the indicator;
When measuring the total hardness, slowly decrease and shake more when approaching the end point;
If the water temperature is too low, should the water sample be heated to 30 ~ 40? 0? 2C, and then measure.
Problem analysis:
1. Should the pipette be wetted with deionized water in this experiment? Should the conical flask be wetted with deionized water?
2. At the end of complexometric titration, the reaction speed of EDTA replacing the indicator in M-In is a little slow, so it is necessary to shake more and drop more slowly.
3. The remaining EDTA solution is recycled (plastic recycling barrel is in the fume hood), which will be used for the design experiment.
Common tools:
Main reagents and instruments
0.02mol/LEDTA, NH3-NH4Cl buffer solution (pH~ 10), 10%NaOH solution, chrome black T indicator.
Basic burette, conical flask, washing flask, volumetric flask, pipette, ear-absorbing ball, beaker, reagent bottle, analytical balance, weighing bottle.