[danielkwn]

Hi. I'm a high school student taking IBDP chemistry course.
I did lime softening experiment for my IA lab, but it seems that there's some problem with it.
I used solid calcium hydroxide to soften the hard water, but when I measured the calcium ions in the softened water by titrating it with EDTA, the concentration of the calcium ions has increased.
Below is the procedure I followed.

1. Pour 500 mL of tap water into a beaker and put 20 g of solid calcium nitrate (to increase the hardness of water. I wanted to use calcium bicarbonate, but there wasn't any at school.).
2. Pour 50 mL of the hard water into a conical flask and add 2 mL of pH 10 buffer solution and 10 drops of EBT indicator.
3. Titrate the hard water in the conical flask with EDTA (3 trials).
4. Pour 50 mL of the hard water from #1 into another beaker. Add enough amount of calcium hydroxide and solid sodium hydroxide (since lime softening only happens at high pH). (I could see the white precipitate at this step)
5. Filter the water from #4 so that the precipitate is removed.
6. Dilute the water from #5 with 800 mL of distilled water (since the range of EBT indicator is from pH 8 to 11).
7. Pour 50 mL of the water from #6 into a conical flask and add 2 mL of buffer solution and 10 drops of EBT indicator.
8. Titrate the water from #7 with EDTA (3 trials).

The amount of EDTA used in #3 was about 7 mL, and in #7 was about 5 mL. But since I diluted the water in #6 so I have to  multiply by (850/50) right? And that means the concentration of calcium ions increased. I don't know what I've done wrong, but I'm guessing it might be related to the sodium hydroxide I added in #4 since I saw the precipitate forming around the pellets.
I'd appreciate if anyone can tell me what went wrong.

[chenbeier]

The reason you cannot find Calcium hydrogen carbonate in the school, it is not existing. This component is only existing in solutions in some caves or by adding carbondioxide gas to saturated solution of calciumcarbonate.

To the problem. I dont see how to decrease the hardness by adding of calciumhydroxide in step 4.
Especially you used calciumnitrate as source.

Better would be to prepare a sample dissove calciumhydroxide, filtrate it, blow CO₂ throw the solution until the white colour dissappears and you have the calciumhydrogen carbonate.
Follow your procedure 1 to 8 and then it should work.