[WillS]

I am in the process of planning a chemical investigation regarding different methods of softening water. For the experiment I would like to create some hard water and then test different methods of softening it (by using ion exchange/magnetic softners). To do this I plan to make up a saturated solution of calcium bicarbonate, as I understand this is naturally how calcium ions enter the water. I will carry out before and end titations with EDTA as an indicator,  for accuracy I would like a large change in calcium ion concentration with in the solution. Firstly, I would like to know if this is a suitable method of creating hardwater and more specifcally will there be enough calcium bicarbonate dissolved to provide adequate results. Is there a list of solubility products on the internet? - I cant seem to find one! As i guess this will enable me to determine exactly how much will dissolve.

Thanks for you help,
Will  

[Borek]

pKsp = 8.35 for calcite and 8.22 for aragonite.

Note however, that the equilibrium will be shifted by atmospheric CO₂ - it will acidify solution and hydrogen carbonate is soluble much better, thus your concentrations will be hard to controll.

Perhaps you should start with CaCl₂ solution, then you will be not CO₂ dependent.

[WillS]

Thanks for the reply. Would this acidification of the water cause huge discrepancies or would the effect be marginal? I guess if the solution was used used relatively quickly the change in solubility would be neglible. Your suggestion of calcium chloride seems a lot better but will the introduction of chloride ions affect my results? Or will they be similar to using natural hard water? And lastly, how would i calculate the solubility of calcium bicarbonate with the figures you gave me?

Thanks, Will

[Borek]

1. Acidification - read about karst, whole karst chemistry is just about hydrogen carbonate/bicarbonate equilibrium.

2. Chloride ions should not interferere with your experiments - they are always present in natural waters. Look for information on permanent and temporary water hardness.

3. For calcium carbonate Ksp = [Ca²⁺ ][CO₃^2- ] - note that in saturated solution made by dissolving of salt [Ca²⁺ ] = [CO₃^2- ].

[kkjc2]

These are pretty standard recipes for preparing hard water samples for lab-investigation purposes:

To prepare Temporarily Hard Water
Take Fresh LimeWater (saturated calcium hydroxide - shake a few grams of Ca(OH)₂ in a litre of water, shake it occassionally over a day and leave to settle, pour off the clear solution on top); Bubble carbon dioxide gas through the limewater (CO₂ can be easily generated by adding dilute acid to some carbonate (eg. baking soda, sodium bicarbonate) in a standarg gas-generation flask set-up).
Bubbling of CO₂ through limewater first turns it milky, then after a prolonged period of bubbling, it turns back to clear once again. This is your Calcium bicarbonate solution - which can be used as temp. hard water for all of your purposes.
(Ca bicarbonate is not a stable compound and cannot be isolated in a pure solid form - you wouldn't find a 'bottle' of that stuff and make up its solution.)

To prepare Permanent Hard Water,
Dissolve calcium chloride or magnesium sulfate (or both) in water, (about 0.1 M should be sufficient)

To prepare Hard Water similar to naturally occuring ones, Take 3 volumes of the temp. hard water and 1 vol. of the perm. hard water and add 2 vol of dist. water. That should give a pretty good mixture, if it is found to be too weak, omit the dist. water.

[kkjc2]

Also to point out one mistake in the original post:

EDTA is not your indicator for the titration, it is the reactant with your metal ions.

You would usually use 'Erichrome black T' or other similar dyes as the indicator for EDTA titrations. I think Erichrome black is a suitable Ca/Mg²⁺ indicator.
 

[kkjc2]

About the solubility products:
You can use the K_sp of Ca(OH)₂ = 5.5X10^-6 mol³dm^-9
to work out how much Ca²⁺ will go into solution for your limewater, then all of these will end up in the calcium bicarbonate solution if the preparation was done correctly.

[Borek]

Topic split and moved to Analytical Chemistry Forum:

http://www.chemicalforums.com/index.php?board=8;action=display;threadid=6131

[Organishe]

Thank you, Borek  

[TabrG]

I am also trying to create my own hard water. Do you have any advice? I have no prior information on how to do this.
Thanks

[Borek]

Just dissolve some CaCl₂.

[Sircodekill]

I am in the process of planning a chemical investigation regarding different methods of softening water. For the experiment I would like to create some hard water and then test different methods of softening it (by using ion exchange/magnetic softners). To do this I plan to make up a saturated solution of calcium bicarbonate, as I understand this is naturally how calcium ions enter the water. I will carry out before and end titations with EDTA as an indicator,  for accuracy I would like a large change in calcium ion concentration with in the solution. Firstly, I would like to know if this is a suitable method of creating hardwater and more specifcally will there be enough calcium bicarbonate dissolved to provide adequate results. Is there a list of solubility products on the internet? - I cant seem to find one! As i guess this will enable me to determine exactly how much will dissolve.

Thanks for you help,
Will

Calcium bicarbonate does not exist, as far bicarbonate means HCO₃^- which is impossible with Ca²⁺. As someone said, having a concentrated solution of CO₃^2- will mean parallel reactions (for example, increasing pH) which will interfere with your experiment due the possible formation of insoluble salts, as Mg(OH)₂, or other metal oxides/hidroxides.

I vote for CaCl₂ too, heat the solution if it not dissolves completely.

EDIT: Didn't look at dates.

[TabrG]

Thank you Mr. Borek