sodium sulfate and lime doesn't react?

[mifriend]

I have a 10g/l sodium sulfate solution
By adding lime ( water lime, milk, etc.) at various concentrations, nothing happends ..
why ?
Solutions ?
thank's a lot

 

[25362]

What quality is the water used ? Salts as NaCl affect the solubility of gypsum or anhydrite.

 

[mifriend]

thank's 25362 for your answer
water quality is surface water:
Cl :30 ppm
hardness :250 ppm
M: 200ppm
did the test with demin: same result
does existing cristals affect cristallisation ?

 

[bimr]

Not exactly sure what you are trying to do. If you keep adding lime, you will get calcium sulfate to precipitate out of solution.

Calcium sulfate solubility is 1290 mg/l at 32 Deg F. Calcium hydroxide solubility is 2390 mg/l. If you add enough lime, it will precipitate.

If you are trying to figure an economical method to precipitate calcium sulfate out of wastewater, you can forget about it, because it is not practical.

 

[25362]

Was demineralized water used also for preparing the lime solution ?

 

[25362]

By mixing two solutions one gets a more diluted one.
The solubility products are for CaSO₄ and Ca(HO)₂, respectively:

4.93*10^-5 and 5.02*10^-6​

Thus, if the molar (ionic) concentrations don't reach the values as expressed by the constants it is possible that no precipitate forms.

 

[mifriend]

Did the tests many times in addin lime at various temp (water lime, milk lime ) in stoechiometric conditions
it seems calcium solubility in Na2so4 solution never achieve precipitation concentration or CaSO4 is soluble in such Na2SO4 solution ??

 

[bimr]

Just keeping adding more lime. Believe me, it will precipitate.

 

[25362]

Calcium hydroxide at, say, 20^oC is less soluble than anhydrite. When adding a diluted lime (albeit saturated) solution to the given solution of sodium sulfate, one dilutes even more the sulfate and the hydroxide.

As long as the products of the ionic concentrations do not reach the solubility products one cannot get a precipitate.

Have you tried a richer solution of sodium sulfate ?

 

[25362]

If the sodium sulfate solution is a given unchanging factor, then try the precipitation with a richer calcium chloride solution.

 

[bimr]

Reread your post and still not sure what you are trying to do. What is "water lime" and "milk lime"?

If these are limestone products, you should get Calcium sulfate solubility is 1290 mg/l at 32 Deg F. Calcium hydroxide solubility is 2390 mg/l. If you add enough lime, it will precipitate.

Have you tried just using hydrated lime?

Also, as 25362 suggested, adding calcium chloride will provide the calcium if your solution is deficient in calcium.

 

[25362]

The solubility of calcium hydroxide drops at higher temperatures. Thus at 20-30^oC it is even less soluble than calcium sulfate. From Lange's Handbook of Chemistry the solubilities in grams per 100 grams of water are:

^oC 0 10 20 30

hydroxide 0.189 0.182 0.173 0.160
CaSO4.2H2O 0.223 0.244 0.255 0.264
CaSO4 (est.) 0.176 0.193 0.202 0.209

The values for CaSO4 were estimated by pro-rating the molecular weights.

The solubility products at 25^oC, by CRC are:

calcium hydroxide: 5.02*10^-6
calcium sulfate: 4.93*10^-5

 

[25362]

I'd like to emphasize the diverse ion effect, well known to chemists performing gravimetric analyses.

This effect is, in fact, a reduction in ion concentration (ie, reduced activity) due to the presence of other ions that shield the dissociated species of interest, in this case: SO₄^2-, Ca²⁺.

The published values of standard solubility products K_sp^o hold for "pure" water, not for the solubility product in the presence of other ions.
The standard value is divided by the product of the activities (generally -but not necessarily- lower than one) to obtain an increased K_sp.

See

htp://

http://www.iugaza.edu/users/Abdellatif/Chem 2310/Contents/Gravimetric_Analysis.htm#

Lately I found a "new" value for K_sp^o for calcium sulfate at 25^oC of 1.9 [×] 10^-4. This value should be divided by the product of the activity coefficients a_Ca⁺⁺ and a_SO4⁼ to obtain the correct K_sp.

As an example, the solubility of barium sulfate is almost tripled in the presence of 0.1 M NaNO₃. It appears that the increase in solubility is greater with precipitates containing multiply charged ions.