Sciencemadness Discussion Board

Calcium bicarbonate

deltaH - 24-2-2016 at 12:16

I was intrigued by the wikipedia article on calcium bicarbonate (particularly it's high solubility), so I performed the following experiment:

100g Calcium carbonate (purified 5 micron powder of white marble) was charged into a soda stream bottle with 853g tap water. This was refrigerated for several hours and then using a soda stream machine, CO2 was repeatedly sparged in. The machine constantly leaked CO2, but in spite of this, high pressure appeared to be maintained for the most part during the run until fully discharging the gas cylinder (270g CO2) over the course of several minutes. In the end, the bottle was removed and capped. A before and after weighing of the bottle and contents indicated an uptake of CO2 of 8g.

Why did so little CO2 dissolve?

blogfast25 - 24-2-2016 at 12:46

Firstly the equilibrium:

CO2 +H2O < === > H2CO3

... is strongly left-leaning, [H2CO3]/[CO2] = 1.7 10<sup>-3</sup> at STP.

The rest of the mystery is found here: pH. Scroll down to b. Calcum carbonate:

http://www.sciencemadness.org/talk/viewthread.php?tid=65303#...

deltaH - 24-2-2016 at 13:11

Quote: Originally posted by blogfast25  
Firstly the equilibrium:

CO2 +H2O < === > H2CO3

... is strongly left-leaning, [H2CO3]/[CO2] = 1.7 10<sup>-3</sup> at STP.

The rest of the mystery is found here: pH. Scroll down to b. Calcum carbonate:

http://www.sciencemadness.org/talk/viewthread.php?tid=65303#...


Very nice :)

Ok so assuming a pH of 3.6, that means your correction factor is 2291 and the solubility of calcium carbonate is thus 0.233M. Assuming that one mole equivalent CO2 is taken up to dissolve this (forming predominantly the bicarbonate), that is a mere 8.7g CO2 in the volume of water I had.

In reality the pH was probably higher because of the buffering effect of the calcium carbonate leading to a lower value of CO2 uptake observed.

Makes sense now, thanks.

Conclusion, wiki solubility for calcium bicarbonate is... #$%@

[Edited on 24-2-2016 by deltaH]

blogfast25 - 24-2-2016 at 13:21

@deltaH:

Bear in mind that actual concentrations of all ions will be slightly higher due to ionic strength, compared to the 'simple' equilibria based calculation:

http://www.sciencemadness.org/talk/viewthread.php?tid=65303#...

deltaH - 24-2-2016 at 13:24

Yes, what I was really after was seeing an appreciable dissolution of calcium carbonate as the wiki entry suggests by its solubility numbers, but alas, poor Yorrick... :)

[Edited on 24-2-2016 by deltaH]

deltaH - 24-2-2016 at 13:29

PS. It tastes rather nice, like soda water without the edge and it's very fizzy.

[Edited on 24-2-2016 by deltaH]