August 06, 2020, 03:56:53 AM
Forum Rules: Read This Before Posting


Topic: Why is calcium chloride basic/alkaline, with a pH of 8-9?  (Read 85319 times)

0 Members and 1 Guest are viewing this topic.

Offline AhBeng

  • Regular Member
  • ***
  • Posts: 51
  • Mole Snacks: +5/-1
Why is calcium chloride basic/alkaline, with a pH of 8-9?
« on: September 20, 2009, 11:49:37 PM »
A variety of sources, including Wikipedia and several chemistry websites, stated (without explanation) that the pH of both anhydrous and hydrated calcium chloride, are basic/alkaline.

Based on the polarizing power of Mg2+ ion, we expect a solution of MgCl2 to be slightly acidic. Based on similar charge density of the Ca2+ ion with Na+ ion, we expect a solution of CaCl2 to be neutral (like NaCl).

So why is CaCl2(s) and CaCl2(aq) basic/alkaline?

Thanks in advance for any replies!

Offline BluRay

  • Full Member
  • ****
  • Posts: 154
  • Mole Snacks: +9/-2
Re: Why is calcium chloride basic/alkaline, with a pH of 8-9?
« Reply #1 on: September 21, 2009, 10:15:24 AM »
A variety of sources, including Wikipedia and several chemistry websites, stated (without explanation) that the pH of both anhydrous and hydrated calcium chloride, are basic/alkaline.

Based on the polarizing power of Mg2+ ion, we expect a solution of MgCl2 to be slightly acidic. Based on similar charge density of the Ca2+ ion with Na+ ion, we expect a solution of CaCl2 to be neutral (like NaCl).

So why is CaCl2(s) and CaCl2(aq) basic/alkaline?

Because it's not... :)
Ca2+ + 2H2O <--> Ca(OH)2 + 2H+
« Last Edit: September 21, 2009, 10:34:24 AM by BluRay »

Offline MrTeo

  • Chemist
  • Full Member
  • *
  • Posts: 312
  • Mole Snacks: +31/-9
  • Gender: Male
Re: Why is calcium chloride basic/alkaline, with a pH of 8-9?
« Reply #2 on: September 21, 2009, 12:39:44 PM »

Ca2+ + 2H2O <--> Ca(OH)2 + 2H+


Er... what do you think of this?

Cl-+H2O=HCl+OH-

I've never seen something like this... probably beacuse both HCl and Ca(OH)2 are strong acids/bases so no hydrolysis occurs  ;D

Anyway I don't think that CaCl2 is alkaline too (or if it is there's a reason unknown to me)...
The way of the superior man may be compared to what takes place in traveling, when to go to a distance we must first traverse the space that is near, and in ascending a height, when we must begin from the lower ground. (Confucius)

Offline renge ishyo

  • Chemist
  • Full Member
  • *
  • Posts: 403
  • Mole Snacks: +67/-14
Re: Why is calcium chloride basic/alkaline, with a pH of 8-9?
« Reply #3 on: September 21, 2009, 02:19:57 PM »
This reminds me of that topic where I got into trouble suggesting to a beginning chem student that NaCl was slightly basic:

http://www.chemicalforums.com/index.php?topic=35427.msg135549#msg135549

The best answer is that CaCl2 is neither basic nor acidic (it's a salt).

If you were pressed to explain why the CaCl2 solution would be "slightly basic" then you can "rationalize it" I suppose by the fact that for every one calcium ion that is released you release two chloride ions. These chloride ions are VERY weakly basic and so they can liberate OH- ions to some extent. The problem there as before is that Cl- is simply very weak. So much so that the acidic byproduct of the reaction of Ca2+ with water actually competes with it. In fact, you can argue as Borek did before that the final result should be the other way around. So it is best to classify CaCl2 as neither acidic or basic if you possibly can.

In reality, I think the basic pH of 8-9 that can be observed when CaCl2 is added to solution probably arises in a complex way. For example, after dissolving calcium chloride in water it can form calcium hydroxide as shown by blue ray above. Calcium hydroxide can further react with CO2 in the air to form calcium carbonate which in water will dissolve to some extent to form bicarbonate ion (HCO3-). The presence of this ion would also contribute to a slightly raised pH. The extent of all of the above depends on the concentrations of the species involved as well. For instance, the wiki also lists that CaCl2 can be present at a slightly acidic pH (see the wiki for the hydrated compound which can be between 6.5-8) depending on the circumstances. Saying CaCl2 is neither acidic or basic is still the best answer.



Offline BluRay

  • Full Member
  • ****
  • Posts: 154
  • Mole Snacks: +9/-2
Re: Why is calcium chloride basic/alkaline, with a pH of 8-9?
« Reply #4 on: September 21, 2009, 02:49:35 PM »

Ca2+ + 2H2O <--> Ca(OH)2 + 2H+


Er... what do you think of this?

Cl-+H2O=HCl+OH-
No.
Cl- does not hydrolize (in significant amount), Ca2+ does (even if very little).
Remember that Ca(OH)2 solubility is small:
Ca(OH)2  ::equil:: Ca2+ + 2OH-;   Ksp ~ 10-5

However the following equilibrium is more clarifying:
Ca2+ + H2::equil:: (CaOH)+ + H+;   Ka ~ 10-12.7
(from: G.Charlot - "Qualitative Chemical Analysis")

With the previous reaction only, an 1M solution of CaCl2 would have a pH = 6.4 (if I made the computation correctly :)). So it's just slightly acidic, but it is.
« Last Edit: September 21, 2009, 03:08:22 PM by BluRay »

Offline MrTeo

  • Chemist
  • Full Member
  • *
  • Posts: 312
  • Mole Snacks: +31/-9
  • Gender: Male
Re: Why is calcium chloride basic/alkaline, with a pH of 8-9?
« Reply #5 on: September 21, 2009, 03:41:15 PM »
However the following equilibrium is more clarifying:
Ca2+ + H2::equil:: (CaOH)+ + H+;   Ka ~ 10-12.7
(from: G.Charlot - "Qualitative Chemical Analysis")

With the previous reaction only, an 1M solution of CaCl2 would have a pH = 6.4 (if I made the computation correctly :)). So it's just slightly acidic, but it is.

Now that's more like it, even if I think that Mr.Charlot should write those H+ as H3O+  ;D
Anyway I simply wanted to state that an equation like that was wrong (you can't image Ca2+ hydrolysing that way, moreover taking 2OH- from water... just as you can't think Cl- taking H+ (or you can hardly imagine it  ;)))
The way of the superior man may be compared to what takes place in traveling, when to go to a distance we must first traverse the space that is near, and in ascending a height, when we must begin from the lower ground. (Confucius)

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 25876
  • Mole Snacks: +1692/-401
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Why is calcium chloride basic/alkaline, with a pH of 8-9?
« Reply #6 on: September 21, 2009, 04:38:06 PM »
Just below 7 would be my bet without checking.

Could be whoever states that it is slightly alkalic refers to CaCl2 industrially available in bulk quantities - it is produced by heating CaO/H2O/NH4Cl mixture in Solvay process, so it can contain some tiny amounts of Ca(OH)2. But that's my guess only.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

Offline BluRay

  • Full Member
  • ****
  • Posts: 154
  • Mole Snacks: +9/-2
Re: Why is calcium chloride basic/alkaline, with a pH of 8-9?
« Reply #7 on: September 22, 2009, 09:56:32 AM »
Just below 7 would be my bet without checking.

Could be whoever states that it is slightly alkalic refers to CaCl2 industrially available in bulk quantities - it is produced by heating CaO/H2O/NH4Cl mixture in Solvay process, so it can contain some tiny amounts of Ca(OH)2. But that's my guess only.
Actually, I think I've understood what the OP and renge ishyo refers to: wikipedia says the the *pKa* of calcium chloride is ~ 8-9, not that the solution has such values of pH!!!!!!

http://en.wikipedia.org/wiki/Calcium_chloride

For the OP: pKa = 9 means Ka = acidity constant = 10-9.

For comparison, ammonium ion NH4+ has a Ka = 5.8*10-10 and so would be less acid than Ca2+.
« Last Edit: September 22, 2009, 10:07:11 AM by BluRay »

Offline renge ishyo

  • Chemist
  • Full Member
  • *
  • Posts: 403
  • Mole Snacks: +67/-14
Re: Why is calcium chloride basic/alkaline, with a pH of 8-9?
« Reply #8 on: September 22, 2009, 01:42:00 PM »
You are correct Blueray! Yeah, i just glanced at the numbers and didn't even notice the pKa next to it! That would be where the confusion comes from. Yeah, that's not equal to the pH ranges for the salt.

Offline AhBeng

  • Regular Member
  • ***
  • Posts: 51
  • Mole Snacks: +5/-1
Re: Why is calcium chloride basic/alkaline, with a pH of 8-9?
« Reply #9 on: September 24, 2009, 01:25:21 PM »
Thanks a lot, BluRay and everyone else who replied!

Amazing how misreading / misinterpreting a little word can lead to sooooo much frustration. Heh.

Sponsored Links