Reef Chemistry Question of the Day #195 Alkalinity Additives

[Randy Holmes-Farley]

Reef Chemistry Question of the Day [HASHTAG]#195[/HASHTAG]

Which of the following solutions will boost the alkalinity the most when added in equal small volumes to a reef aquarium (say, 1 mL per gallon). Note that 1 mM means 1 millimolar, or 0.001 moles per liter.

A. 1 mM sodium carbonate
B. 1 mM sodium bicarbonate
C. 1 mM calcium hydroxide
D. 1 mM sodium hydroxide

You may elect more than one if you believe two or more have the same effect.

Good luck!

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[JimWelsh]

C & D

EDIT: C only, because stoichiometry.

 

[Cory]

I was going to say d, but jims making me
Think...

 

[Kershaw]

C

 

[DamianOZ]

C by 2x that of A or B or D

 

[DamianOZ]

I change my answer to A and C

A: CO3-2 + H+ > HCO3 + H+ > CO2 + H2O
B: HCO3- + H+ > CO2 + H2O
C: 2 (OH)- + 2H+ > 2 H2O
D: OH- + H- > H2O

 

[Randy Holmes-Farley]

And the answer is...A. 1 mM sodium carbonate and C. 1 mM calcium hydroxide

As DamianOZ points out, each mole of sodium carbonate (Na2CO3) and each mole of calcium hydroxide (Ca(OH)2) provide 2 units of alkalinity.

Na2CO3 --> 2 Na+ + CO3-- and this carbonate provides 2 equivalents of alkalinity, which can be seen in this way:
CO3-- + H2O --> 2HCO3-

Ca()H)2 --> Ca++ + 2OH- and this hydroxide provides 2 equivalents of alkalinity, which can be seen in this way:
2OH- + 2CO2 --> 2HCO3-


Sodium bicarbonate (NaHCO3) and sodium hydroxide (NaOH) provide only 1 unit each.

NaHCO3- --> Na+ + HCO3-
NaOH --> Na+ + OH-

So A and C provide twice as much as B and D.

This article has a more extensive discussion of such issues:

Chemistry And The Aquarium: The Relationship Between Alkalinity And pH ? Advanced Aquarist | Aquarist Magazine and Blog
http://www.advancedaquarist.com/issues/may2002/chem.htm

Happy Reefing!