PH, Alkalinity, and total carbonate concentration

[jetsetter]

Homework Statement

If the pH of a natural water is 7.8 and the alkalinity was measured to be 125 meq/L, determine the following:
-Alkalinity as mg/L CaCO3: Answer = 6250 mg/l CaCO3
-Approximate the total carbonate concentration CT, in moles per liter: Answer = 0.125 moles per liter

Homework Equations

Not certain

The Attempt at a Solution

I was able to determine the alkalinity by finding the equivalent weight of CaCO3, 50 g/eq. I then converted 125 meq/L to eq/L. Then I multiplied (50 g/eq)(0.125 eq/L)(1000 mg/g) and got an answer of 6250 mg/L of CaCO3. I am having trouble approximating the total carbonate concentration. Could someone help please?

 

[Borek]

What is the definition of alkalinity? How is it related to the carbonate concentration?

 

[jetsetter]

Well, alkalinity is the buffering capacity of the liquid. The resistance of the liquid to pH change. The greater the carbonate concentration the greater the buffering capacity.

 

[Borek]

Let me put it differently then. How is carbonate alkalinity measured and how - in result of the way it is measured - does it depend on the carbonate concentration?

I am not asking about qualitative description, but about a simple quantitative one.

 

[jetsetter]

I realize it is going to be something like this:

Alkalinity = (1 eq/mol)([HCO3]) + (2 eq/mol)([CO3]) + (1 eq/mol)([OH]) - (1 eq/mol)([H])

The influence of the OH and H are minimal and easy to calculate. [H+] = 10^(-7.8), [OH-] = (10^(-7.8))/(1 x 10^(-14))

 

[jetsetter]

I also know that ultimately we will get something like this:

Alkalinity = f1Ct + f2Ct + [OH-] - [H+]

f1 and f2 need to be calculated in order to ascertain Ct.

 

[Borek]

You start with pH of 7.8. What do you have to neutralize?

Note: I don't like the answer given, IMHO it is slightly off.

 

[jetsetter]

We have to neutralize an H+ donating acid.

 

[Borek]

Which is?

You are giving the simplest possible answer, one that doesn't require any insight, and you wait for another hint. Try harder.

 

[jetsetter]

Carbonic Acid (H2CO3) I believe.