# PH, Alkalinity, and total carbonate concentration

1. Oct 13, 2012

### jetsetter

1. The problem statement, all variables and given/known data
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

2. Relevant equations
Not certain

3. 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?

2. Oct 14, 2012

### Staff: Mentor

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

3. Oct 14, 2012

### 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.

4. Oct 14, 2012

### Staff: Mentor

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.

5. Oct 14, 2012

### 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))

Last edited: Oct 14, 2012
6. Oct 14, 2012

### 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.

7. Oct 14, 2012

### Staff: Mentor

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.

8. Oct 14, 2012

### jetsetter

We have to neutralize an H+ donating acid.

9. Oct 14, 2012

### Staff: Mentor

Which is?

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

10. Oct 14, 2012

### jetsetter

Carbonic Acid (H2CO3) I believe.