PH, Alkalinity, and total carbonate concentration

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Discussion Overview

The discussion revolves around the concepts of alkalinity, pH, and total carbonate concentration in natural water, with a focus on a homework problem involving calculations related to these parameters. Participants explore definitions, relationships, and quantitative measures of alkalinity and carbonate concentration.

Discussion Character

  • Homework-related
  • Conceptual clarification
  • Mathematical reasoning

Main Points Raised

  • One participant calculates alkalinity as mg/L CaCO3 and approximates total carbonate concentration but seeks assistance with the latter.
  • Another participant defines alkalinity as the buffering capacity of a liquid, suggesting a relationship between greater carbonate concentration and increased buffering capacity.
  • A different participant requests a quantitative explanation of how carbonate alkalinity is measured and its dependence on carbonate concentration.
  • One participant proposes a formula for alkalinity that includes contributions from bicarbonate, carbonate, hydroxide, and hydrogen ions, noting that the influence of hydroxide and hydrogen ions is minimal.
  • Another participant suggests that the calculation of alkalinity will ultimately depend on determining specific factors related to total carbonate concentration.
  • There is a discussion about the need to neutralize an acid, with one participant identifying carbonic acid as the relevant acid in this context.

Areas of Agreement / Disagreement

Participants express varying levels of understanding and approaches to the problem, with no consensus reached on the exact methods for calculating total carbonate concentration or the implications of alkalinity definitions.

Contextual Notes

Some participants express uncertainty about the definitions and calculations involved, and there are unresolved aspects regarding the influence of different ions on alkalinity and the specific calculations needed to ascertain total carbonate concentration.

Who May Find This Useful

This discussion may be useful for students and professionals interested in water chemistry, environmental science, or related fields, particularly those dealing with alkalinity and carbonate systems in natural waters.

jetsetter
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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?
 
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What is the definition of alkalinity? How is it related to the carbonate concentration?
 
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.
 
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.
 
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:
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.
 
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.
 
We have to neutralize an H+ donating acid.
 
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
Carbonic Acid (H2CO3) I believe.
 

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