Alkalinity (As of CaCO3 to As of HCO3-): Practical guide on unit conversion

Alkalinity (As of CaCO3 to As of HCO3-): Practical guide on unit conversion

This article is for all young geochemist starting their work in consulting world! I went through the pain of discovering the facts that are not really taught in schools or in books. You can only learn them by playing with actual laboratory sheets and various geochemical problems!

Alkalinity is the total acid – neutralizing capacity of the water. Alkalinity is also one of the most common parameter you will observe in any kind of water quality analysis. But alkalinity is commonly reported as of CaCO3 mg/L. To use this value for any kind of calculations we must convert it to proper unit.

For example, say we are plotting a piper or stiff diagram. We will need activity of all major cations and anions to be used in plot making. In Piper or Stiff diagram, one of the input parameter is HCO3- (bicarbonate). We need to convert bicarbonate ion concentration (as of CaCO3) to bicarbonate ion activity.

Well, here is the trick.If you pH is less than about 8.4, just multiple the Alkalinity as of CaCO3 by 1.22 to convert to HCO3. We assume that all alkalinity present in the solution is as HCO3-. Which is true in most cases as HCO3- is truly most dominant component of alkalinity in natural water.

So, for example, if we have 100 mg/L Alkalinity as of CaCO3, we have –
HCO3- mg/L = 100 x 1.22 = 122 mg/L HCO3 in the system.
To convert to activity, divide 122 by (61*1000).

You can also use the spreadsheet to calculate the activity of Bicarbonate in your system.
Activity, molality, normality calculations-Download Spreadsheet

If you want more sophistication in your calculation use the following equations to calculate bicarbonate in your system.

I am attaching the “Magic” Spreadsheet that will help you to calculate the HCO3 as mg/L just with few simple entries. Change the Alkalinity value and pH from your lab sheet to the spreadsheet and get the result.

Download : Alkalinity (as of CaCO3) to HCO3 mg/L conversion Spreadsheet


Note: As I told before, if you are below
pH 8.4, just multiply your alkalinity by 1.22 to make it simple. Check the table using sophisticated HCO3 calculation in various pH range:

Alkalinity VS pH

You can see that withing pH Range 5 to 9 the sophisticated HCO3 calculation differ in only 122-110.89=11.11 when we have 100 mg/l as of CaCO3 in the system. So, within that range, multiplication by 1.22 will be close to the “actual value” calculated by the sophisticated equation. But when you go to pH 10 multiplication by 1.22 will not be valid any more as you can see that we have only 60 mg/L HCO3- instead of 122 mg/L!

Why so?
At higher pH carbonic acid becomes more stable than the bicarbonate ion.

4 Comments

  1. Dick Waller

    The graph of pH against HCO3 doesn’t match the values in the table immediately above: has the pH scale been halved by mistake?

    It would also be helpful to show clearly on the graph that the HCO3 is mg/l, not as CaCo3

    Reply
    1. admin (Post author)

      I will check. Soon.

      Reply
  2. chris p

    The graph data selection is off by one row.

    Reply
  3. Rakesh

    Can you recommend a book dealing with Charge balance chemistry

    Reply

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