You can understand it by the figure attached herewith.

Thank you @Pankaj.

Actually I want to calculate the concentration of bicarbonate from the concentration of Total alkanity and pH i.e., if there is any equation or emperical formula for clculation then please share.

It can be calculated exactly but the method depends a bit on the conditions of the water.

• Do you know that alkalinity is only found as carbonate and bicarbonate? (no silica, borate and other unusual chemicals in solution)
• Do you know if the water is in equilibrium with CO2 in the atmosphere?
• Which pH range are you in? (at 6-9 it is easier)

Kindly find attached.

Here P is Phenolpthalein Alkalinity

and T is Total Alkalinity.

pH range of my sample ranges from 6-9@ Henrik Rasmus Andersen

About the rest of the water chemistry. Where are your samples from? River, groundwater etc.

Sir I have river samples@ Henrik Rasmus Andersen

please refer to this link

https://en.wikipedia.org/wiki/Alkalinity

Simplified summary.

Alkalinity roughly refers to the amount of bases in a solution that can be converted to uncharged species by a strong acid.

measures the ability of a solution to neutralize acids to the equivalence point of carbonate or bicarbonate. The alkalinity is equal to the stoichiometric sum of the bases in solution

the total alkalinity = bicarbonate and the value of bicarbonate become zero when pH = 4.5 because at this point, all the bases of interest have been protonated to the zero level species.

author, James Drever

If it is your main buffer you can use a simple charge and mass balance to calculate the concentration of both species.

Henrik Rasmus Andersen

May you please explain why CO2 will lower the pH but will not affect alkalinity? As i know, CO2 dissolves in water will form carbonic acid, which will then dissociates into HCO3- and H+. From this process, isn't the concentration of bicarbonates increasing due to the CO2?

Looking forward to your explanation.

Thank you!

Eileen Tan , Then CO2 dissolve in water it does react with water as you say.

CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3− ⇌ 2H+ + CO32−

One of the simplified definition of alkalinity describes the effect on alkalinity:

Alk = [HCO3−] + 2[CO32−] - [H+]

As you can see formation of a hydrogen carbonate is coupled with release of a proton and the two cancel each other in the alkalinity definition. Thus pH can decrease while alkalinity remains the same.

Akshay Shende thank you! By chance will you have the reference of the book? Thanks again!

Akshay Shende, This general overview of carbonate alkalinity is described at different in many textbooks. Generally anyone who works with natural water chemistry and water treatment need to understand this. I studied with James F. Pankow's

Book Aquatic Chemistry Concepts

Some care is needed with the precise definition of concentration, used with either total alkalinity (AT) or with pH calculations, even if units are the same (not necessarily):

i) AT can be defined in terms of total concentrations of selected contributing chemical species (*). Subscript T (with square brackets) may unambiguously identify such concentrations, but is often omitted. These are not, necessarily, actual ('free') molar concentrations. Instead; they should be understood as measured/measurable concentrations, hence possibly not accounting for speciation. Particularly in oceanographic/hydrographic literature, [H+]T often collectively denotes the concentration of several proton-donor species, besides H3O+.

(*) cf. https://en.wikipedia.org/wiki/Alkalinity

ii) Equations involved in a typical pH problem should in principle ultimately translate conserved quantities (such as atomic molar balances or charge balance) and/or equilibrium relationships between the actual (also called 'free') molar concentrations of the relevant chemical species. These concentrations are usually expressed with square brackets (without the subscript T, meaning total ― see § above), as they account for speciation ― unlike nominal molar concentrations, viz. formality. Concentration units used with AT are often other than molarity.