Question

Alkalinity Concepts Assuming no solids form or precipitate, does the alkalinity of a 1 liter sample of a typical natural water increase, decrease, or stay the same if you:

a.). Add a small amount of CaCl2(s) to it?

b.). Add 10 mL of 0.1 M NaOH and 10 mL of 0.1 M H2SO4 (sulfuric acid) to it?

c.) Titrate it with acid to the H2CO3 endpoint (pH = 4.3)?

d.) Degas the solution to remove dissolved CO2(g) before starting the titration?

Answer 1

Alkalinity is used for aquatic chemistry. According to this concept

1) It is a conservative quantity which remains unaffected by change in tmperature.

Ions like Na⁺, K⁺ , Ca²+ , Mg ²⁺, Cl- , SO42+ and NO32-

are regarded as conservative.

thus option a) on adding CaCl2 Alkalinity remain same

2) It is used for measuring water's ability to resist pH change due to strong acid and base.

3) It is easy to titrat with strong acid until an end point is obtained.

In typical ground water or sea water the measured Alkalinity is set equal to

A_At = [HCO3-]_t + 2[CO2-]_t + [B (OH)₄^-]_t+ [OH-]_t+ 2 [PO₄^2-​]_t+[HPO4 ^²-]_t+ [SiO(OH)₃ ^-]_t - [ H⁺]- [HSO4 -]

Where At = Alkalinity

t = total Alkalinity

This shows that, in option b) on adding NaOH and H2SO4 , all bases have been protoned to zero level speices. Hence, they no longer cause Alkalinity, thus stays same.

For option c). In the carbonated system bicarbonate ion and carbonate ion have become carbonic acid at pH 4.2 - 4.5 . This pH is called as CO2 equivalent point where major dissolved component is CO2 which is converted to carbonic acid in aqueous solution.

thus Alkalinity remain same.

4) The Alkalinity of solution is its acid neutralizing capacity. When the end point of titration is CO2 equivalent point.

hence dissolution of CO2 , before titration does not change the Alkalinity thus for option d) alkalinity remain same.