Carbonate System and pH

Question 1

Which dissolved carbonate species dominates at which pH-values? There are two pH-values where important switches between the dominating carbonate species occur. Which are these pH values?

Answer 1

pH < 6.35 H2C03.
6.35 < pH < 10.33 HCO3-.
pH > 10.33 CO32-

6.35 and 10.33 important!

Question 2

(2) Explain how pH is regulated in freshwater. Which role does photosynthesis play for the pH?

Answer 2

pH is log[H+] and therefore depends on the carbonate buffer system where free H+ are created in the different reaction steps. When there is photosynthesis CO2 is consumed, which leads to more CO2 forming from H2CO3 (carbonic acid), which in turns means less free H+ together with less bicarbonate and carbonate ions. Therefore, pH gets higher. From the atmosphere CO2 is dissolved into the water, which has the opposite effect and decreases the pH.

Question 3

(3) Is pH going up or down if you add NH4+ to oxidised surface water?

Answer 3

pH will go down because of increased H+.
Nitrate and free H+ will form

Question 4

Explain how photosynthesis affects the solubility of calcite in natural waters.

Answer 4

From the carbonate ions, minerals and shells can be formed which means less carbonate ions and therefore a higher pH. Weathering can cause an increased amount of carbonate ions in the water which also impacts pH. So, if we have a lot of photosynthesis there will be an increased solubility of Ca+ as more HCO3- is needed to form more CO2.

Question 5

How do the phosphates vary with pH?

Answer 5

H3PO4 until pH 2.14,
then H2PO4- until 7.20,
then HPO42- until 12.37,
then PO43-

Question 6

How does ammonia vary with pH?

Answer 6

NH4+ until 9.1, then NH3