It seems the numbers are off; I'd check the testing and confirm them. High alkalinity and low pH are common but 22 dkh and 7.4pH are extraordinary and very unlikely.
The relationship between alkalinity and pH is not clearly linear in reality;there are a number of moving parts. Foremost, pH is driven by CO2 from the surrounding air as well as biological activity's effect on CO2 levels in the water.
As Alk neutralizes H+ and reduces it , over the course of a few hours the air equilibrates with the water altering the CO2 level back to near where it was before the alk addition, adding or subtracting CO2 and consequently H+. The effect of alkalinity additions to raise pH generally have a short lived effect on pH ,unless alk is very low at the start , but do raise the alkalinity.
Alkalnity is a measure of several things( carbonate, bicarbonate, borate, PO4 etc) which have an effect on the solution of tank water's ability to neutralize acid. In reef tanks it is mostly carbonate alkalinty, around 97%.
PH is a measure of H+ in the water driven by CO2 levels which vary via gas exchange and depend largely on the CO2 level of the air surrounding the tank along with biological activity related to CO2 production and consumption.
Following is an oversimplified explanation.
The carbonate alkalinity continuum looks like this:
(carbonate) (bicarbonate) (carbonic acid)
CO3-- <-----> HCO3- <------>H2CO3
Note: CO3 (carbonate/aka soda ash)has two negative charges --;HCO3 (bi carbonate/aka) has only one -; H2CO3(carbonic acid) has none.
Thus, carbonate adds 2 units of alkalinity ; bi carbonate adds one .
As CO2 enters the water it hydrolizes freeing H from the H20 ,the extra two H+ lowers pH . CO2 +H2O= CO3-- and 2H+. The CO3 adds 2 units of alkalinity but the 2H+ moves the carbonate continuum toward carbonic acid using the 2 units , leaving the alk unchanged. The extra H+ ,in the water lowers the pH though.
This article by Randy H Farely gives a much more detailed and iformative look at the relationship:
http://reefkeeping.com/issues/2006-06/rhf/index.php
