Will raising PH also raise Alk?

FireEMT · Sep 28, 2016

I was wondering if raising PH will cause Alk to rise. I open my windows in the house today to try to raise PH and noticed when I tested Alk tonight it was higher, even when lowering my dose of soda ash to try to lower Alk. I got my PH to raise from 7.78 to 8.05 by opening the windows, which means I am going to run fresh air to my skimmer.

jason2459 · Sep 28, 2016

No.

You can have high pH and low Alk.
You can have low pH and high Alk.

However, when you increase Alk you will basically (ha!) also increase pH though temporarily. Why dosing for and chasing pH numbers is tough and generally not recommended to do.

But using freshair or a CO2 scrubber is fairly safe way to increase pH with out increasing Alk. I say fairly as polutants from the outside could enter the tank this way. I would put some catalitic carbon in line to hopefully help reduce that risk.

Randy Holmes-Farley · Sep 28, 2016

To expand a bit, if the pH is changed by adding or subtracting CO2, it has zero impact on pH.

Here's a copy and paste from one of my articles on this topic:

Alkalinity Facts
There are several facts about total alkalinity that follow directly from the definition. Unfortunately, some of these have been misunderstood by some hobby authors.

One of these facts is termed The Principle of Conservation of Alkalinity by Pankow ("Aquatic Chemistry Concepts", 1991). He shows mathematically that the total alkalinity of a sample CANNOT be changed by adding or subtracting CO2. Unfortunately, there is an article available on line, which claims otherwise, and encourages people to "lower alkalinity" by adding CO2 in the form of seltzer water. This is simply incorrect.

Forgetting the math for the moment, it is easy to see how this must be the case. If carbonic acid is added to any aqueous sample with a measurable alkalinity, what can happen?

Well, the carbonic acid can release protons by reversing equations 1 and 2:

(5) H2CO3 ==> H+ + HCO3-

(6) HCO3- ==> H+ + CO3--

These protons can go on to reduce alkalinity by combining with something that is in the sample that provides alkalinity (carbonate, bicarbonate, borate, phosphate, etc). However, for every proton that leaves the carbonic acid and reduces alkalinity, a new bicarbonate or carbonate ion is formed that adds to alkalinity, and the net change in total alkalinity is exactly zero. The pH will change, and the speciation of the things contributing to alkalinity will change, but not the total alkalinity.

This is not true for strong acids, however. If you add hydrochloric, sulfuric or phosphoric acids (or any acid with a pKa lower than the carbonic acid endpoint), there will be a reduction in the alkalinity.

Another interesting result of the Principle of Conservation of Alkalinity is the equation for determining the total alkalinity when two different aqueous solutions are mixed together. If you mix (a) parts of a solution with total alkalinity A with (b) parts of a solution of total alkalinity B, the resulting alkalinity is just the weighted average of the two samples:

TAmix = [a(A) + b(B)]/[a + b]

Equation 7 can be used to calculate changes in TA for water changes in a tank, for additions of limewater, for dilution of tank water with pure water, and a host of other situations where you might want to know what the final alkalinity will be. It can also be used for calculating reductions in alkalinity caused by strong acids, where the alkalinity of the acid is just the normal strength of the acid as a negative number.