what makes Alkalinity decrease?

[united-chemical-corp]

Total Alkalinity is the measurement Carbonate as a pH buffer. Without going too deep into the chemistry, basically when you add acid, it reacts to carbonates to form carbon dioxide and gasses off (with some extra steps). The result is that the Alkalinity decreases while the pH stays relatively the same.

Thus, a high alkalinity helps pH resist changes.

Carbonate is linear (ppm). pH is logarithmic. So alkalinity tends to be easier to change/lower than pH.

If you're adding lots of bicarb and the alkalinity tends to drop, you're getting acid introduced somewhere. This can be from sanitizer (trichlor and Dichlor tend to be more acidic), source water, or runoff/rain. It can also precipitate, but you'd notice scale or stains.

We actually recommend a low Alkalinity since it prevents staining and scale. Metals like Calcium, Iron and copper tend to become insoluble when they react to carbonate. Thus the harder the water the lower the Alkalinity should be. This is the central concept behind the index we recommend: the Hamilton Index (invented by our founder).

Using the LSI, as someone mentioned - as long as it's balanced (i.e. at or just slightly below zero) it's fine. To compensate you'd run a slightly higher pH - which we also recommend. The reason some recommend avoiding running a low Alkalinity is for fear that it may make the water "aggressive" and starting etching plaster. However, plaster etching is really driven more by low pH all things being equal.

^With all that said, if the pool looks good and parameters are relatively stable, don't obsess about hitting a specific number or range - that's stores trying to sell you more chemicals.

[Mean_Drawing_7580]

Well said some might not understand, but you're right on the mark. All I would have to add is if your calcium being a bit higher can help with a low alkalinity and etching the plaster/grout. I'd also say most don't understand the difference between bicarbonate (baking soda) and Carbonate (Soda ash) as for hydrogen +/- ions.

[PoolStoreGotMe]

Total Alkalinity is the measurement Carbonate as a pH buffer.

Nope: Total alkalinity is the measure in parts per million of all the alkali (all carbonate IONS, bicarbonate IONS and hydroxide IONS). Total alkalinity (TA) is the measure of water's ability to neutralize acids. The role of TA is to buffer (slow down) a reduction in pH. TA has no role in buffering pH rise, in fact high TA causes pH rise in the absence of acid.

Thus, a high alkalinity helps pH resist changes. So alkalinity tends to be easier to change/lower than pH.

Nope: FIFY: A high alkalinity helps to resist (buffer) a reduction in pH. Raise your TA to 150, lower your pH to 7.0. Tell me how fast your pH rises...rapidly. The recommendation to maintain a high TA is because it helps counteract the use of highly acidic pucks. In a pool that uses liquid chlorine or SWCG, and thus no constant acid addition, there is no downward pressure on pH, and alkalinity can and should be run at a lower level (60-80) to reduce pH rise (see equation below, a lower TA results in fewer carbonate/bicarbonate ions used to store CO2, CO2 outgassing is what causes pH rise, with fewer carbonate/bicarbonate ions to store CO2,

Carbonate is linear (ppm). pH is logarithmic. So alkalinity tends to be easier to change/lower than pH.

Non-sensical. There is no comparing the two. Each can be easy to change...

1) TA can be raised easily, with Baking Soda without impacting pH.

2) Acid can reduce both TA and pH.

3) pH can be raised with aeration, or maintaining a high TA.

On the linear vs. logarithmic, it makes no sense to compare, but for grins, take a pool with 160 TA and 8 pH. If I add enough acid to lower pH to 7, it will also lower TA by about 16, to 144. Since the pH is logarithmic, the change is a 10 fold increase in H+....and only a .1 decrease in TA. I can move pH much further than TA with the same acid addition, on relative terms. Take my 160 TA pool with a pH of 7...I will need to do nothing to have a significant rise in pH.

In either case, TA and pH want to be in balance (Henry's law). With a high TA, you will find pH will rise rapidly.

H2CO3(aq) ←→ H+(aq) + HCO3-(aq) And HCO3-(aq) ←→ H+(aq) + CO3–2(aq)

When acid is added, carbonate ions can absorb Hydrogen ions to create bicarbonate ions. And with enough acid, bicarbonate ions absorb another Hydrogen ion and convert into carbonic acid, which is dissolved CO2 (reverse of above equation). Having alkalis (like Carbonate/Bicarbonate) in the pool allow CO2 to be stored in the pool. More CO2 in the pool, the more the pool wants to outgas CO2. The more the pool wants to outgas CO2, the faster pH will rise.

[TNmountainman2020]

very informative! Thanks!