We have 500 gal hot tub and have had it for about a year now. I have yet to be able to get PH and alkalinity into spec at the same time - as a result I have focused on PH.

I have yet to find anyone locally who can help with this issue.

If Ph is in spec, say about 7.6, my alkalinity will be about 20ppm. If I bring ALK into spec, say 100, my PH is north of 8.6. When I bring my PH back down (PH down/sodium bisulate), ALK goes too low.

When I raise ALK, I am using Sodium Hydrogen Carbonate (baking soda) which I have read is supposed to move PH minimally - in my tub, it moves PH up a lot.

I have had my ColorQ system verified by a local shop so I know it is not my tester giving me false readings. For example, the sample I brought to them yesterday, had PH at 8.1 with ALK at 47ppm. I followed their recommendation to bring both in spec(ph down and ALK up), I tested this morning and had a PH of 7.7 and ALK of 25.

Has anyone had this issue? Ideas?

Thank you


  • My chem lessons were a very long time ago, but if I am not mistaken PH measures alkalinity, and ppm is the amount of stuff in a solution. Using more than one chemical should change ppm numbers from what they should be. I would start with clean water again, measure the water first, and then try to do the adjustment. Pure water should be at 7.0 PH, but water is never pure out of a tap.
    – crip659
    Mar 25, 2023 at 18:24
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    Alkalinity and pH are the same thing - something is wrong with your testing regimen. You shouldn't be using random-ass chemicals, you should be using chemicals designed and approved for treatment of hot tubs. That may also explain why your testing isn't agreeing. Mar 25, 2023 at 21:10
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    I’m voting to close this question because it does not have anything to do with home improvement.
    – RMDman
    Mar 25, 2023 at 21:22
  • Thank you, JD74 and Rohit for your constructive responses. Rohit, baking soda is commonly used as alkalinity up and even sold as such ("Natual Chemistry" brand ) under the IUPAC name sodium hydrogen carbonate - 'Baking Soda' (Harper that's for you too!).
    – RTC
    Mar 26, 2023 at 10:00
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    I don't understand why this is off-topic. Many people have hot tubs at their homes which need repair and maintenance, and there isn't a pools and hot tubs site on stack exchange.
    – Mark
    Mar 26, 2023 at 15:11

2 Answers 2


I have a salt water pool and have to adjust ph and alkalinity. The ph likes to climb and I use muriatic acid to lower it. It will also lower alkalinity. Alkalinity at the right level should buffer and stabilize your ph. My best advice is to make slow adjustments or you wind up overshooting. Take your time and only add half or less of what you think you need to add. Give it time to adjust. If you are on Facebook there a couple of different sites related to pools and hot tubs you can join. They can be great sources of information. Also it's better to buy a test kit that you put drops in rather than us test strips.


You have to use chemicals from the Pool Shop. Baking Soda is not going to do it economically as it changes both pH and Alkalinity.

Crudely speaking, Alkalinity (Buffer) is the amount of free H+ ions. pH is the ratio of H+ to OH- ions. To minimise changes to pH, you need to have the Alkalinity where it is supposed to be, first. Alkalinity is a misleading name for it. It should be called Buffer.

Here is a good article on it. I am associated with the website as it belongs to a friend. Nevertheless, you should read it.

How to Look After Your Pool and Spa?

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