Muriatic acid not bringing PH down

[-+Cox]

We have a 1,600 l spa that I filled with softened water (under 100 ppm hardness). PH was tested at 8.7 and TA at somewhere around 350.

Decided to change to muriatic acid to bring the PH down to 8.0; which is the recommended level for the natural enzymes that we use, instead of the "PH Down" that we used in the past (Expensive - need almost 2 kg of it). We also use a UV light in conjunction with the enzymes and shock every couple of weeks with industrial strength hydrogen peroxide (H2O2). Changed to H2O2 because I developed an allergy to DiChlor.

Using online resources, it was suggested that 40 ml to 50 ml of MA would bring the PH level down to the desired level. Well, over a period of two weeks, I have now dumped a litre of MA into the Spa, about 70 ml per day. PH drops but bounces back up overnight and we are at perhaps 8.2 right now after a litre of the stuff; measured with test strips as well as drops.

I don't get it lol. With that amount of MA the water should be acidic enough to dissolve corpses (sorry, bad timing for this kind of joke).

[The Wizard of Spas]

I am not sure about conversions, so I will not speak to the volumes you've added.  I'll speak to the process, however-

When trying to reduce pH / alkalinity, when adding the reducing agent (in this case- muriatic acid) it is most beneficial to add the agent while the water is calm to the deepest area.  With muriatic acid, I'd dilute with water first.  After adding, leave the water calm for an hour or more.  This will yield the best results for pH / alkalinity reduction.  I stress- This doesn't mean it won't work if you let the jets run.  Just a more effective process is all.

Leaving the air on with your jets will also aid in pH increasing.  Turning off the air (and thus leaving just 100% water running through the jets) should be something you can look into to help aid in your process.

Regardless- I'd concentrate on reducing your alkalinity first:  When your alkalinity level is HIGH, the pH will creep up.  The higher it is, the faster it creeps up. 

Moving forward, lets say you added up all the amounts of muriatic acid you've put in over the past week or so.  My advice is to total it all up and then add it all at once, via the method I laid out, and then re-test after a spell.  Then, I'd write down everything you did, so you can refer back to it and know what the proper amounts, process, etc. to follow.

I hope this makes sense.  Good luck moving forward. 

[castletonia]

What is your total alkalinity now?  High alkalinity can drag your pH up and also prevent it from dropping.  Also keep in mind that aeration causes pH to rise.

[Tman122]

There are a ton of different strengths of MA also. What does the label say?