Sodium bisulfate is the most common ingredient used in 'pH down' products, used to lower a hot tub's pH or alkalinity. With a high pH value or too much alkalinity, your sanitizer will not be effective, and you'll eventually see green algae growing in the water.
But what if you're out of sodium bisulfate, and wondering what you could put in your hot tub instead? You know you need to keep the total alkalinity and the pH down to ensure your tub is balanced, but what else can you use to do this?
Muriatic acid will do the same job as sodium bisulfate, lowering the pH value of the spa, and also reducing the alkalinity.
It's sometimes also called hydrochloric acid, although hydrochloric acid is actually a purer and more toxic form. Either will help to bring down a high pH—although there are some safety concerns when handling these strong chemicals.
Is muriatic acid a good alternative to sodium bisulfate?
Muriatic acid's only byproducts are water and salt, which actually makes it preferable to sodium bisulfate in some ways. You also need much less of the chemical to get the same effect, as it is more concentrated.
On the other hand, muriatic acid is very potent, and therefore much more dangerous to handle. It comes in liquid form, which means there's a higher risk of spilling it. You need to wear a mask, boots, goggles, and acid-proof safety gloves to handle it. Do not handle muriatic acid without this safety gear.
Sodium bisulfate does not carry these risks, which is why most people prefer to use this in their hot tubs. It's easier to use and much safer to handle.
It's possible to use sulfuric acid to lower a hot tub's pH or alkalinity.
However, sulfuric acid is even more potent and toxic to handle than muriatic acid, and therefore shouldn't be used by anyone other than professionals. It is more commonly used in commercial pools and spas for this reason.
Vinegar is also an acid, and so it can be used to lower the pH (and to a lesser extent, alkalinity) of your hot tub.
It is less potent than a dry acid like sodium bisulfate, so you would need to add about 4 cups of white vinegar (NOT apple cider or wine vinegar!) per dose—much more than the teaspoon or two of sodium bisulfate that would be required to achieve the same effect.
In most cases, although adding vinegar to your hot tub will have an effect on the pH value or the alkalinity readings, it isn't an effective long-term replacement for sodium bisulfate. You need to add much larger quantities to get the readings you want, and it can also give your spa an unfortunate vinegary smell.
You can technically use muriatic acid, sulfuric acid, or even vinegar to achieve the same effect as sodium bisulfate if you haven't got it. But not all of these are recommended, due to either the risks of handling them or (in the case of vinegar) the amount you would need to get the effect you want.
Even with regular hot tub chemicals, make sure you take safety precautions by wearing protective gear (gloves and goggles at a minimum) and read the instructions that come with each product before you use it. Never handle strong acids without the appropriate protection, as you could seriously hurt yourself.