Just some friendly advice for the OP, to heed the advice above from others as they had to guess what you meant because what you said made no sense (but they could extrapolate what you had meant). Allow me to explain...
You said:
- My pool was having excessive need for acid so I discovered the alkalinity was low.
That makes no sense to us for a few reasons, not the least of which is the pool never "needs acid", per se, except when LOWERING alkalinity (or lowering the pH).
Even so, the alkalinity being low would mean your pH ceiling would be low which means you'd need LESS acid; not more acid.
You then said:
- I have put several lbs of baking soda into it (12, 000 gal) and it finally was on the verge of 70.
That makes sense if your carbonate alkalinity was low, which you now have at total alkalinity of 70 ppm which is just fine.
After that you said:
- I put more acid in to get it to 7.4 and all was good last night.
You probably mean pH of 7.4, which is just fine, although with a CYA outdoor pool, the pH isn't all that important for disinfection.
The pH only matters, for the most part, on an outdoor CYA pool for your saturation index situation to work on scaling/aggressiveness.
As people said, simultaneously adding acid (which lowers the pH) and baking soda (which raises the pH) seems to be counterproductive.
But bear in mind what I said about the pH not being all that important in an outdoor CYA pool for disinfection purposes.
Then you said:
- This morning the acid was 7.79 so I decided to add another lb of baking soda and acid.
Again, you mean pH most likely, where a pH of 7.8 is just fine for an outdoor pool with CYA since pH doesn't matter to disinfection.
The pH only matters for the calcium saturation where you don't seem to know about that yet so for you - for now - stop chasing pH.
Then you said:
- Now 2 hrs later I check and the pH is 7.5 but the alkalinity is 40.
If you added acid, then you "burned" off your carbonate alkalinity; so it's to be expected that your total alkalinity will drop.
As people said, if you're simultaneously adding acid & carbonate alkalinity, they'll defeat each other.
Then you asked:
- How did it drop to 40 in about 12 hrs with nothing really added?
I suspect the acid burned off the carbonate alkalinity, which turned it into carbon dioxide & water, which lowered the total alkalinity readings.
Then you assessed:
- I'm overwhelmed at this point and don't want to fill the pool with even more baking soda.... I tested the alkalinity twice and then once again with new water.
I understand. You're chasing all the wrong numbers it seems. What you need to do is stop chasing ranges.
Take the advice of people here to do one thing and one thing only.
- Report your pool chemistry numbers
- People here will then tell you what you need to do (if anything).
Also note the pool math calculators will also tell you what you need to do
But you have to be careful to not simply chase ranges
Lastly, you noted:
- We did get a bit of rain, would this cause a problem with alkalinity?
Rain water is essentially distilled water, which will basically dilute almost everything.
Therefore, depending on the amount of rain, the answer is it will lower your alkalinity.
Overall, my recommendation for all newbies is the following basic (but sound) advice:
- Test your pool chemistry yourself (and report the results here where people will kindly help you).
- (most people would recommend you use a good pool-math calculator on your mobile device)
- Most would also recommend you use only liquid chlorine (no tablets, no granules, no powders) for sanitization
- (keep your cyanuric acid level as low as feasible, where 30 ppm is a good inflection point to aim for)
- For sanitation, maintain free chlorine at least 7.5% higher than the CYA, and then plus 1 or 2 ppm)
- (maintain free chlorine at less than 7.5% of your CYA - e.g., half that - only if your phosphates are extremely low)
- (this eclectic algae assessment is rarely discussed but it came from either Richard Falk or Bob Lowry, as I recall)
- For saturation, maintain the water's equilibrium within +/- 0.33 of the zero-crossing point
- (generally that means aim for calcium saturation & carbonate alkalinity to be within typical ranges)
- (not many people think about it this way, but a common ratio is 1:4 alkalinity ppm to calcium ppm)
- (keep in mind the six factors which affect the saturation index are temp, pH, carbonate alkalinity, calcium hardness, CYA alkalinity & TDS)
- (but you have little control over the temperature & not much control over the pH and/or TDS)
- (the CYA alkalinity is about 1/3rd of the CYA ppm & the carbonate alkalinity is about 10% lower than the total alkalinity)
- (but the pool math calculators take all those equations & square roots into account for you)
- If you don't fully understand any of those basics, don't worry as everyone here knows them inside & out
- (so you can read up on each of them & ask away if necessary).
