Rates of Addition for Various Chemicals

[bluenoise]

Hello, all-

When I need to make some adjustments to things like calcium hardness, total alkalinity, etc. I have to add certain quantities of chemicals. I've seen mentioned at various times to "add no more than XXX ounces per day" or other indications that there is a limit to how much can be used at once. Now that I am trying to find that info, however, I'm unable to find the specifics. In the spirit of this, I'm hoping some of the chemistry experts here can put all the important information on maximum rates of addition for the various chemicals we use to balance our pools. Likewise, it would be useful to know which chemicals can be added together or in the same pump cycle.

If this exists already, I couldn't find it.

 

[JasonLion]

I don't know of any table that lists everything in one place. Here are a few of the restrictions/interactions I remember at the moment.

Baking soda - I suggest 2 lbs per 10,000 gallons max per addition, wait four hours and retest to see where you are. The reason for this escapes me at the moment.

Calcium - You don't want to add calcium on the same day as anything that raises the PH (soda ash, baking soda, shocking the pool), I would give it a full 24 hours to be safe. Calcium can percipitate out and cloud the pool if added too quickly or if the PH is raised too soon after an addition and won't go away if the CSI is positive (to fix add acid to lower CSI and get it to redisolve). If your CSI is positive you might need to add in small increments to avoid clouding.

CYA - takes up to a week to disolve.

 

[chem geek]

If a pool is somewhat near calcium carbonate saturation (i.e. saturation index near 0), then adding pH Up / Sodium Carbonate / Arm & Hammer Super Washing Soda, or Alkalinity Up / Sodium Bicarbonate / Arm & Hammer Baking Soda, or Calcium Chloride (anhydrous or dihydrate) should be done slowly with good mixing in the general body of pool water because clouding can occur. This is because locally where you add the product, the concentration of what you are adding is very high so you force the water into being over-saturated in calcium carbonate. In fact, if you dump in pH Up quickly, you can easily form hard calcium carbonate chunks, though it will slowly dissolve unless the water is over-saturated. I've done this

I'm not sure why baking soda would have a limit on the amount of addition, assuming one accurately calculated how much to add. It is true that the calculations for changing pH are complex so the recommendations on the bottles of acid and base (pH Up) are only for standard water parameters. For example, at a TA of 100, adding an amount of acid that lowers the pH from 7.8 to 7.5 will only lower the pH from 7.2 to 7.07. Likewise, adding an amount of pH Up that raises the pH from 7.5 to 7.8 will only raise the pH from 7.0 to 7.13. However, baking soda adds to TA independent of the starting pH or TA so I'm not sure why it would be necessary to only add a smaller amount at a time.

Richard

 

[JasonLion]

I got the 2 lbs per 10,000 gallons of baking soda rate from Ben, here. He doesn't explain his reasoning, though he sounds fairly adamant about it. Ben's advice is usually quite good.

When adding baking soda there will be some PH change, though it only seems to be significant when TA and/or PH start very low. The PH changes are somewhat self limiting, as the PH change becomes smaller as either TA or PH rise, which adding baking soda will do. The most extreme situation I could come up with that was vaguely plausible (16 lbs of baking soda per 10,000 gallons, starting at a TA of zero) raised PH from 7.5 to 7.9, or from 7.7 to 8.0. That wouldn't be good but it wouldn't be a disaster either.