Citric Acid's effect on pH & Chlorine Neutralization

[smallpooldad]

Going to help a friend remove slightly heavier than normal iron staining from a 20,000 gallon pool. We will be using 5 lb of Citric Acid.

The question is by how much will the pH go down if we start at 7.3, per 1 lb of Citric Acid added to 10,000 gallons, and how many ppm of chlorine, will 1 lb neutralize for 10,000 gallons.

Normally I use Ascorbic Acid, however Citric Acid is 50% less expensive here in Honolulu. Is it harder than Ascorbic Acid to re-balance the Chlorine, in other words will we need more.

Although I found this interesting post from chem geek, regarding Citric Acid, pH, Chlorine, and Chloroform, unfortunately no definitive numbers in this post:

http://www.troublefreepool.com/adde...-will-it-consume-chlorine-t37914.html#p310434

And this from Lenntech:

http://www.lenntech.com/processes/disinfection/swimming-pool-/swimming-pool-disinfection-health.htm

Fortunately we have more than enough wind in Hawaii, most of the time, to remove Chloroform.

Thank you to those that reply.

 

[chem geek]

Citric acid is over 3 times as acidic as ascorbic acid on a molar basis. Citric acid anhydrous weighs 10% more than ascorbic acid on a molar basis.

It takes 3 moles of hypochlorous acid to oxidize 1 mole of citric acid to carbon dioxide, chloroform and malonic acid. It takes 1 mole of hypochlorous acid to oxidize 1 mole of ascorbic acid to dehydroxyascorbic acid. So it should take roughly 3 times as much chlorine to react with an amount of citric acid compared to ascorbic acid.

 

[smallpooldad]

Chem geek,

Thank you for the very interesting reply.

For each 10,000 gallons using the following assumptions, if items are purchased in Honolulu:

1. Chlorine is neutralized to zero using either Thio-Trine, or Hydrogen Peroxide. No difference in cost here.

2. One uses say 2 lbs of Citric Acid, instead of 1 lb of Ascorbic Acid. Total cost of CA about $8.00, AA about $19.00. Saving $11.00.

3. Chlorine required to bring it back up to 6 ppm. With AA, about 2 jugs of Walmart regular bleach (from personal experience), total cost about $8.50. With CA, 3 x 2 jugs, about $25.50 (no experience). About $17.00, more expensive.

4. Borax to raise pH back to 7.3, is about 1 box for AA, about $6.00. I am assuming that with CA, one might need about the same amount of Borax, as the increased amount chlorine required is three times as much, and should raise the pH by a greater amount.

In total the overall cost would be greater with CA than with AA, by about $6.00.

Do these numbers/measurements look about right to you? I am sure as I have no personal experience with CA, they might be incorrect.

If, however CA is actually more expensive than AA overall, then obviously It might be better in our case, with our costs, to go the AA way. This is particularly true if one considers that AA has no issues with chloroform.

One last question, would the CA as it is over 3 times more acidic than AA, drop the 7.3 pH from where AA normally takes it to 6.8, to either 6.6, or is it 3 times the difference of 7.3 to 6.8 (0.5) meaning the pH would drop to 5.8? This then might require more Borax.

Fortunately he has not yet bought the CA.

Thank you for your help.

 

[chem geek]

4. Borax to raise pH back to 7.3, is about 1 box for AA, about $6.00. I am assuming that with CA, one might need about the same amount of Borax, as the increased amount chlorine required is three times as much, and should raise the pH by a greater amount.
:
One last question, would the CA as it is over 3 times more acidic than AA, drop the 7.3 pH from where AA normally takes it to 6.8, to either 6.6, or is it 3 times the difference of 7.3 to 6.8 (0.5) meaning the pH would drop to 5.8? This then might require more Borax.

Actually, the oxidation of citric acid by chlorine raises the pH when it produces chloroform. It's not the same as when oxidizing nitrogenous compounds where the pH drops due to creation of hydrochloric acid. So you might be able to use less borax to raise the pH.

For the pH drop, it is not proportional because pH is a logarithmic scale and the pH buffering from the TA (bicarbonate/carbonate) gets stronger as the pH gets lower. If you have the pH normally drop from 7.5 to 7.3, then with 3 times the acidity it would drop to around 7.0.

 

[smallpooldad]

Chem geek,

Thank you for the very quick reply, I do understand.

Taking everything into consideration, I think it might be better go with AA, for three reasons; one it is a little less expensive, two there is no chloroform produced - albeit a minor issue if not doing the treatment too often, and most importantly I am used to dealing with AA.

Thank you.