Effects of adding Ascorbic Acid?

[cledee]

@Leebo I know that Ascorbic Acid affects PH/Alkalinity, but I'm never sure how much it lowers PH/Alk,

Can this be added to the PoolMath app?

 

[Texas Splash]

@Donldson, I'm curious if you would know of any way for a user to estimate how much of an impact AA might have on a pool's pH and/or TA? Key points like the pool details, current pH, TA, and amount of AA being dispensed would effect the results. But since AA isn't captured in the Poolmath APP or the old Poolmath webpage, could there be any way for a user to know - ballpark?

 

[JamesW]

176.124 grams of ascorbic acid dissolved in water produces 1 mole of hydrogen ions.

Muriatic acid is about 10 moles per liter for 31.45% Hydrochloric acid.

So, 176.124 grams of ascorbic acid dissolved in water is equivalent to about 100 ml of 31.45% (20º Baumé) hydrochloric acid.

0.388 lbs = 3.38 oz 31.45% (20º Baumé) hydrochloric acid.

1 lb. of ascorbic acid = 8.71 oz 31.45% (20º Baumé) hydrochloric acid.

 

[JamesW]

Every pound of ascorbic acid in an 18,000 gallon pool will lower the TA by about 1.9 ppm.

 

[Texas Splash]

Now there's some math. Thanks James.

 

[JamesW]

Actually, the effect on the TA is lower due to the pka of ascorbic acid.

1÷(1+10^(4.17 – 4.5)) = 0.68.

So, the percentage that remains as ascorbate is 68%.

So, the effect on the TA is actually 32% of the calculated value.

So, 1.9 X 0.32 = 0.68 ppm.

The effect on the pH is the same as the original calculation.

A weak acid with a pka at or above 6.5 like carbonic acid (carbon dioxide) or cyanuric acid will have no effect on the TA because the hydrogen ions that dissociate when the weak acid is added to the water will be picked up by the conjugate base as the sample is titrated down to a pH of 4.5.

However, for acids with a pka between about 2.5 and 6.5 there will be a partial effect on the TA.

 

[JamesW]

As the ascorbic acid neutralizes the chlorine, the reaction can produce more hydrogen ions.

One gram of ascorbic acid will neutralize 1 milligram per liter of chlorine per 100 gallons of water.

2C5H5O5CH2OH + HOCL + OCl → 2C5H3O5CH2OH + H + 2Cl + 2H2O

Ascorbic acid + Hypochlorous acid + hypochlorite → Dehydroascorbic acid + Hydrogen ion + chloride + water.

The Dehydroascorbic acid probably breaks down over time as the carbon gets oxidized.

2C6H6O6 + 6H2O + 9OCl + 9HOCl --> 12HCO3 + 21H + 18Cl.

2C6H5O6 + 9OCl +9HOCl --> 12CO2 + 6H2O + 7H.

So, the reaction with chlorine produces another 0.5 moles of hydrogen ions per mole of ascorbic acid and the breakdown of the Dehydroascorbic acid produces another 3.5 moles of hydrogen ions per mole of ascorbic acid for a total of 5 moles of hydrogen ions per mole of ascorbic acid added.

So, the long term effect of adding 1 pound of ascorbic acid to 18,000 gallons of water should be a 9.5 ppm reduction in the TA.

C6H8O6 + 5OCl + 5HOCl --> 6CO2 + 4H2O + 5H + 10Cl.

 

[cledee]

Thank you @JamesW @Texas Splash ! This is really helpful. I have to do a AA treatment about every 2 years and I always noticed a big drop in PH with just the Ascorbic Acid and Sequestrant so I would never have to add any MA to get it to 6.8 - 7.0.

So 3lbs of Ascorbic Acid would drop TA by 28.5 ppm? @JamesW

 

[JamesW]

The ascorbic acid loses one hydrogen ion (deprotonates) when the ascorbic acid is added to water and that lowers the pH and TA.

That converts the ascorbic acid to ascorbate.

C₆H₈O₆ --> C₆H₇O₆^- + H⁺

So, 1 lb. of ascorbic acid = 8.71 oz 31.45% (20º Baumé) hydrochloric acid for this part or 1.9 ppm reduction in TA for 18,000 gallons.

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The ascorbate reacts with chlorine and this produces another 0.5 moles of hydrogen ions per mole of ascorbic acid added.

So, 1 lb. of ascorbic acid = 4.355 oz 31.45% (20º Baumé) hydrochloric acid for this part or 0.95 ppm reduction in TA for 18,000 gallons.

2C₆H₇O₆^- + HOCL + OCl^- → 2C₆H₅O₆^- + H⁺ + 2Cl^- + 2H₂O

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The dehydroascorbate reacts with chlorine and this produces another 3.5 moles of hydrogen ions per mole of ascorbic acid added.

The dehydroascorbic acid (dehydroascorbate) breaks down over time as the carbon gets oxidized and this produces more acid and it uses up more chlorine.

So, 1 lb. of ascorbic acid = 30.485 oz 31.45% (20º Baumé) hydrochloric acid for this part or 6.65 ppm reduction in TA for 18,000 gallons.

2C₆H₅O₆^- + 9HOCL + 9OCl^- → 12CO₂ + 6H₂O + 7H⁺ +18Cl^-

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The total reaction can be written like this.

C₆H₈O₆ + 5HOCL + 5OCl^- → 6CO₂ + 4H₂O + 5H⁺ + 10Cl^-

So, 1 lb. of ascorbic acid = 43.55 oz 31.45% (20º Baumé) hydrochloric acid for this part or 9.5 ppm reduction in TA for 18,000 gallons.



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[JamesW]

So 3lbs of Ascorbic Acid would drop TA by 28.5 ppm? @JamesW

That is the total drop with the breakdown of the dehydroascorbic acid, but we do not know how long that takes.

The initial drop is 2.85 ppm per pound or 8.55 ppm for three pounds immediately (Acidity + Chlorine Reaction).

The breakdown reduces the TA by 6.65 ppm per pound or 19.95 ppm for three pounds.

The total reduction in TA should be 28.5 ppm for 3 pounds of ascorbic acid added to 18,000 gallons of water.

Some people report that they cannot get the chlorine level back up after an ascorbic acid treatment and this might be due to the reaction with the dehydroascorbic acid.

If you do another treatment, record the results and report them to see it the results match the calculations.

If the dehydroascorbic acid breaks down quickly, then the TA should drop and the FC will be harder to maintain.

However, if the dehydroascorbic acid takes a long time to break down, then the effect will be gradual and less noticeable.

At this point, I do not know if the dehydroascorbic acid breaks down in hours, days, weeks, months or years.

 

[JamesW]

So, to simplify the answer somewhat, expect a TA drop of about 10 ppm immediately and another 20 ppm "eventually" if you add 3 pounds of ascorbic acid to an 18,000 gallon pool.

If the pH and TA drop as you begin to bring the FC back to normal after the treatment, then the dehydroascorbic acid is probably breaking down especially if the FC does not seem to be coming back as quickly expected.

 

[JoyfulNoise]

Excellent analysis.

Unfortunately this would not be easy to add to pool math as there are too many unknowns around how the acidic chlorine reactions would happen. I suppose one could put in a simple estimate of the effect with some strong warnings about what to expect. AA treatments are not universally used by most pool owners (I’ve never had to use it), so it has a limited audience.

 

[cledee]

Excellent analysis.

Unfortunately this would not be easy to add to pool math as there are too many unknowns around how the acidic chlorine reactions would happen. I suppose one could put in a simple estimate of the effect with some strong warnings about what to expect. AA treatments are not universally used by most pool owners (I’ve never had to use it), so it has a limited audience.

I'm going to do a treatment in the fall, 2 weeks before I close and report back on initial TA loss, TA after 24 hours, TA when starting to add Chlorine, and TA once Chlorine starts to hold.

 

[JamesW]

Dehydroascorbic acid spontaneously decomposes to 2,3-ketogulonate and then to threonic acid and oxalate.

Oxalic acid/oxalate gets further oxidized to carbon dioxide.

HOCl + HC₂O₄^- --> H₂O + 2CO₂ + Cl^-

Oxalic acid can also remove rust stains.

Fe₂O₃ + 3H₂C₂O₄ --> Fe₂ (C₂O₄)₃ + 3H₂O

Iron Oxide (Rust) + Oxalic Acid = Iron(III) Oxalate + Water



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