Why di Soda Ash turn pool cloudy

[eastonfmly]

I have 25K gallon vinyl pool and have been taking care of it for over 20 years. Lately with all of the rain, I had a PH of 6.5 and Alkalinity of 40. I usually use Baking Soda but decided to use Soda Ash instead because it was more convenient. I added 8 lbs premixed into the deep end right above the main drain. I instantly saw a white fog engulf the pool. It has taken a couple of days to clear up but has left white sediment on the bottom which I am brushing up. I have been through the forums looking at other people who have posted and no one has really explained why it turns the pool cloudy. Can anyone explain the chemistry as to what made my nice clear pool into a white fog and is it normal?

Thanks so much

Jim

 

[jaduck]

This sounds like CaCO3 (Calcium Carbonate) precipitation. The chemistry goes like this: CaCl2 (Calcium Chloride) + Na2CO3 (Soda Ash) --> CaCO3 (Calcium Carbonate) + 2NaCl (Salt). When I add Soda Ash (which is rarely) I do so very slowly, in multiple batches over several hours. Dumping in 8 Lbs of Soda Ash, even predissolved, can cause this kind of precipitation.

 

[JoyfulNoise]

Another way I've seen it added is in several small batches that are pre-dissolved in a 5 gal bucket of water and then poured very slowly into the skimmer. That way any calcium carbonate formed has a good chance of being caught by the filter. Doing it in batches lets you add a few and then retest pH to see if you need to keep going or not.

Raising pH is always tricky as you need to be careful not to raise it too far too fast in order to avoid calcium scaling.


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[chem geek]

When added to a pool that has some calcium in it, the soda ash (aka washing soda) will raise both pH and TA. It's very high locally so the saturation index is high and precipitation or cloudiness from calcium carbonate is the result.

Na₂CO₃ + H₂O ---> 2Na⁺ + HCO₃^- + OH^-
Sodium Carbonate + Water ---> Sodium Ion + Bicarbonate Ion + Hydroxyl Ion
...................................................................... +TA ................ +pH +TA

8 pounds of soda ash added to 25,000 gallons with starting pH of 6.5 and TA of 40 ppm results in a pH of 7.25 and TA of 76 ppm, but locally until it fully dilutes in the water the saturation index is much higher. In 100 gallons the saturation index (with CH 300 ppm) is +2.56, in 1000 gallons it is +2.46, in 10,000 gallons it is +1.46 so calcium carbonate is formed and doesn't dissolve until the dilution is into at least half of the pool water.

The reason this doesn't happen as much with baking soda is that it raises the TA but doesn't raise the pH as much.

NaHCO₃ ---> Na⁺ + HCO₃^-
Sodium Carbonate + Water ---> Sodium Ion + Bicarbonate Ion
...................................................................... +TA

12.7 pounds of baking soda (for the same TA increase as the above soda ash) added to 25,000 gallons with starting pH of 6.5 and TA of 40 ppm results in a pH of 6.77 (ignoring carbon dioxide outgassing) and TA of 76 ppm, but locally until it fully dilutes in the water the saturation index will be somewhat higher, but not nearly as high as with the soda ash. In 100 gallons the saturation index (with CH 300 ppm) is +1.79, in 1000 gallons it is +1.07, in 10,000 gallons it is -0.37 so there is far less calcium carbonate formed.

 

[eastonfmly]

Thank you to everyone, I probably will stick to Baking Soda in the future. But the pool stores sure have alot of this stuff in stock.

 

[woodyp]

Thank you to everyone, I probably will stick to Baking Soda in the future. But the pool stores sure have alot of this stuff in stock.

At the prices and markup they sell it at, wouldn't you?

 

[chem geek]

For raising TA, you generally want to use baking soda anyway. For raising pH with no change in TA, you can aerate the water. If you want to raise the pH without raising the TA as much as with pH Up, then you can use 20 Mule Team Borax.