Oxalic Acid Treatment and Calcium Oxalate Worries

[solarboy]

The start of the conversation was split off of this topic. JasonLion

The nice thing about oxalic acid is that chlorine will oxidize it to carbon dioxide whereas ascorbic and citric acids will leave some soluble organic remnants (though the amount you use is small so not a problem).

Does this mean my TA will rise ? It was 100 before I started. I'm thinking maybe I'll need to aerate.

 

[chem geek]

Re: Ascorbic Treatment to rid Pool of metal stains

No, adding carbon dioxide to the water does not change the TA, but it does lower the pH. Yes, aeration will raise the pH with no change in TA so if you find that your pH is too low then that is what you can do.

 

[solarboy]

Re: Ascorbic Treatment to rid Pool of metal stains

Thanks Richard. What started off as "lets try a bit of that Oxalic acid in a sock and rub off that iron stain" rapidly turned into more than I was willing to get involved in at that moment (Pool went milky, probably Calcium and pH dropped to 7.0). A few searches and I now know my concrete shell isn't dissolving before my eyes
I think I'll aerate overnight and add Na2CO3 if needed in the morning.

 

[solarboy]

Re: Ascorbic Treatment to rid Pool of metal stains

From Wikipedia......The "gritty mouth" feeling one experiences when drinking milk with a rhubarb dessert is caused by precipitation of calcium, abstracted from the casein in dairy products, as calcium oxalate.
I'm guessing that's what the milkyness in my pool is, so if I flocc this and filter it out I should see a drop in the CaCl2 I just spent money on to lower my CSI. Hopefully it will slowly dissolve back into solution. At the moment it just seems to be precipitating in large white clumps on the bottom of the pool.
Seems like a nifty way of dropping Calcium levels for some people.
http://en.wikipedia.org/wiki/Oxalate

 

[solarboy]

Calcium Oxalate worries

I've just used Oxalic acid the try and remove some stains as described here:

ascorbic-treatment-to-rid-pool-of-metal-stains-t2298-60.html

As I explained in that post, the water turned cloudy, probably due to precipitation of Calcium Oxalate which I then got to clump together with flocculant and is now collecting in large drifts on the floor of my pool.

So in my research to try and get the Calcium back in solution (it's somewhat useful to keep my CSI in check) and the Oxalic acid to combine to form CO2 and I came across the wiki for Calcium Oxalate;

http://en.wikipedia.org/wiki/Calcium_oxalate

It sounds like a horrific chemical to have in drifts across the floor of my pool so I'm (probably totally overreacting) starting to worry what I should do with it?

Any help greatly appreciated.

I promise not to mess with chemicals without researching them first.
I promise not to mess with chemicals without researching them first.
I promise not to mess with chemicals without researching them first.
I promise not to mess with chemicals without researching them first.
I promise not to mess with chemicals without researching them first.
I promise not to mess with chemicals without researching them first.

 

[JasonLion]

Re: Calcium Oxalate worries

Do you add any sequestrant at any point?

 

[solarboy]

Re: Calcium Oxalate worries

No Jason, we don't have that available here, my plan was to shock and floc the iron to waste as I normally do with iron. In my experience it doesn't stain as it may with plaster pools (there are no plaster pools that I know of here).

 

[chem geek]

Re: Ascorbic Treatment to rid Pool of metal stains

The solubility product of calcium oxalate (CaC₂O₄) is 2.32x10^-9 for the monohydrate to 1.5x10^-8. Oxalic acid is essentially fully dissociated at pool pH -- it's essentially a strong acid for your purposes which is why you saw the pH drop so much. The calcium concentration at 300 ppm is around 2.9x10^-3 (accounting for ionic strength) so precipitation might start occuring when the oxalic concentration gets above 8x10^-7 or only 0.07 ppm. So it's not surprising you saw it cloud up. It basically limits how much you can realistically add. Given the range of K_sp and the over-saturation usually needed, I'd say you might be able to get away with around 1 ppm oxalic acid before you start running into trouble. That would be only 1.3 ounces weight per 10,000 gallons -- I bet you used a lot more than that.

Now the good news is that as chlorine oxidizes the oxalate, the precipitate should dissolve so if you waited long enough you could get rid of it, but of course if there is a lot of it then it's probably better just to filter it out.

As for being a Calcium Hardness (CH) remover, that's interesting and sounds like it should work -- one would make sure the chlorine is low when starting since the chlorine is just going to get used up in the process. Let us know, however, if you notice any calcium oxalate scale forming on surfaces or if it, hopefully, just clouds the water and forms a precipitate that can be removed via filtration, possibly aided by clarifier or flocculant. The good news is that once it is removed, any remaining oxalate can be removed by chlorine. Of course, removing any substantial amount of CH would produce a lot of precipitate -- probably too much to filter so flocculation would be the main approach.

 

[solarboy]

They say a little knowledge is a dangerous thing !
I think I added around 8 ounces per 10k Gallons which seems to put me in the trouble zone . That has put my mind at ease knowing that chlorine will oxidise the oxalate and I think I will keep FC at shock levels until there is no sign of any cloudiness or precipitate.
The precipitate doesn't seem to have formed any scale but has clumped together (I added 3 x standard dose of aluminium sulphate related flocculant) and settled on the bottom. I'll vacuum it tomorrow which should give me time to come up with an excuse for my darling wife.
Thanks fellas

 

[solarboy]

Curiouser and curiouser. I found that the flocculated sediment mainly went straight through my sand filter, even with DE, and returned to the pool leaving me back where I started, unable to see the main drain. Some was staying in the filter and as pressure rose I did a short backwash of around 80 gallons. I then upped my FC to 15 (shock level for my CYA of 40, I used 10 oz of dichlor as I'm upping my CYA) and this seemed to work the best, oxidising the Calcium and the water started to clear. Then I started to do a full test of all levels, just to report back what had changed. pH had dropped from 7.5 to 7.0, Calcium had dropped from 260 to 220, TA stayed roughly the same around 60-80 but the strange thing was the CYA reading, which had shot up from 40 to 100! I did the test twice (disappearing drop test) and I tested with pool water with no tablet to check I wasn't just seeing the murky water. I then tried a test strip (Aquacheck) and got a reading of 40 more or less. As I've filled the pool just in March I have records of every chemical with CYA I have added, and using The Pool Calculator I was able to work out that all the dichlor and trichlor I have added should have added a total of 56ppm to my water. So it seems this Oxalic treatment process I have stumbled through has affected the dot-based cya test. I guess I'll wait and see what the reading is when the water returns to normal.

 

[chem geek]

So it seems this Oxalic treatment process I have stumbled through has affected the dot-based cya test. I guess I'll wait and see what the reading is when the water returns to normal.

I'm guessing that between the precipitate in the water and the floc in the water, they have conspired to cause more cloudiness in the melamine-cyanurate test. That test has a pH buffer that makes the pH low so perhaps that reacts with the floc. 'Tis interesting.