after adding 8 lbs of baking soda

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tstex

Silver Supporter
Aug 28, 2012
2,177
Houston, TX
Hello to all, [And Happy St. Patrick's Day]

Needed to raise the TA so it called for 7.8lbs of sodium bicarbonate. I typically pour 2lbs [from a 4lb box] into a 5 gal bucket gradually as I add water up to 4 gal and stir rapidly. Then toss across the top of the pool...the bottom of the bucket has some white ooze on it, so I add another gal to dissolve and toss across the top.

After the final 2 of 8lbs added, I peered on the bottom and didn't see any white globs, so it's pretty well mixed, then pump to circulate.

The next day, [and I didn't notice it any other time], when the pump stopped it's 2nd of 4 cycles, I noticed the spillway of spa into pool when it dried looked like a heavy Calcium build-up, which couldn't happen that fast. My question is how long do any particulates from the sodium bicarbonate /H2O solution stay in solution? What I think I saw was some residual that dried on the spillway.

Thank you very much,
tstex
 
What were your chemistry numbers before and after? Baking soda will raise TA but, depending on aeration, can also cause the pH to go up as well. Bicarbonate additions will not cause scale but high pH can.
 
Hello James and Matt,

The pH was good...about 7.55-7.6, then a bit over 7.8 when taken 2-3 hrs afterwards. The TA was about 40...my CYA was so low bc I have been using all 8.25% bleach and no pucks - lots of rain too here in Houston, so I added pucks over a span of 14-18 days to raise CYA [and get a sm break from adding bleach :) ] The CYA also helped to keep the pH a little lower during the Sodium Bi-carbonate add as well. CH is good.

W summer coming soon, I wanted to raise the CYA to stabilize the FC, but surely needed to raise the TA, even though the pool water was pretty stable/equilibrium w regard to not having to add any MA, but for the CSI and temp increasing, I did need to raise the TA...pebbleTech looks good and nothing visually out of whack. I've been using spa twice a week and as recommended, add a very small amt of MA, but surely not enough to raise the ppm when recirculated in pool.

Thanks guys [and I changed "don't to didn't" :)

PS - if it was only carbonate, then they labeled the packing incorrectly...
 
Is the "scale" still there? If you turn off the spillway and let it dry, do you still get scale? Can you scrape off a sample?

You could have efflorescence developing. Post a picture of what you're seeing, that will help.
 
tstex said:
Will do...I will have to do so tomorrow...what is efflorescence from a pool perspective? what causes it?
Click to expand...

From our good friends at Wikipedia -

In chemistry, efflorescence (which means "to flower out" in French) is the migration of a salt to the surface of a porous material, where it forms a coating. The essential process comprises the dissolving of an internally held salt in water, or occasionally in another solvent. The water, with the salt now held in solution, migrates to the surface, then evaporates, leaving a coating of the salt.
Click to expand...

Essentially, this is a phenomenon where water infiltrates into or through a porous material and absorbs minerals from that material. In the case of most pool construction materials this is either concrete or some form of porous limestone. Porous stones typically have open and penetrable cellular structure (think of it like a foam) and water can easily infiltrate into these porous spaces. Once held inside the pore, the water will attempt to come into chemical equilibrium with the surrounding material. So, if the porous space is composed of limestone (calcium carbonate), then pool water sitting inside this space will begin to absorb calcium because the pool water can be easily undersaturated with calcium relative to it's equilibrium value. As the water moves through the material, either by penetrating from one side to another OR returning to the original surface it entered from, the newly saturated water can come back into contact with air and evaporate. As the water evaporates, the salts in it become more and more concentrated until they reach a point where they will precipitate out of solution. This precipitation can lead to surface deposits of salt mixtures (chlorides, carbonates, sulfates, silicates, etc) OR, if the evaporation and concentration changes takes place in the subsurface region, the salts can be deposited in the pores themselves. This recrystallization of salt materials can often lead to internal stress build up which can cause the near surface layers to shear away from the bulk. This is often what leads to spalling and cracking in stone materials.

In your specific case, it would not surprise me if the water was getting into your spillway stone work (since it is constantly submerged) and then, when the flow of water stops, the stones dry out. Once they begin to dry, a thin layer of deposited salts will be left on the surface (and they will not necessarily re-dissolve when the water turns back on). Over time and with many on/off cycles, the scale will build up and become noticeable.

Efflorescence will happen no matter what your water balance is. So it's always good to keep an eye on where deposits tend to build up the most and then try to treat just those areas. Sealing the stone spillway can help to stop the deposits but you would first have to stop running the spillway long enough to let the stonework dry out and then clean off whatever deposits are there. Then, after the stone is cleaned, you can seal it with a penetrating stone sealant designed to lock up the porosity and keep water form infiltrating the stone. Often, the best and fastest way to remove efflorescence and scale is to use soda blasting to mechanically remove it. Chemical treatments work as well but often involve fairly acidic solutions to remove scale. Scale can be composed of both carbonates and silicates and so acidic etching isn't always possible.
 
Thanks Matt - that's a very detailed and informative answer. I hope you save this in your file bc the next time someone asks the same or near same question, I would surely cut and paste.

My stone is tumbled travertine, both very porous and calcium-carbonate based...therefore, this could be what is happening. I will try to get some pic's today and display them here...just need to do so after the spillway dries and before the pump starts.

Again, thank you very much Matt for the great info - it is immensely appreciated - tstex
 
Ok, here are the pool readings:

FC = 5
CH = 275
TA = 70 [this is after adding 8 lbs of Sodium Bicarbonate earlier in the week and another previous 4lbs about 2 weeks prior]
pH = 7.5
CYA = 50
Pool temp = 74 [my inclination is that both my air and pool temps always read a little high, more so the air]

Let me know what you guys think? Thank you, tstex
 

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This seems to have happened quickly, so I don't think that it's efflorescence or calcium scale. Typically, effloresence takes time and calcium scale can only happen that fast if calcium chloride and bicarbonate or carbonate are added at the same time.

Can you post a picture?

What happens if you put a drop of acid on a test spot?
 
I'll take some pic's this afternoon.

Since I'm all travertine, the acid would etch my stone. maybe I can scrap off some into a glass and drop some MA into glass. If it bubbles and dissolves, that's one answer. If it does nothing, then that's another.
 
Yeah, you definitely do not want acid anywhere near travertine....

Look forward to seeing the pics.


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