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Boric Acid + Sodium Carbonate Buffer Operation & Intro


Doug Huffman
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I installed a HotSprings Jetsetter spa on April Fools Day for relaxing and bathing, because I can drain it to the yard rather than to our 2K gallon holding tank, just like a boat's, for black and gray water. I soon shifted to generic chemicals, rather than branded tradename compounds, to be frugal (or cheap). I maintain chemistry using Taylor Technologies' test kit and direction from their 'Pool & Spa Water Chemistry', PN #2004B.

In my career (15 years retired) I supervised and approved chemistry maintenance and evolutions for small nuclear power plants, but had no hands on involvement. In other words, I had lots of theory but no practice and a long time ago.

The buffer implicit in 'Total Alkalinity' (p13) seems to have little capacity, because I was constantly adding HCl or sodium bi-carbonate as the pH drifted up and down. I found a patent describing the boric acid + sodium carbonate system and, when I next changed the 'water', I installed the buffer. I have had to change the 'water' about every 45 days due to percipitants clouding it. Water is untreated well water through a charcoal filter column dedicated to filling the tub. Softened water is available.

I added it about three weeks ago, after adjusting 'Total Calcium' to 160 ppm indicated and 'Total Alkalinity' to 100 ppm. Bromine is close but high due to the small mass needed and I'd rather be a bit high, ~10 ppm. I use potassium monopersulfate oxidizer (and would like to find it as Oxone in cheaper quantities).

I added 400 grams total, 95.8% boric acid, and 4.2% sodium carbonate approximately (kitchen digital scale indicates in half-gram increments) and pH indicated 7.0. I adjusted using sodium carbonate from Taylor's Table D for 12 drops Base Demand in a 800 liter volume and pH is 7.5.

Now the 'introduction' of me and my spa is done.

The Taylor Demand Reagents and colorimetry is convenient but does not mention boric acid. Can someone direct me to a table or help me adjust Taylor's Table E for HCl to boric acid?

Thanks for any assistance. Doug Huffman, Washington Island, Wisconsin. (PS; yes, I plan to use the spa through the winter)

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The pool calculator is handy for calculating the correct doses of chemicals to add.

The correct amount of borates is 50 ppm. Boric acid is only added at the beginning. It is not used for adjusting the pH. Boric acid is mostly pH neutral due to its high pKa of 9.14.

See this post for how to do a three step bromine program. Make sure that you add sodium bromide salt at each fill up.

The total alkalinity needs to be wherever it keeps the pH stable. If the pH is constantly too high, then the TA (Total Alkalinity) is too high. If the pH is constantly too low, then the TA is too low. When the TA is right, the pH will remain stable. Do not feel like you have to set the TA at any predetermined level or range. A TA of 100 ppm is usually too high for a hot tub. A high TA causes the pH to rise due to the off gassing of carbon dioxide, which is in equilibrium with the bicarbonate ions.

It is important to measure "Calcium Hardness", not "Total Hardness". A Calcium Hardness of 160 ppm is a bit high for a hot tub. I recommend a lower level of closer to 100 ppm.

Use the pool calculator to calculate your CSI. If your tub is plaster or has grout, then you want a CSI of 0.0 to about +0.2. If your tub is plastic or fiberglass, you want a CSI of -0.3 to 0.0. A high CSI will cause scale and precipitation of calcium carbonate.

I would use the softened water and then add back in the correct amount of calcium.

To raise the pH, try aerating first. Aerating causes the carbon dioxide to off gas, which raises the pH.

HCO3- + H+ < > CO2 + H2O

If aerating does not work, you can use Borax to raise the pH. I recommend against using sodium carbonate as it is most likely to cause a precipitation reaction.

If your pH is very low or chronically low, add baking soda to increase the TA.

I highly recommend that you get the Taylor K-2106 FAS-DPD test kit.

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Thanks. I'll digest that.

You wrote

Aerating causes the carbon dioxide to off gas' date=' which raises the pH.[/quote']Did you mean perhaps that aeration causes CO2 absorption?
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Pool and spas are intentionally over-carbonated in order to provide a pH buffer and, for plaster (and fiberglass) pools, to saturate the water with calcium carbonate to protect pool surfaces.

Because of this over-carbonation, there is more carbon dioxide dissolved in the water than there would be at equilibrium with air so this excess carbon dioxide outgasses (i.e. leaves the water and goes into the air). Aeration speeds this process up by increasing the surface area between the water and air boundary. The pH rises when carbon dioxide leaves the water because you are essentially removing carbonic acid from the water.

Some groundwater has higher TA because it dissolves calcium carbonate in rocks, but the pH is often higher as well and for underground sources the pressure is higher -- all of this prevents the carbon dioxide from escaping. However, when you have the water in your pool or spa, you lower the pH and that shifts more of the carbonates in the water towards carbon dioxide thus making things more out-of-equilibrium. The rate of outgassing seems to be proportional to the square of the TA level so even though a lower TA level provides less pH buffering in a somewhat linear way, the far lower outgassing ends up making the pH more stable overall.

The borates are an additional pH buffer system that does not have the problem of causing the pH to rise. That is, TA from carbonates is a SOURCE of rising pH itself. This is why one can get into the vicious cycle of adding acid to lower pH and then adding bicarbonate to increase TA and then have the pH rise again, etc. The way to break out of this cycle is to lower the TA level and keep it there (i.e. don't raise the TA if the pH tends to rise).

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  • 10 years later...

I know this is an old post, but thanks for that Chem geek!

One item of clarification I would like: It seems obvious that this implies that once the level of TA that results in stable pH is found, if the water saturation index is not balanced that the calcium hardness should then be adjusted to obtain approx. zero saturation index. Is this correct?

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On 1/10/2021 at 6:40 PM, Spasome said:

if the water saturation index is not balanced that the calcium hardness should then be adjusted to obtain approx. zero saturation index. Is this correct?

Only for a plaster spa. Calcium saturation index is not applicable for acrylic spas. Calcium should be high enough to help deter foaming but not high enough to cause scaling. If your fill water CH is above 120 ppm you are good to go, if it's lower add calcium. If it's higher than about 300 ppm be aware that scaling might be a problem. The only way to lower CH is by filling from an ion exchange water softener (the kind that uses salt). Weekly use of a phosphonate based metal sequestrant can help high calcium from depositing as scale to a small extent when there is no other way to lower the CH.

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  • 3 months later...

Sorry for the late reply, yes, it's been 3 months! I kept meaning to check back but I would only remember when I was drifting off to sleep at night and then I would forget the next day.

I had never heard the Saturation Index was not applicable to acrylic spas! I knew there was no plaster to worry about but I just assumed balanced water was good for other reasons! Still, I would assume you mean a negative Saturation Index is fine but that a positive Saturation Index could be problematic for metal corrosion? Is that correct?

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For plaster a negative saturation index is aggressive to the plaster surface. A positive saturation index is not. Calcium saturation index has NOTHING to do with metal corrosion in pools and spas. Low pH is the primary factor that causes problems with metal parts (and vinyl liners), Positive saturation index that is too high can lead to scale formation, which is NOT desirable, even for plaster surfaces.

Calcium saturation index is only applicable to plaster. However, there is some empirical evicence that higher calcium levels in fiberglass pools and spas (NOT acrylic shells with fiberglass baking reinforcement but actual fiberglass gelcoat exposed to the water) might help prevent or slow cobalt spotting and the formation of iron staining, both of which fiberglass is prone to.

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