pH keeps creeping up

[cnix]

OK--Thanks--

I'll get the borate up higher. If you said 40# puts me at 18 ppm. Then I need to add 85# to get to 50ppm. I can get it at my local Farmer's Coop in 50# bags (It's used to supply trace Boron mostly for Alfalfa, I think). BTW, Baking soda is usually available at Coops too in 50# bags.

One bit of clarification--I knew I was close to scaling conditions--I had understood that this would help preserve plaster from normal erosion. I know Calcium is essential for this, but does the Carbonate play any role in plaster preservation?

Charles

[chem geek]

It will take 70-71 pounds to get you to 50 ppm Borates, not 85 pounds.
( (50-18)/18 ) * 40 = 71.

It will take 67 cups (4.2 gallons) of Muriatic Acid to compensate for the pH rise from the additional Borates. You can add them one after the other in batches (say, 10 pounds Borax, then 9.5 cups acid, etc.).

Preventing dissolving of plaster/gunite/grout surfaces requires saturation of the water with calcium carbonate. Over-saturation is not necessary and can lead to scaling or cloudiness. Being a little under-saturated is fine -- it's only when you are way out of balance that problems are seen (i.e. well out of the +/- 0.5 range for saturation index -- problems are usually not seen until at least 0.7 to 1.0 or higher in either direction). It is the combination of calcium and carbonate that is important -- not each individually. The reason is that it is calcium carbonate that is in plaster/gunite/grout and that dissolves -- this originally comes from limestone (which is calcium carbonate) and is used as part of the cement making process.

Richard

[cnix]

I did a lot of reading over the weekend, that has brought up additional questions.

Richard, your explanation of saturation with calcium carbonate only being necessary to protect the plaster makes perfect sense. so:

1) How does one determine when the water is fully saturated?

2) If CYA contributes 1/3 of its ppm to TA, how much does Borate contribute to TA? (Assuming the Methyl Red/Bromcresol Green indicator--so that we know what the pH endpoint is)

3) How is Saturation index determined?

[chem geek]

1) How does one determine when the water is fully saturated?

One uses the Langelier Saturation Index (or the equivalent Calcite or Calcium Carbonate Saturation Index). See below for more info.

2) If CYA contributes 1/3 of its ppm to TA, how much does Borate contribute to TA? (Assuming the Methyl Red/Bromcresol Green indicator--so that we know what the pH endpoint is)

Borates contribute a negligible amount to TA. At a pH of 7.5, 50 ppm Borates contributes 5.7 ppm TA so figure an 11% contribution to TA.

3) How is Saturation index determined?

There are many online calculators, some that are good and some that are not, but I believe that Michael Smith's BleachCalc calculates the saturation index. It is a function of pH, TA, CH, TDS and temperature. When the index is near 0, your pool is at saturation for calcium carbonate. Negative numbers indicate risk of dissolving plaster/gunite/grout but only when it's quite negative -- don't even think of worrying until it gets to -0.5. Positive numbers indicate risk of scaling or precipitating calcium carbonate (including cloudiness). Again, don't even think of worrying until it gets to at least +0.5 and realize that most pools don't see any effect until at least +0.7 and some until +1.0 or more.

For a non-salt pool (i.e. a TDS around 500-1000), an ideal saturation (0.0) occurs at a pH of 7.5, TA of 100, CYA of 40, CH of 300, and temp of 80F. These are the mid-point values for the recommended ranges from NSPI. The pH directly affects the index so a 0.1 change in pH changes the index by 0.1 in the same direction. The TA and CH affect the index logarithmically so a change in adjusted TA (that is, TA reduced by one-third the CYA at a pH of 7.5) or CH by a factor of 10^0.1 = 1.26 changes the index by 0.1 in the same direction. The variation with temperature is small -- index increases by about 0.1 with a 10F temperature increase. The Langelier index, or at least the approximation used by the pool industry, has the index change by -0.08 when going from 525 to 3200 TDS (i.e. from non-salt to salt pool) while my calculations show that calcium carbonate saturation really changes by -0.22 instead. The temperature and especially the TDS calculations in the traditional industry formula are the least accurate portions of that formula. My calculations track the Taylor Watergram so if you have a Taylor K-2006 test kit then you can just use the watergram (using adjusted TA) to get an accurate index.

Richard