Questions about dosing with Borax to increase borates

[Water_man]

1. Proteam’s dosage chart for their Supreme product (99.5% Sodium tetraborate pentahydrate)
shows 3.35 Lbs per 1000 gal for 50 ppm borates increase, which means 10.7 Oz per 10 ppm.
The dosage suggested in this forum for Borax (Sodium tetraborate decahydrate) is 12 Oz
per 10 ppm per 1000 gal.

Based on the ratio of the molecular weights of the two products, and assuming that only the tetraborate is the active ingredient, if Proteam’s dosage chart is right, then Borax should be dosed at 14 Oz, not 12.
This is about 17% more than suggested. Can anyone comment?

2. I wonder what “ppm borates” exactly means.
If 12 Oz Borax are needed to raise 1000 gal water by 10 ppm of borates, the resulting solution is about
100 ppm of Borax. Since the molecular weight of "Borax" is 382, it means that this measured "borates" species should have a molecular weight of about 38, in order to have a 10 ppm of measured "borates" in a 100 ppm Borax solution. 38 corresponds to B2O but I can't see a species in borates chemistry that corresponds to this stoichiometry. Any ideas?

Obviously all the above is moot if I made a Boo Boo in the calculations.

 

[Guest]

1. Proteam’s dosage chart for their Supreme product (99.5% Sodium tetraborate pentahydrate)
shows 3.35 Lbs per 1000 gal for 50 ppm borates increase, which means 10.7 Oz per 10 ppm.
The dosage suggested in this forum for Borax (Sodium tetraborate decahydrate) is 12 Oz
per 10 ppm per 1000 gal.

Based on the ratio of the molecular weights of the two products, and assuming that only the tetraborate is the active ingredient, if Proteam’s dosage chart is right, then Borax should be dosed at 14 Oz, not 12.
This is about 17% more than suggested. Can anyone comment?

2. I wonder what “ppm borates” exactly means.
If 12 Oz Borax are needed to raise 1000 gal water by 10 ppm of borates, the resulting solution is about
100 ppm of Borax. Since the molecular weight of "Borax" is 382, it means that this measured "borates" species should have a molecular weight of about 38, in order to have a 10 ppm of measured "borates" in a 100 ppm Borax solution. 38 corresponds to B2O but I can't see a species in borates chemistry that corresponds to this stoichiometry. Any ideas?

Obviously all the above is moot if I made a Boo Boo in the calculations.

PPM borates is referring to boric acid since the majority of borates will be in the form of boric acid at normal pool pH. The titration test for borates that is used is actually a boric acid titration using mannitol and bromthymol blue. I am not sure of the chemistry used in the strips but since the majority of borate strips have a color change in the tan range I suspect it is something similar. Much of this has been discussed to great extent at PF, btw.

 

[Water_man]

PPM borates is referring to boric acid since the majority of borates will be in the form of boric acid at normal pool pH. The titration test for borates that is used is actually a boric acid titration using mannitol and bromthymol blue. I am not sure of the chemistry used in the strips but since the majority of borate strips have a color change in the tan range I suspect it is something similar. Much of this has been discussed to great extent at PF, btw.

This explains my second question. Sodium tetraborate has a mol weight of 202 and boric acid is 62. The measured and reported "borates" level is actualy a third of the true boric acid level (202 devided by 10 is about 20, and 20 is about a third of 62. In this calculation the water content of the solid sodium TB-10-hydrate is not taken into account because it doesn't contribute to the BA.)
Still, it beats me why the true level of boric acid is not reported, instead of the titrant value, which is a third. But if this is the convention then everybody is happy, provided they all compare apples to apples.
Couldn't find details about these two topics at the PF.

 

[chem geek]

The ppm for Borates is actually for Boron with a molecular weight of 10.8 g/mole. Boric Acid is 61.8 g/mole and is what was incorrectly used in the BleachCalc program. My spreadsheet and The Pool Calculator both correctly use the ppm Boron number. So 50 ppm Borates in pool water is mostly 286 ppm Boric Acid -- a molecular amount roughly similar to Calcium Hardness (CH).

One sodium tetraborate molecule produces FOUR boric acid molecules or another way of looking at it is that one sodium tetraborate contains 4 boron. The pentahydrate form in Proteam Supreme has a molecular weight of 291.35 g/mole for FOUR boron so the ratio is 291.35 / (4 * 10.8) = 6.74. In Borax, the decahydrate form has a molecular weight of 381.38 g/mole so 381.38 / (4 * 10.8) = 8.83. If you are converting from a weight ratio of product to get ppm you most certainly have to count the hydrated water since it is part of the weight of the product.

It takes 11.7 ounces weight of 20 Mule Team Borax or 9.00 ounces weight of Proteam Supreme per 1000 gallons to raise the Borates by 10 ppm (Boron). I don't know where Proteam Supreme gets their 10.7 ounces from since their product seems more than the implied 84% purity from their recommendation.

Na₂B₄O₇•5H₂O + 2H₂O --> 2Na⁺ + 4B(OH)₃ + 2OH^-
Sodium Tetraborate Pentahydrate (Proteam Supreme) + Water --> Sodium Ion + Boric Acid + Hydroxyl Ion

Na₂B₄O₇•10H₂O --> 2Na⁺ + 4B(OH)₃ + 2OH^- + 3H₂O
Sodium Tetraborate Decahydrate (20 Mule Team Borax) --> Sodium Ion + Boric Acid + Hydroxyl Ion + Water

It should be noted that the nearly pH neutral Proteam Supreme Plus product is mostly Boric Acid with some Sodium Tetraborate Decahydrate since at the pH of pools near 7.5 the Boric Acid is mostly undissociated.

Richard

 

[Water_man]

The ppm for Borates is actually for Boron with a molecular weight of 10.8 g/mole. Boric Acid is 61.8 g/mole and is what was incorrectly used in the BleachCalc program. My spreadsheet and The Pool Calculator both correctly use the ppm Boron number. So 50 ppm Borates in pool water is mostly 286 ppm Boric Acid -- a molecular amount roughly similar to Calcium Hardness (CH).

One sodium tetraborate molecule produces FOUR boric acid molecules or another way of looking at it is that one sodium tetraborate contains 4 boron. The pentahydrate form in Proteam Supreme has a molecular weight of 291.35 g/mole for FOUR boron so the ratio is 291.35 / (4 * 10.8) = 6.74. In Borax, the decahydrate form has a molecular weight of 381.38 g/mole so 381.38 / (4 * 10.8) = 8.83. If you are converting from a weight ratio of product to get ppm you most certainly have to count the hydrated water since it is part of the weight of the product.

Na₂B₄O₇•5H₂O + 2H₂O --> 2Na⁺ + 4B(OH)₃ + 2OH^-
Sodium Tetraborate Pentahydrate (Proteam Supreme) + Water --> Sodium Ion + Boric Acid + Hydroxyl Ion

Na₂B₄O₇•10H₂O --> 2Na⁺ + 4B(OH)₃ + 2OH^- + 3H₂O
Sodium Tetraborate Decahydrate (20 Mule Team Borax) --> Sodium Ion + Boric Acid + Hydroxyl Ion + Water

It should be noted that the nearly pH neutral Proteam Supreme Plus product is mostly Boric Acid with some Sodium Tetraborate Decahydrate since at the pH of pools near 7.5 the Boric Acid is mostly undissociated.

Richard

I made the calculation and now the "measured borates" = level of "Boron" fits perfectly the dosage scheme of 12 Oz Borax per 10 ppm "Boron" per 1000 gal.
This means, BTW, that at least as far as the stoichiometry is concerned, Proteam's dosage for the Supreme product is too high.

 

[chem geek]

Yup, and I don't know why it's too high but it wouldn't be the first time a manufacturer recommendation isn't exactly right.

By the way, boric acid is interesting because it does not dissociate to release a hydrogen ion but rather combines with a hydroxyl ion as follows (with the equilibrium mostly to the left at a pH of 7.5):

B(OH)₃ + OH^- <--> B(OH)₄^-
Boric Acid + Hydroxyl Ion <--> Borate Ion

or equivalently

B(OH)₃ + H₂O <--> B(OH)₄^- + H⁺
Boric Acid + Water <--> Borate Ion + Hydrogen Ion

 

[Water_man]

By the way, boric acid is interesting because it does not dissociate to release a hydrogen ion but rather combines with a hydroxyl ion as follows (with the equilibrium mostly to the left at a pH of 7.5):

B(OH)₃ + OH^- <--> B(OH)₄^-
Boric Acid + Hydroxyl Ion <--> Borate Ion

or equivalently

B(OH)₃ + H₂O <--> B(OH)₄^- + H⁺
Boric Acid + Water <--> Borate Ion + Hydrogen Ion

pKa = 9.25, which explains why the BA system is a good buffer for pH "upward pull" towards the 8 to 9 values.
Unrelated to the topic at hand, I read somewhere that BA works as a disinfectant by reacting with water to produce small amounts of hydrogen peroxide. I wonder if this is true.

 

[chem geek]

There is no such thing as an "upward pull" or tendency or anything like that for a buffer system to move towards a pH closer to its pKa value. I don't know where people get that idea, but I've seen that said in several places. All that happens is that when the pH is equal to the pKa there is an equal amount of the undissociated acid with its dissociated ion (salt) and that the amount of buffering is greatest (i.e. the pH moves least from a fixed amount of acid or base that is added) and that the buffering capacity is equal in each direction. If you leave a buffer system alone, it won't spontaneously move in pH. The pH in pools rise because carbon dioxide gets outgassed, not because of anything to do with the buffer system being away from pH = pKa.

I can easily make the pH go higher or lower than the pKa value through addition of a pure base or acid and this is just as true for the carbonate buffer system as it is for the borates (boric acid) buffer system. The carbonate buffer system has a pKa(1) of 6.3 and a pKa(2) of 10.3 so the first one is the primary one in pools and has more capacity against a drop in pH though still has plenty of capacity preventing a rise as well, but as one lowers the pH the buffering becomes greater which is why it takes more acid to lower the pH from 7.2 to 7.0 than from 7.5 to 7.3. For the Borate buffer system, it becomes stronger as one approaches a pH of 9.2 and that's what makes it useful to prevent scale from alkaline sources such as high hypochlorite levels in The Liquidator and from high pH levels at the hydrogen gas generation plate in the SWG.

As for hydrogen peroxide, it will react with borax to create sodium perborate, but I don't see where hydrogen peroxide itself is produced from Boric Acid. The combination of hydrogen peroxide with boric acid does make a more powerful oxidizer, but I suspect that's just perborate. Also, sodium perborate can hydrolyze into hydrogen peroxide so perhaps that is what you read. You can read about various Boron products and their toxicity here and there is nothing about hydrogen peroxide associated with Boric Acid -- only with sodium perborate (NaBO₃ though it does form peroxy bridges from water as in Na₂B₂O₈H₄). The oxidation state of the Boron is +5 in perborate but is +3 in Boric Acid. Sodium Tetraborate (Na₂B₄O₇) also has Boron in the +3 oxidation state. It is the higher oxidation state of perborate (and it's free energy difference) that makes it a more powerful oxidizer.

The Boron article I linked to says "There is evidence in both in vitro and in vivo systems that boric acid has an affinity for cis-hydroxyl groups, and this may be the mechanism that explains the biological effects of boric acid." It does not appear that a definitive mechanism is known, however.

Richard

 

[Water_man]

There is no such thing as an "upward pull" or tendency or anything like that for a buffer system to move towards a pH closer to its pKa value. I don't know where people get that idea, but I've seen that said in several places. All that happens is that when the pH is equal to the pKa there is an equal amount of the undissociated acid with its dissociated ion (salt) and that the amount of buffering is greatest (i.e. the pH moves least from a fixed amount of acid or base that is added) and that the buffering capacity is equal in each direction. If you leave a buffer system alone, it won't spontaneously move in pH. The pH in pools rise because carbon dioxide gets outgassed, not because of anything to do with the buffer system being away from pH = pKa.

I can easily make the pH go higher or lower than the pKa value through addition of a pure base or acid and this is just as true for the carbonate buffer system as it is for the borates (boric acid) buffer system. The carbonate buffer system has a pKa(1) of 6.3 and a pKa(2) of 10.3 so the first one is the primary one in pools and has more capacity against a drop in pH though still has plenty of capacity preventing a rise as well, but as one lowers the pH the buffering becomes greater which is why it takes more acid to lower the pH from 7.2 to 7.0 than from 7.5 to 7.3. For the Borate buffer system, it becomes stronger as one approaches a pH of 9.2 and that's what makes it useful to prevent scale from alkaline sources such as high hypochlorite levels in The Liquidator and from high pH levels at the hydrogen gas generation plate in the SWG.

As for hydrogen peroxide, it will react with borax to create sodium perborate, but I don't see where hydrogen peroxide itself is produced from Boric Acid. The combination of hydrogen peroxide with boric acid does make a more powerful oxidizer, but I suspect that's just perborate. Also, sodium perborate can hydrolyze into hydrogen peroxide so perhaps that is what you read. You can read about various Boron products and their toxicity here and there is nothing about hydrogen peroxide associated with Boric Acid -- only with sodium perborate (NaBO₃ though it does form peroxy bridges from water as in Na₂B₂O₈H₄). The oxidation state of the Boron is +5 in perborate but is +3 in Boric Acid. Sodium Tetraborate (Na₂B₄O₇) also has Boron in the +3 oxidation state. It is the higher oxidation state of perborate (and it's free energy difference) that makes it a more powerful oxidizer.

Richard

Your bold blue words are exactly what the claim is, and this is what I meant. You'll find it in all textbooks.

 

[Water_man]

It should be noted that the nearly pH neutral Proteam Supreme Plus product is mostly Boric Acid with some Sodium Tetraborate Decahydrate since at the pH of pools near 7.5 the Boric Acid is mostly undissociated.

Richard

According to my calculation, based on BA's pKa, the pH of 50 ppm "borates" (="Boron") of Boric Acid in water is about 4.9.
Please tell me if this is incorrect. It's been a looong time since I touched this stuff.

 

[chem geek]

Sorry to have misinterpreted what you meant by "upward pull". That's just the same terminology some people mean when they think there is some sort of drift towards a pH closer to the pKa and I wanted to make sure no one thought that was the case.

The pH of 50 ppm Borates (Boron) from Boric Acid in pure water is 5.77 and takes 381.8 ounces weight of Boric Acid per 10,000 gallons. To make this pH neutral takes only 6.1 ounces weight of Sodium Tetraborate Pentahydrate (for an end result Borates level of 50.7 ppm). So for the Proteam Supreme Plus product to be pH neutral, it would need to be 98.4% Boric Acid and 1.6% Sodium Tetraborate Pentahydrate. That's why I wrote it's mostly Boric acid with some Sodium Tetraborate Pentahydrate (I wrote Decahydrate as with Borax, but Proteam uses Pentahydrate). In practice, they may add more tetraborate at a rate of 19.7 ounces weight per 10,000 gallons to get to a pH of 7.5 since that's the typical pH in a pool. So that would be more like 95.1% Boric Acid with 4.9% tetraborate. The MSDS is shown here but doesn't give the percentages nor even the pH. The dosage is 2 pounds per 1000 gallons for 30-50 ppm borates (boron) which works out about right if it's mostly boric acid.

I just realized that I shouldn't do the calculation on the basis of what happens in pure water since what one wants is to be pH neutral in buffered pool water at a pH of 7.5. In that case, the amount of Sodium Tetraborate Pentahydrate it takes to keep the pH at 7.5 after adding 381.8 ounces weight of Boric Acid per 10,000 gallons is 21.8 ounces weight of Boric Acid (with final Borates level of 52.4 ppm). That's 94.6% Boric Acid with 5.4% tetraborate. By the way, just adding pure Boric Acid to the pool to get to 50 ppm Borates isn't a problem for pH as it will only drop from 7.50 to 7.31. The price for Proteam Supreme Plus is roughly $4/pound here while pure Boric Acid can be obtained for roughly $2/pound here and for even less here though this varies a lot based on quantity purchased.

The rough computation for pH of Boric Acid in pure water using a pKa of 9.2 is as follows:

B(OH)₃ + H₂O <--> B(OH)₄^- + H⁺
pK_a = 9.2 = -log₁₀( [B(OH)₄^-] * [H⁺] / [B(OH)₃] )
H₂O <--> H⁺ + OH^-
pK_w = 14 = [H⁺] * [OH^-]
[B(OH)₃] + [B(OH)₄^-] = 50 ppm = (50 mg/liter) / (1000 mg/g) / (10.8117 g/mole) = 4.62x10^-3 moles/liter

We can do a quick approximation assuming [H⁺] >> [OH^-] and [B(OH)₄^-] is approximately [H⁺] and [B(OH)₃] is approximately the initial concentration of Boric Acid. So,

[H⁺]² = 10^-9.2 * 4.62x10^-3 = 2.92x10^-12
[H⁺] = 1.71x10^-6
pH = -log₁₀( [H⁺] ) = 5.77

Checking our assumptions, we see that they are valid since
[H⁺] >> [OH^-] is 1.7x10^-6 >> 5.9x10^-9 and
[B(OH)₄^-] / [B(OH)₃] = 10^-9.2 / 10^-5.77 = 3.7x10^-4 so [B(OH)₃] is indeed approximately the initial concentration of Boric Acid (from the mass balance equation) and
[B(OH)₄^-] = [B(OH)₃] * 3.7x10^-4 or 4.62x10^-3 * 3.7x10^-4 = 1.7x10^-6 which is the same as our calculated [H⁺].

Richard

 

[Water_man]

Sorry to have misinterpreted what you meant by "upward pull". That's just the same terminology some people mean when they think there is some sort of drift towards a pH closer to the pKa and I wanted to make sure no one thought that was the case.

The pH of 50 ppm Borates (Boron) from Boric Acid in pure water is 5.77
Richard

Our calculations are basically the same. The only difference is that I used the moler concentration of water in the Ka equation (as I said, I haven't done this stuff for a long time.) I guess that the 55.56 M for water is a part of the Ka.

 

[Guest]

1. Proteam’s dosage chart for their Supreme product (99.5% Sodium tetraborate pentahydrate)
shows 3.35 Lbs per 1000 gal for 50 ppm borates increase, which means 10.7 Oz per 10 ppm.
The dosage suggested in this forum for Borax (Sodium tetraborate decahydrate) is 12 Oz
per 10 ppm per 1000 gal.

IF you check out their chart for other water volumes and other ppm concentrations you will find that they do a lot of 'rounding off' in their dosage chart. On the container they recommend dosing at 2 lbs per 1000 gallons which will only bring you up to about 30 ppm. I am not sure why but I suspect it's because the exact dosage is not critical as long as it is in the ballpark. It's been found that borates are effective above 30 ppm in chlorine and bromine systems and there is not much added benefits above 50 ppm. In biguanide systems the range is 50 to 80 ppm.
Also, since the actual volume of a pool is never going to be known unless the water is being metered as the pool is filled so it's going to be a ballpark figure anway. Also, the testing method that is used for boron is a titration using mannitol as an indicator (it complexes with boron). For accuracy this titrations should be done with a pH meter after all carbonate alkalinity is destroyed by converting it to CO2 by acidifying and then heating to drive out the CO2 but in actual practice bromthymol blue is used as an indicator for both the acidifying step and the actual titration with sodium hydroxide (boric acid is being titrated). Ingenious but a further source of errror.
Bottom line is this, we are talking about a swimming pool, not a lab experiment. Sometimes people lose sight of that.

 

[chem geek]

I guess that the 55.56 M for water is a part of the Ka.

Yup. The pK_w of 14 for water has the constant concentration of water built into it.