Phosphate Buffer And Saturation Index

[trigear]

I bought Control the Ph from spa depot active ingredient: phosphate buffer (as KH2PO4/K2HPO4). What is the difference between this and adding borates to the water. How can I calculate saturation index with this added to the water?

[polyvue]

I bought Control the Ph from spa depot active ingredient: phosphate buffer (as KH2PO4/K2HPO4). What is the difference between this and adding borates to the water. How can I calculate saturation index with this added to the water?

The best way to control pH is be sure the spa has the right level of Total Alkalinity (TA) and make only modest adjustments when adding acid (reducing pH and alkalinity), sodium bicarbonate (increasing TA) or aerating (increasing pH). Borates is recommended to buffer (resist) sharp elevations in pH.

But you have an interesting question. I would like to know the answer to this, as well. It's evident that borates are accounted for in the carbonate alkaline portion of the CSI calculation (see chem geek's 2007 post: http://www.troublefreepool.com/viewtopic.php?p=14664#p14664) but I'm not sure how potassium (di)hydrogen phosphates would figure in.

[chem geek]

First of all, realize that a phosphate buffer system is incompatible with saturating the water with calcium carbonate because calcium phosphate (or related compounds) gets precipitated first. The CSI calculation does not account for phosphates and phosphates do count towards TA so would need to be adjusted, but it's really a moot point since they prevent any reasonable amount of Calcium Hardness (CH) in the water.

[trigear]

First of all, realize that a phosphate buffer system is incompatible with saturating the water with calcium carbonate because calcium phosphate (or related compounds) gets precipitated first. The CSI calculation does not account for phosphates and phosphates do count towards TA so would need to be adjusted, but it's really a moot point since they prevent any reasonable amount of Calcium Hardness (CH) in the water.

Any way to calculate the amount of TA supplied by the phosphate buffer. If so, could you simply insert that value and perform theCSI calculation.

[chem geek]

As I said, it's not relevant since adding a phosphate buffer will precipitate calcium in the water so you won't be able to raise your CH high enough to saturate the water with calcium carbonate to get to a near zero saturation index. That is, use of a phosphate buffer will force you to have a negative saturation index -- not because of how it effects the calculation (i.e. TA), but because it prevents you from having much CH in the water. Adding a phosphate buffer to water with more than a small amount of CH results in the water being cloudy until the calcium phosphate filters out.

This is why phosphate buffers are NEVER used in plaster/gunite pools and generally are only found in residential spas since those typically do not have exposed plaster/gunite/grout.

You will note that Control the pH says "Not recommended for use with hard water." This is because adding a phosphate buffer to hard water does more than just get a little cloudy -- it can create a precipitated mess both in the spa and clogging the filter.

As for the phosphate buffer contribution to TA, it depends a lot on the pH since one of the equilibriums is at 50/50 near a pH of 7.2 meaning about half of the added phosphate is counting towards TA at that point. But as I said, this isn't relevant.

[trigear]

At http://www.watershedpoolandspa.com/ht.php they state "If you use pH Anchor and brominating tablets, it's a good idea to raise the total alkalinity to 200ppm." Ph Anchor contains phoshates in its formulation. I have not used this product however.

I assume that by raising the TA you can lower the saturation index. Does this make sense?

[polyvue]

At http://www.watershedpoolandspa.com/ht.php they state "If you use pH Anchor and brominating tablets, it's a good idea to raise the total alkalinity to 200ppm." Ph Anchor contains phoshates in its formulation. I have not used this product however.

I assume that by raising the TA you can lower the saturation index. Does this make sense?

When I played around with the numbers a bit, using my pool (with typical CH, TA, CYA, chlorine and pH levels) as a model, increasing TA inside the CSI calculations results in an increased saturation index. It doesn't lower it.

[chem geek]

Yes, raising the TA raises the CSI. Since the CH can't be raised and is effectively lowered, this is there way to somewhat compensate for the CSI getting low, but it's not a great fix and over time the pH will still rise due to the high TA, albeit the rise will be slow (though when it does rise, it will take a LOT of acid to lower it back down).