The PH test and high CYA levels

[09659]

The part that Taylor is missing is that the "quickly and completely" occurs when there is no CYA in the water. With CYA in the water it appears that this conversion process takes longer, perhaps 30 seconds or a minute, depending on the FC/CYA ratio.

I routinely deal with cyanuric acid levels above 200 ppm (I recently tested water for a potential customer that had 700 ppm cyanuric acid. They refused to do anything about it. I declined the account). I try to keep the free chlorine level at 11.5% of cyanuric acid.

So, at 300 ppm cyanuric acid, and 35 ppm free chlorine, will the pH test be inaccurate? It seems that it should be fine if the test sample is tested quickly.

Thanks

 

[chem geek]

Re: Accurate pH test during shock levels with R-007?

Hard to say. In theory, the pH may be OK at first but we don't deal with this situation often enough to know. Also, the FC/CYA ratio rule ignores the smaller oxidizing effect from chlorine bound to CYA, but for the kinds of high FC and CYA levels you are talking about that probably can't be ignored. It means that the ratio probably doesn't need to be as high and that if it is that high then it may react with the pH test faster. You are in uncharted territory. Confirming with an accurate pH meter would probably be the best way to tell the true pH.

 

[09659]

Re: Accurate pH test during shock levels with R-007?

Also, the FC/CYA ratio rule ignores the smaller oxidizing effect from chlorine bound to CYA, but for the kinds of high FC and CYA levels you are talking about that probably can't be ignored.

I am unaware of these effects. Please tell me more about this.

Thanks

 

[chem geek]

Chlorine bound to CYA has a small oxidizing capability. In the Certified Pool Operator (CPO) training -- what is not taught thread in the "Chlorine/CYA Relationship" section there is this paper referenced when describing CYA's effect on chlorine's rate of oxidizing organics. The reaction rate of oxidation of monochlorodimedone (MCD) by H₂ClCy was estimated at 0.5×10⁵ M⁻¹ s⁻¹ compared to that from free chlorine of >7.6×10⁶ M⁻¹ s⁻¹. That's a factor of over 152 difference. So we normally ignore this. For example, with an FC that is 3 ppm and a CYA of 30 ppm, the HOCl and OCl^- together at pH 7.5 are 0.087 ppm while the chlorine bound to CYA is 2.91 ppm. If we divide the chlorine bound to CYA by a factor or 150, then it is equivalent in oxidizing power to 0.019 ppm so a fraction of the free chlorine amount.

From a disinfection point of view, we know it's even a bigger difference since the dominant species at pool pH is HClCy^- which like OCl^- is negatively charged so has a harder time getting into cells (it's also a bigger molecule and unlike HOCl doesn't "look like" water). It is unknown whether the killing of algae is more similar to the HOCl effects of getting inside cells and disrupting metabolic and reproductive processes or whether it is more related to the oxidizing capability, but I suspect it's the former. For the pH test, however, it would be about oxidation rates for oxidizing the pH dye.

However, using your 35 ppm FC with 300 ppm CYA numbers we have HOCl and OCl^- together at 0.11 ppm while chlorine bound to CYA is 34.9 ppm so dividing by 150 we have 0.23 ppm which is actually larger than the free chlorine amount. So it's possible that the pH dye gets oxidized almost 3 times faster than at the same FC/CYA ratio but lower absolute FC and CYA levels. Of course, even this 3 times faster is still slower than not having any CYA at all where the numbers would be the FC level itself. Keep in mind that this is very speculative since the rate of chemical reactions is species-specific so the rate of reaction with dye may not be the same as the relative rates of reaction with MCD.

 

[09659]

Thanks for the explanation.

pH meters are surprisingly cheap. I'll have to look into getting one.

 

[chem geek]

With a pH meter, just be sure to calibrate it regularly. And most important is to store it in appropriate buffer solution -- you don't want the measurement cell to dry out.

 

[09659]

I've read a number of reviews about various pH meters. They seem notoriously unreliable and seem to need constant calibration.

 

[JasonLion]

Inexpensive PH meters are next to useless. Starting around $100 you can find PH meters that are quite good as long as you are methodical about keeping them in calibration and never letting the tip dry out. For homeowners that level of care is normally impractical, but for professionals using the meter every day it is plausible that the required level of care can be worth the trouble.

 

[jblizzle]

That is true for all digital testers which is why we do not typically recommend them.

 

[09659]

I think I'll dump my electronic salinity tester for the Taylor K-1771. 2 fluid ounces per bottle.

The K-1766 has 0.75 fluid ounces per bottle.

 

[jblizzle]

Well you only use 1 drop for one of the reagents and the other is 200ppm per drop. 2 oz seems like overkill to me.

 

[09659]

I was thinking about using a larger test sample to get to at most 100 ppm increments.

 

[jblizzle]

Why? And how often do you think you need to test salt levels? If the SWG is happy, that is all that matters. I can not imagine testing more than once a month and even that might be overkill.

 

[chem geek]

I use a 25 ml sample size so each drop is 80 ppm salt instead of 200 ppm using a 10 ml sample size. But that's because I'm not measuring 3000 ppm for a saltwater chlorine generator and I want to measure smaller differences, but usually such accuracy isn't necessary.

 

[JasonLion]

Using a larger sample size with the Taylor tests doesn't give you as much accuracy as you might imagine. The measurement is still +-10% at best due to things like variation in drop size and varying potency of the reagents. The standard procedure is +-400 around 3000 ppm, while the best you can possibly do with a larger sample size is +-300.

 

[09659]

Thanks

 

[09659]

How will the salt test kit reagents react with bromide?

 

[chem geek]

The salt test uses silver nitrate to precipitate silver chloride but it will also precipitate silver bromide as well. After all the chloride and bromide is used up, then the silver reacts with the chromate indicator dye to produce the red silver chromate that tells you when you are at the end of the test. So the salt test measures the sum of chloride and bromide in units of "sodium chloride" (ppm NaCl).

 

[09659]

So, there's no way to differentiate between the chloride and bromide? The test doesn't show the ratio of chloride to bromide, only the sum of both?

 

[chem geek]

That is correct. The test does not distinguish between chloride and bromide. For lower levels of bromide, such as with a bromide bank in a spa, there is a complicated way to distinguish bromide from chloride by adding chlorine to produce bromine from the bromide and then to add ammonia which will produce monobromamine from the bromine and monochloramine from the chlorine. One can then use an ammonia test kit that actually measures monochloramine as well and the difference between that and a CC reading is the amount of bromide originally present (I told you it was complicated).