TA reading with high CYA question.

[Dz7]

Hoping one of the experts can help me out with this question. I work in the industry and test 100+ pools a week. I've noticed when I get a CYA level that is way over 100 the TA test is pretty much useless. I'll retest TA 3-4 times with a high CYA and get different results every time, sometimes ridiculously different results. Why does high CYA affect the TA reading? We use the Taylor K-2006 with titration for TA and CH tests. Is there any way to get an accurate TA reading with a sky high CYA level?

Also, when the CYA is that high I always recommend draining and refilling or switching to a liquid chlorine diet. Almost always customers aren't willing to drain because it's expensive. Is there any other advice I could be giving them they may listen to?

 

[Frankhugus]

I started in December with CYA over 200, thanks Les. Since then it's been all bleach or liquid C and, a little drain and fill. It's August now and I'm at 90 on my way to 50. It just takes time and and daily testing which I'm will to do. #nomorepucks!

 

[pooldv]

I have not heard that CYA can interfere with the TA test. FC can interfere, maybe the FC is high to compensate for the cya being high?

From here, Pool School - Extended Test Kit Directions
R-0007 is thiosulfate, used to neutralize chlorine so it won't interfere with the test. Extra R-0007 can be added when the FC level is very high to help prevent the dye from bleaching out.

From Taylor,
High halogen level may change indicator reaction from green/red to blue/yellow; to prevent, add thiosulfate prior to testing.

 

[camueller]

Also, keep a wet paper towel around when doing the TA test. Wipe the tip of the stuff you add dropwise each time you put a new drop in. Supposedly static builds up and can affect drop sizes.

 

[max2k]

I have not heard that CYA can interfere with the TA test. FC can interfere, maybe the FC is high to compensate for the cya being high?

From here, Pool School - Extended Test Kit Directions
R-0007 is thiosulfate, used to neutralize chlorine so it won't interfere with the test. Extra R-0007 can be added when the FC level is very high to help prevent the dye from bleaching out.

From Taylor,
High halogen level may change indicator reaction from green/red to blue/yellow; to prevent, add thiosulfate prior to testing.

I think it's testing technique problem- OP cannot obtain consistent results over multiple tests of the same sample.

 

[JamesW]

Cya is one of the types of alkalinity that comprises the total alkalinity. However, the reading for TA should be consistent.

Try a speedstir and titrate slowly.

If you're using a program that calculates adjusted alkalinity from the TA and CYA, then the adjusted alkalinity will be affected by high cya.

 

[duraleigh]

1. I do not believe High CYA and erroneous TA results have any relationship.

2. Wiping the dropper tip for the TA test is only necessary within the first few weeks of purchasing a kit. The phenomenon dissipates after that.

3. Look up your local water rates...probably 5-8 dollars per 1k. So a 20k pool might cost around $100 to drain and refill. Not a freebie, but I would not call that expensive considering the benefit.

 

[Dz7]

I have not heard that CYA can interfere with the TA test. FC can interfere, maybe the FC is high to compensate for the cya being high?

From here, Pool School - Extended Test Kit Directions
R-0007 is thiosulfate, used to neutralize chlorine so it won't interfere with the test. Extra R-0007 can be added when the FC level is very high to help prevent the dye from bleaching out.

From Taylor,
High halogen level may change indicator reaction from green/red to blue/yellow; to prevent, add thiosulfate prior to testing.

I'm quite comfortable testing TA and I add thiosulfate whenever the FC is high. It is not FC affecting the test. I do LOTS of water testing and my results are consistent. We use a speed stir with titration and I go slow with the drops.

Im telling you it does affect the TA reading. I don't know why or how, but it does when the CYA is really high.

 

[JamesW]

The same sample tested several times should give consistent results. Try mixing the sample water before testing.

 

[JoyfulNoise]

Im telling you it does affect the TA reading. I don't know why or how, but it does when the CYA is really high.

Since you're making this assertion then it is up to you to prove it. There is no interfering chemistry here. Cyanuric acid, when dissolved in pool water, exists as a mixture of chlorinated and unchlorinated cyanuarte ions. The first acid dissociation reaction looks like this -

[C(O)NH]₃ <--> [C(O)NH]₂[C(O)N]⁻ + H⁺ (pKa = 6.88)

Cyanuric acid <--> Cyanurate ion + Hydrogen ion

The pKa is 6.88 which means that, when your water's pH is 6.88, half of the cyanuric acid exists as cyanuric acid and the other half exists as cyanurate ion. The cyanurate ion acts as an acid buffer and it adds to the total alkalinity of the sample water. So the TA of your sample is -

TA = [OH^-] + [HCO3^-] + 2*[CO3^--] + [C(O)NH]₂[C(O)N]⁻ + [B(OH)₄^- - [H⁺]

So, as you can see, the cyanurate only serves to increase the baseline level of the TA but there is no chemical interaction between cyanurate and any other ion that would cause the TA to fluctuate during testing. As you add drops of the R-0009 titrant, the acidity of each drop is consuming alkalinity. First goes the hydroxide alkalinity, then the cyanurate alkalinity gets used up and then, finally, all of the carbonate alkalinity is consumed (borates, at pH range of TA testing, only add a very small amount to the alkalinity). Once the pH is less than 4.5, the dye changes color from green to red signaling the end of the test.

As others have stated, it's the testing methodology that's a problem. If you use a 25mL water sample then each drop of R-0009 consumed 10ppm of TA. For a high TA over 100ppm, that's 10 or more drops. It is very easy to lose count of drops or squeeze out drops that are too big or too small and that will directly affect the test. The problem gets worse if you use a 10mL sample size with a titrant precision of 25ppm/drop. In that case, you'll reduce the potential drop count inaccuracy but the droplet volume uniformity will become critical.

 

[JamesW]

Assuming a well mixed 25 ml sample and proper test procedures, your tests should all be within a 20 ppm range (assumes each test is +/- 10 ppm). For example, if you’re getting 90 then 100 and then 80, that’s not ideal, but within test precision.

However, if you’re getting 20 then 100 and then 150, you’re definitely doing something wrong.

All other tests should also be within the range of the test precision of each test.

For example, if you weighed the sample of water on a scale that had a precision of +/- 0.1 lb and you got 0.9 lbs then 1.0 lbs then 1.1 lbs, then that would be ok. However, if you got .5 lbs then 1.5 lbs and then 2.3 lbs, your testing contains some sort of error with procedure, equipment or interference.

How much difference are you getting between tests?

Give some examples of tests that you are referring to. What were all of the results for each test and is the difference only with the TA test or with the other tests as well?

How high was the CYA level? Are you doing a diluted CYA test for levels over 100?