CyA and TA measurement

[Knetsel]

Hello,
A question regarding the correction factor to be applied at Total Alkalinity measurement in order to get Carbonate Alkalinity : does this correction factor applies whatever the measurement type for TA ?
Said otherwise, do we get (approximately) the same results for TA whether we use a photometer, test strips, drops, etc.. ?

 

[Mdragger88]

Actual TA is what you get when testing.
At least with the drop test (recommended) & strips (not recommended).
Not sure about a photometer (not recommended).
All the results may be different & using the drop test is the most accurate.
Adjusted ta is simply a calculation.
Use actual measured TA &
PoolMath to make adjustments to ph & ta.

TA - Further Reading

www.troublefreepool.com

 

[HermanTX]

The adjustment is based on pH value. So it find the most accurate method to test TA.
Pool Math app does the calculation
This is from an article
The adjusted alkalinity is the TA - (CYA x cf).
cf is the correction factor, which is based on the pH.

pH........cf
7.0.......0.22
7.1.......0.24
7.2.......0.26
7.3.......0.28
7.4.......0.30
7.5.......0.32
7.6.......0.33
7.7.......0.34
7.8.......0.35
7.9.......0.36

 

[Knetsel]

Hello,

Thank you for your answers.
I know how to get Carbonate alkalinity from TA in the presence of CyA by using the pH dependent correction factor or via pool math app.
As I'm using a photometer, I would like to know if the value for TA obtained with it is the same as the one obtained with drops (at least theoretically) and can thus be used as a correct input value..

 

[mgtfp]

The idea is that different measurement methods are supposed to deliver the same, i.e. Total, Alkalinity. And the same correction factor would be required to calculate Carbonate Alkalinity.

Unfortunately, strips are almost always out. And my confidence in photometers is also limited. I did experiment with one photometer, the PoolLab, and the Alkalinity measurement seemed to be the worst of all, completely off, and different value each time.

Just stick to the Taylor/ TFtestkits drop test, it's the most reliable test.

The manual Carbonate Alkalinity calculation is really only required when calculating the Calcium Carbonate Saturation Index manually, for example with the Taylor Water Wheel.

When using PoolMath to calculate the Saturation Index, then PoolMath will calculate the correction factor and carbonate alkalinity internally.You just enter all test results as you get them from your Taylor/ TFtestkits kit.

 

[Knetsel]

OK with you regarding the poor quality of test strips measurements.
No doubts that a test made with drops is the best one.
My photometer is a Lovibond 620 and I think that one can trust the measures made with it; the question is just "does it takes CyA alkalinity into account" ?
May be the best way to get a reliable answer is to ask the question directly to Lovibond.

 

[ILSBJ]

if you really believe everything in the photometer, measure the TA of tap water in a bucket, then dissolve 60 ppm CYA in it and measure the TA again
at 60 ppm CYA, the TA should rise by about 20 ppm compared to the measurement without CYA
by the result, you will know whether the photometer is correcting TA

or you will know that you don't trust him anymore

ILSBJ

 

[mgtfp]

As I had said, all Alkalinity tests test Total Alkalinity. Otherwise they wouldn't actually do an Alkalinity measurement.

CYA Alkalinity is real Alkalinity. If your photometer measures Alkalinity, then this should include the CYA part (and the Borates contribution if Borates are present).

Even aquarium test kits that often say on the package that they test for Carbonate Alkalinity, actually test for Total Alkalinity. They just make the assumption that there are no other contributors to Alkalinity in aquariums (you don't add CYA or Borates to aquariums, fish don't really appreciate this), and in the case of aquariums you assume Total Alkalinity = Carbonate Alkalinity. But the actual test principal in these aquarium kits is the same as in the Taylor drop test: You add an indicator that changes colour around pH 4.5, and then add drops of acid until the indicator changes colour. This gives you Total Alkalinity, doesn't matter what the source of the Alkalinity is. The only difference in pool tests vs aquarium tests is that you also have to add a chlorine neutralizer, as chlorine interferes with the used indicator dyes.

A colorimetric Alkalinity measurement is quantifying the same thing: How much does pH change when adding acid.

In case of titration you add acid until you reach a specific pH value.

In case of colorimetry you add a fixed amount of acid and quantify the change in pH. This adds the difficulty of having to decide between colour shades, rather than having a sharp colour transition.

I can only give you the experience I had with my PoolLab photometer. It sometimes gave a comparable result to titration. Sometimes it wasn't even close to the titration result. Starting a new blister pack of the reagent tablets suddenly gave different results. In the end I gave up on the idea of using a Photometer to test for TA.

One problem could be chlorine interference. I don't know how the used pH dyes react to chlorine, especially once you are leaving the "traditional" range of FC 1-4 to follow the TFP method. I also don't know if fixing a chlorine interference with Thiosulfate (as we do with the Taylor titration test) would work, as Thiosulfate messes with the pH.

I would strongly recommend to try to source a titration based test kit for TA. I know this can be challenging in Europe.

I'd prefer an aquarium titration test over a photometric test. But you need to source thiosulfate, as this isn't part of aquarium kits. You could buy R-0007, it shouldn't be import restricted due to hazardous ingredients as other Taylor reagents are (like R-0009, R-0010 or R-0011). Thiosulfate is also sold with hiking water purification systems to remove the chlorine after disinfecting the water.

See for example this discussion (also with recommendations for CH, when clicking on the name above the quote, you get to the thread and can follow the full discussion):

Well done!

For TA and CH you can try aquarium kits.

For CH for example this one:

sera calcium-Test (Ca) | sera

sera calcium-Test - Water test for aquariums ✔ determine the calcium level in water ✔ extremely precise in steps of 20 mg/l (ppm) ✔Discover now!

www.sera.de

It will give you the Ca content in mg Ca per litre which you'll need to multiply with 2.5 to convert it to mg CaCO3 per litre which is "our" ppm for CH.

For TA you can try this one:

sera kH-Test | sera

sera kH-Test - Water test for aquariums ✔ test aquarium water in an uncomplicated way ✔ check carbonate hardness ✔ monitor water parameters ✔ Find out more now!

www.sera.de

It tells you that it tests Carbonate Hardness, but that's only because there are no other types of Alkalinity in aquariums, in a pool it will give you Total Alkalinity, including Alkalinity contributions from CYA and Borates (if present), in the same way that the Taylor TA test does. It will give you Alkalinity in dKH which you'll have to multiply with 17.9 to convert it to mg CaCO3 per litre (ppm).

You might want to use 10ml samples instead of 5ml samples to get similar resolutions than the Taylor tests.

Only problem is that you'll need some Thiosulfate (like Taylor R-0007) with the TA test (CH test should be fine without) to remove the chlorine which would bleach the indicator (chlorine is not an issue in aquariums).

Thiosulfate is sold for hikers to remove chlorine after using disinfection tablets, for example this one here:

Amazon.de

I believe this is quite powerful and probably higher concentrated (seems to be 6%) than Taylor's R-0007. I guess it would be easy to dilute with distilled water, but I don't know the concentration of R-0007. The datasheet says that 3 drops are required to dechlorinate 1 litre of water, so for 10ml sample sizes you'll want something like a 10:1000 dilution.

I hope this helps a bit.

 

[Knetsel]

I compared results of total alkalinity tests made with :
a) my Lovibond 620 and alkalinity-m tablets
and
b) a Hanna total HI3811 alkalinity kit with drops of acid and bromophenol blue as indicator.
Both give coherent results with or without ~ 20 ppm Cya in pool water.

These tests were performed in a bucket using, as a base, water from my pool where there is no Cya .

Thank you very much for your explanations and your help.

 

[mgtfp]

When you add CYA, then the acidity of the CYA compensates exactly the added alkalinity. Effectively you are replacing Carbonate Alkalinity with CYA Alkalinity.

Example:
You start with TA 100ppm that contains no CYA, all of the TA is in the form of Carbonate Alkalinity. Now you add 60ppm of CYA. This will replace about 20ppm (about 1/3 of 60) of Carbonate Alkalinity with CYA Alkalinity. The water will still have TA 100ppm, but split into 80ppm Carbonate Alkalinity and 20ppm CYA Alkalinity.

So in your test, both should show unchanged alkalinity before and after. But the the Alkalinity contribution of 20ppm CYA is pretty small anyway, only about 7ppm.

 

[Knetsel]

Hello,
This test confirms, at least to myself, that alkalinity measured via my photometer is coherent with the classical drop count test.
I will try to make a similar test with much more stab in the water so as to have a higher contribution for Cya Alkalinity.

Thank you again for your contribution and the relevance of your comments.