Clear Choice TA titration drops vs 4 in 1 drops?

tomfrh

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Jan 30, 2018
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I’m have two test kits, Aussie gold, and clear choice labs.

They each have a TA test kit, which includes an indicator, and a titration bottle.

The Aussie gold has blue-clear indicator. The Aussie gold has green magenta indicator.

The kits give different answers. Clear choice says TA is 50. Aussie gold says 90. The titration bottles are different strength. The Clear choice titration bottle will change the pH sample from 7.8 to 7 with 1 drop. The Aussie gold titration bottle takes two drops. So it seems the clear choice titration bottle is stronger?

What are these titration bottles? Some form of dilute acid? Should the they be standard across kits? Is there a way to gauge if the titration bottle is accurate? Eg the effect of 1 drop on the pH test kit?
 
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brendio

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May 18, 2020
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I have the CCL test kit, as well as a Blue Devil Kit I inherited with the house. From what I have seen, the Blue Devil seems very similar to the Aussie Gold kit. My comments are based on the Blue Devil kit.

Firstly, the two kits use a different sample size for the tests. My Blue Devil uses a 15 mL sample, whereas the CCL test uses a 25 mL sample for the high sensitivity test. The CCL also has a lower sensitivity test with a 10 mL sample size, but the high sensitivity test is the comparable one, given that for both this test and the Blue Devil/Aussie Gold test 1 drop = 10 ppm TA. You are correct that the titration bottle contains a dilute acid (we can ignore the different colour indicators and assume both have a similar colour-change pH value). Because the CCL test is using a larger sample size, the acid will need to be more concentrated in the CCL test than in the BD/AG test (technically, we should be speaking of the number of moles of acid, which is concentration of the acid times the volume of the drop, which may be different between the bottles, but we can ignore that, since the test kits are calibrated to the drop size such that 1 drop = 10 ppm TA).

Your pH tests are showing a 1:2 ratio of number of drops needed to reduce pH from 7.8 to 7. The ratio of the sample volumes it 1:1.67. That is pretty close.

As to why you are getting different answers, may I check what sample volumes you are using? You can switch the indicator from one test the the other (I did that when I ran out), but you must match your titration bottle the the right sample tube. If you are using the correct volumes, we need to look further. My two test kits agree very well, and I don't know the age of my Blue Devil kit.
 

tomfrh

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Jan 30, 2018
489
Australia
Thanks for the detailed reply.

The sample size is 15ml for Aussie gold.

10ml and 25ml for the CCL.

What about the chlorine neutraliser in the Aussie gold kit, and the TA buffer in the CCL kit. Are these similar things? What are they doing? Also, I just realised I made a mistake with the CCL test, and only used 1 drop of the buffer, whereas the instructions say two.
 

mgtfp

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The buffer in the CCL kit's buffer is the same chlorine neutralizer, sodium thiosulfate. Chlorine interferes with the test, must be neutralized first. With chlorine remaining in the sample, the test can show low, as far as I know. Depending on your FC, you might need 2 or 3 drops in the 25ml CCL test.

I got a blue devil this week, because I wanted the pH comparator. Just tried the BD TA test, and it showed 10ppm higher than the CCL test, probably within the tolerance.

The titrating reagents are different, CCL uses sulfuric acid (like Taylor), BD uses hydrochloric acid.

Not sure if I got the BD test right. The instructions say to titrate until it turn clear or slightly yellow. Well, it first turned clear and with the next drop it went do slightly yellow. I assume, the clear was my endpoint. Taking the slightly yellow as endpoint would make it 20ppm higher than CCL.
 
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mgtfp

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There is one issue to keep in mind with the TA-test, at least with the Taylor/CCL test: Especially with new reagent bottles there can be an electrical charge at the dropper that results in smaller drop sizes, which means you will add more, but smaller, drops to get to the endpoint, giving a false high reading:


In your case, the Aussie Gold shows the higher reading, maybe that same problem can occur also with Aussie Gold.

The recommendation is to wipe the tip with a damp cloth or tissue before each drop to remove potential electrical charges. Maybe try if that helps.
 

AUSpool

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Geday Tom,

The CCL test is better in that it uses better reagents. Try using the CCL’s high resolution test with the sample filled to the upper line.
 

mgtfp

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Geday Tom,

The CCL test is better in that it uses better reagents. Try using the CCL’s high resolution test with the sample filled to the upper line.

Good points. I always use the high sensitivity CCL test for TA (for CH the standard test is sufficient, I think). And if in doubt, trust CCL.
 

tomfrh

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I tried it with both the high sensitivity and the low sensitivity. Same result.

I’ve since added some bicarb to bring the Total Alkalinity up and the tests disagree by similar margin, with the CCL saying 30-40ppm lower.

Does anyone know the strength of the CCL titration acid?
 

AUSpool

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AUSpool

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What are these titration bottles? Some form of dilute acid? Should the they be standard across kits? Is there a way to gauge if the titration bottle is accurate? Eg the effect of 1 drop on the pH test kit?

You cant compare results and accuracy between test kits via titrating acid strength. The basic test principle is the same but thats where the comparison stops. Manufacturers start with the bottles and then formulate the titratant to match the drop size. Not all bottles are equal, there is no standard and some bottles are defiantly better than others. A minor defect will change the drop size and the test out come. You cant verify your accuracy with the equipment you have at home.
 

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mgtfp

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How much bicarb did you add, does the addition show up correctly in the CCL test?

I am not concerned about the Aussie Gold test being not that accurate, and would just say forget about it. But I find it a bit unusual that the margin between the two tests remained constant, rather than the factor between the two tests which could be explained by errors in drop size and/or reagent concentration.

A constant margin indicates more a systematic error you are doing in one or both of the tests, like stopping the CCL test too early and the AG test too late.

Again, I am not worried about the AG test, and I think with this colour transition to clear and then yellowish, the test can easily be overdone.

But make sure that you don't under-do the CCL test. The endpoint is not reached at the first hint of pink, but when another drop (that itself will not be counted) doesn't change the colour any further.

Do you get a consistent result with the TA-standard that came with the CCL kit?
 

tomfrh

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Again, I am not worried about the AG test, and I think with this colour transition to clear and then yellowish, the test can easily be overdone.

But make sure that you don't under-do the CCL test. The endpoint is not reached at the first hint of pink, but when another drop (that itself will not be counted) doesn't change the colour any further.

Yes I know it has to go full pink and not change any further. Regarding the AG test, I think it is bromophenol blue indicator, is that right?

I looked at this chart which I found online. This seems to suggest that the slightest change in the Blue indicator corresponds to the end point of the green/pink test? So clear or yellow will always be a bit too far?

pH_Indicator_Chart.jpg


Regarding the TA standard, I've used it up. Can I make some more with distilled water and bicarb?
 
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mgtfp

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Not sure if it's worth the effort to mix your own standard, might not be exact enough and raise more new questions than it answers. Just get a new one from CCL with your next order.

But it sounds like you have used the standard before? Did the CCL test show the expected 100ppm? If so, then I wouldn't see a reason to not trust it now.

You seem to get the CCL endpoint right, whereas the AG endpoint can easily get over-titrated. If the AG is identical to the BD, then yes, the indicator is bromophenol blue. I don't like that test with the creeping transition via clear to yellow.

I only tried the BD TS test about twice, out of curiosity because I got it as a byproduct to getting the pH comparator. It seems to give more or less the same result as CCL as long as I take the first hint of clearing as the endpoint - which seems to confirm your interpretation of the graph you found. I guess they mention yellow in their instructions as it can happen that the second last drop is just short of changing colours and the next drop skips clear and goes straight to yellow. But I reckon the search for yellow encourages to add a few more drops.

Just ditch the AG test.

Any chance that your CCL reagents are too old? Or that the titrating reagent bottle was kept open for too long and some water evaporated and increased the concentration? If not, then I would trust the test.
 

tomfrh

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As an experiment I mixed up some 100ppm standard TA of my own. The Aussie gold test was pretty accurate. The blue started to fade at 8-9 drops and it went clear/yellow at ten drops.

I’ve run out of the CCL indicator now. I’ll get some more and see how it compares. I’m wondering if the CCL indicator and/or acid drops were out of date...
 

AUSpool

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50ppm TA does seem a tad low, have you been adding lots of acid or have you had lots of heavy rain? If you’ve been logging your results you should be able to roughly guess where you should be. With a concrete/plater pool, SWCG, solar heating and in the middle of summer you evaporation would be high and TA would slowly creep up from the top up water unless you’ve been adding acid or have had lots of rain.

As you’ve probably guessed the end point for the TA test is at a pH 0f 4.5. Bromocresol green and Methyl red as found in the CCL and Taylor TA kit is just a better indicator than the bromophenol blue.

My gut feeling is that you CCL result is a bit low while the AG is a bit high. Organic indicator dyes are the most sensitive to storage and aging and will give inconsistent results when out or date and degraded. The Taylor/CCL TA indicator will start to stain the bottle as it goes ‘off’. If stored properly the acid will last for a very long time.
 

mgtfp

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How did you mix your standard? Did you use PoolMath or the dosing instructions on a bag of Alk-Up? The bag of Alk-up that I once bought and that is primarily being used by my wife in the garden now, says to add 15g per 1000l for a 10ppm TA increase. According to PoolMath, 15g actually only give a 8.9ppm TA increase. Or speaking in smaller quantities: to get a 100ppm standard, you needed to add 150mg of bicarb to 1l of distilled water according to bag-instructions, but according to PoolMath that results only in 89.4ppm - to get to 100ppm you actually need 167.8mg.

I am also not sure how accurate PoolMath actually is when starting the calculation with completely unbuffered water, the implemented approximations might no longer be valid. TA is quite complex, it also contains the concentrations of OH- (increases TA) and H+ (decreases TA). With buffered water, adding bicarb will only have a small impact on pH and the changes in OH- and H+ concentrations can probably be neglected, but I don't think that assumption still holds when starting with distilled (i.e. unbuffered) water. PoolMath for example predicts a change in pH of only 0.2 when adding those 167.8mg of bicarb - I reckon the real change is much bigger, and the whole calculation is outside of PoolMath's range.

AUSPool is probably spot on in thinking that one test is a bit low and the other one a bit high. Curious about the results with fresh CCL reagents.
 

tomfrh

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How did you mix your standard?



I added 5.5g of sodium bicarbonate with distilled water to bring it to 200g. Then diluted by
10, then by 20. So equivalent to 138g per 1000L, which is less than the two numbers you stated.

I may have done it wrong? I did it on the amount that would add 100g of carbonate to 1000kg of water.
 

brendio

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Even though you make up your solution with sodium bicarbonate, total alkalinity is expressed in ppm calcium carbonate, so you need to divide by the molecular weight of NaHCO3 (84 g/mol) and multiply by the MW of CaCO3 (100 g/mol) and then (the bit I often forget) divide by 2 (because carbonate takes 2 protons when titrates, where bicarbonate takes only 1). I get 138 g/1000L /84 *100 /2 = 82 g / 1000 L = 82 ppm TA.

Pool Math agrees with me too, so that's comforting.
 
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mgtfp

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Thanks, Brendio. I just did my hand calculation and came to the same result that you already posted.

You put 168mg of NaHCO3 into 1l of water, which is 0.002 mol/l because of the molar weight of 84 g/mol of NaHCO3. That dissolves into the water as HCO3- and CO32- according to the equilibriums. For counting purposes we just assume it's all HCO3-, i.e. we will have 0.002 mol/l of HCO3- ions.

The definition of TA in units of mol/l is counting all the molecules that can accept H+. If a molecule can accept two H+, it has to be counted twice. And then you subtract the number of H+ ions itself. With Ca in the water, but no CYA or Borates that gives us:

TA (in units of mol/l) = [HCO3-]+[CaHCO3+]+2[CO32-]+2[CaCO3] + [OH-] - [H+] = 0.002 mol/l (neglecting already [OH-] and [H+], which will be much smaller).

We express TA usually in units of mg CaCO3 per liter (which we call simplified "ppm" because of 1 mg/l = 10-6kg/kg (using 1l of water = 1kg of water)) = 10-6 = 1ppm), i.e. how much CaCO3 in 1 l of water would result in the same TA. And because CaCO3 can accept (after dissolving) two H+ ions, you have to multiply with the molar weight of 100 g/mol (= 100000 mg/mol) of CaCO3 and divide by 2 because of the double counting you would have to do with CaCO3 in units of mol/l and you get:

TA = 100 (in units of mg CaCO3 per liter)
 

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