Help with Taylor K2006 PH test

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J

jnorris

0
Apr 9, 2007
68
N.E. South Carolina
I could swear I saw someone with this same problem, but maybe it was over at the PF site? Anyway, everytime I do a PH test, I get a different reading if I do it twice. For a while I was testing FC first, then PH, and that's when I noticed the problem. Then I thought, maybe I'm not rinsing well enough. So I am now doing the PH test first, but it still does it. The first test is ALWAYS lower. For instance today, first test was 7.2, then did it again and got 7.6. A couple of times it was 7.0-7.2ish the first time, and 7.8 -8.0ish the second time, which is WAY different! I'm trying to figure out if it could be the dropper tip, but it really doesn't look like the drops are a different size.

By the way, if I do it a third time, I get whatever was on the second test, so I'm assuming the first test is incorrect.

Anyone else having this problem? Should order a different reagent bottle?
 
Assuming your'e rinsing the comparator tube well, the results of an earlier test can't affect a subsequent one.

My guess is that the drops are not precisely the same size. Try wiping the tip of the reagent bottle before each drop.
 
For the pH test, the drop size shouldn't matter very much since the test compares color hue, not intensity. However, contamination on the dropper could certainly influence the pH, but it would have to be something pretty strong which seems unlikely.

There have been 2 (one on the PoolForum) reports of this varying pH problem with the first test being the one that is different from subsequent tests. I don't know what is going on, but will describe what I do and I'll have to test multiple times to see if I get any variation (I think I've done that a few times and don't recall any variation). Whenever I take a pool water sample, I flip the comparator tube upside down and put my hand about a foot or so (probably 18") below the water surface and then flip the tube so it becomes upright and fills with water. I then take the comparator out (now filled with water), then invert it and dump the water back into the pool. Then I go down again (inverted) and repeat to get a water sample. This sample is the one I then spill out excess into the pool (to get to the appropriate line, which for pH is the 44 ml line near the top). I cap the tube (which I rinse in the pool water first) and then take it back for measurement, adding the 5 drops, capping, inverting a few times, then looking at the color. After I'm done with the test, I dump the sample into the sink and rinse out both the tube and the cap with filtered water (though tap water should be fine -- we just happen to have a filtered water faucet at the kitchen sink where I do my tests so I use it).

If the sample is shaken instead of inverted, then it's possible for the aeration to make the pH rise a little. That's one possibility, though doesn't explain why only the first sample is off. It could be that though the tube is cleaned, the cap is not. That would tend to make the first sample bad if the previous test was not a pH test.
 
After reading your post a few times it sounds like a reagent delivery problem..Moste dripper bottles are desigined to deliver 50 microleters per drip..I have mesured it here and the lab using a scale and found they will have about a 2 to 5 microleter varance when the bottle is held verical while having a 10 to 12 microleter varance when not held verticle. When you take in to account that it takes 5 drops there is a mesureable diffrence.
Steve
 
I can understand the difference in volume affecting the drop counting tests, like FC, CC, TA and CH, but would that really affect the pH test that much since it is mostly looking for a comparison of hue and not intensity (saturation)? I suppose one can just add 2-3 drops and see what it looks like compared to 5 drops and 7-8 drops. If they come out similar in color hue, then the problem isn't the drop size.
 
I just tested with a HTH ph kit that uses 9.5 ml for sample and an Aquatrend test kit that uses 44 ml for testing. I grabbed some water from our DI water source at work and adjusted the pH to 7.4 and brought it home. I then tested with rinse between tests adding a drop at a time in each kit. This made for a total of 8 runs between each methodology.
The HTH kit showed some variance from one drop to the next in color intensity witch could be mistaken for different pH results.
The Aquatrend showed even more variance due to the greater dilution on the test reagent.
I feel that small variances with in test drop delivery can change the test results on the colorimetric testing mythologies due to prozone effect. The prozone is the concretion of reagent to sample volume that matches the color slides we use for comparison. After reading on Taylor’s site they also recommend to hold the bottles vertical to get greater precision for with in run testing.
Steve
 
medvampire said:
KurtV said:
Ockham's Razor...
Click to expand...
I was thinking along the same lines until I read chemgeeks post. I respect his opinion and he is an top notch theory chemist but we have disagreed more than once when it comes to real world chemistry. I do lab testing for a living and had to disagree this time.
Steve
Click to expand...

My post was in no way directed at Richard. I always find his posts insightful and made in a spirit of elightment and/or inquiry.
 

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No offense was taken and for the record, I'm not a chemist. A chemistry geek/nerd, maybe. This is why we have forums and multiple users. None of us gets it all right all the time. It's through joint efforts and matching theory and knowledge with experience that we get to models that help us predict and control our future.
 
I played around with my Taylor pH test tonight while checking the pool. Here's what I did:

Normal test as per instructions: pH 7.2
Repeated without rinsing test vial:pH 7.2
Added an extra drop:pH 7.2 (slightly more color, but no way it could be confused for another shade)
Rinsed and tested with 4 drops:pH 7.2 (slightly less color, .....)

No way could I have been fooled by a reagent volume error of +/- 1 drop or less. I can't say that other people might not have been fooled. If it was between two colors, darkening or lightening the solution MIGHT have led me to guesstimate the wrong way.

Just another data point.
 
I have read the instructions that came with my test kit and it states to hold the bottle vertical. At first, I was holding the sample bottle and swirling it vertically. After doing the test a couple of times I figured they meant the regeant bottle because I noticed the drops were not consistent and in fact sometimes the first drop had an air bubble. So, I came up with some kind of process to test my water. Can some one please chime in and correct me if what I am doing is wrong. Maybe this might help other newbies.

When doing multiple tests I use one of the sample bottles that my local pool store hands out, this way I don't have to keep sticking my hand in everytime and when I empty water from the collector tubes to get the correct volume I usually over do it and have to start over.

First, I take the sample bottle and go to the deep end, stick the bottle upside down about 18-20 inches below the water surface and invert the bottle, which fills up, I then empty and repeat keeping the second sample. I then go over to my patio table with all of my test chemicals and follow all the instructions making sure to swirl the sample while holding the reagent bottle vertical. After each test I use the same pool water from my sample bottle and rinse. I do use the caps that were provide for the OTO tests, CYA and shake to mix, I only swirl when adding chemicals that are supposed to change colors.
 
Go to a beauty supply store and you can get a really good plastic applicator bottle with a cap. You can get all the water you need to do a full series of tests and it has a cap to prevent outgassing between your tests (someone said that was important). Works great for me.

Donnie
 
If you are doing a test that is NOT a titration test (drop count test) then cap the vial and invert it 2 or 3 times to mix it....don't shake it. If you are doing a titration test then you want to swirls for about 10 seconds between drops (30 seconds for the calcium hardness test--this test can take a while if you are doing it right and your calcium is high. I have been doing this test for MANY MANY years. It is also used in salt water aquarium keeping and I've been doing THAT for over 30 years now!) For the CYA test you do want to shake it hard, then let it sit for about 3-5 minutes, then shake again before dispensing into the view tube.

For titration tests you want to keep adding drops until the last drop added does not produce any color change. For example let's use the TA test. You added 10 drop and the indicator turns red. You add an 11th drop and it gets redder. You add a 12th drop and there is no color change. Your endpoint is 11 drops (110 ppm Total Alkalinity).

NOTE THAT SOME TESTS MIGHT NOT FOLLOW THIS. THE SALT TEST IS AT ENDPOINT WHEN THE COLOR FIRST CHANGES TO REDDISH ORANGE AND THE COLOR HOLDS (DOESN'T DISAPPEAR IMMEDIATELY). IF YOU CONTINUE THE COLOR WILL STILL CHANGE TO A BROWNISH RED COLOR AND YOU HAVE GONE TOO FAR.
 
waterbear said:
If you are doing a titration test then you want to swirls for about 10 seconds between drops (30 seconds for the calcium hardness test--this test can take a while if you are doing it right and your calcium is high.
Click to expand...

You're talking 20 minutes to do a test if you're CH is 400. Do you really wait this long? Is the long delay between drops because swirling is an ineffective method of mixing, or because it takes that long for the reagent to properly interact with the liquid under test?

I've seen a unit down at my local Leslie's that has a pill shaped piece that's dropped into the test vial and then rotated by a magnet underneath the test vial for mixing purposes. Would using something like this allow the test to be sped up, or is the 30 seconds between drops still required? Does anyone know if units like these are available for the average consumer?
 
You are tallking about a magnetic stirrer. It is a standard piece of laboratory equipment ( and was an option on some top of the line stoves and cooktops in the 60's!) You can pick one up for under $100 at most laboratory supply companies. They will speed things up a bit but even with a stirrer and a burette to dispense the reagent the drops should not come that fast. Hand swirling is much more inefficient which is why I suggest swirling for 30 seconds. It does take a bit of time for each drop of titrant to react with the calcium in the water. The calcium hardness test is not a fast test. It does take a bit of time, but then again, it's really only necessary to do it maybe once a month since it doesn't change that fast if you pool stays balanced. A magnetic stirrer will mix the reagent more quickly and the drops can be added about 3-5 seconds apart in that case. I know that I might get some people disagreeing with me on this test but I do have many years experience with it using tests from several different companies and doing it this way will give reproducable results.
 

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