Taylor R-0870 test powder (crystals) - hard to measure for anyone?

[aylad]

The refill container I got prior to the one I'm using now was the same way--it came to me already in crystals. You're right, a lot of it does stay undissolved in the water but that's okay. If you get pink, then don't worry about what doesn't dissolve. Your test will still be accurate.

[Mm_putnam]

So if I get any pink when I add the powder/crystals (regardless of qty of powder/crystals) I will get an accurate reading? That surprises me - but pleasantly surprised if that's the case.

[moblaine]

I'm no scientist, but I had been wondering if the crystals all dissolved, would my sample have been a darker pink and would it have taken more drops to turn it clear and therefore would I get a larger FC reading?

[Mm_putnam]

That's what I was thinking. I would think the # of drops of the reagent in comparison the amount of the powder is important - not just in comparison to the presence of the powder. But I also am not a scientist.

[BigDave]

Think of it this way:
The powder color tags the Free Chlorine. It doesn't matter how much because the Free Chlorine takes the color it needs, the color is just there for the ride.
The titrating drops eliminate the color tagged Free Chlorine. When there's no more color tagged Free Chlorine the color goes clear.
The 5 drops for the CC test turn any amount of Combined Chlorine into Free Chlorine which then picks up the color it needs.
Then you use the titrating drops again to eliminate the newly Freed Free Chlorine.

Does this make sense?
I'm probably wrong on the chemistry but I think the analogy works.

[AnnaK]

Does this make sense?
I'm probably wrong on the chemistry but I think the analogy works.

Loved your post!

[waterbear]

The amount of DPD is not critcal. You are doing a titration. It works like this:
1) DPD is added to the sample. If any free chlorine (or free bromine if you are testing bromine) is present it oxidizes the DPD and forms what is known as Wurster Dye which is a magenta colored molecule. If there is a really high level of chlorine prenent then this Wurster Dye is futher oxidize to a colorless imine (the bleachout that is seen with DPD) adding more DPD power will shift this chemical reation to reform the Wuster Base (only possible with FAS-DPD testing and not with DPD testing because with DPD testing we are looking at how deep the pink color is, i.e. the AMOUNT of Wurster Base formed in our sample. With FAS-DPD testingi we just need it as an indicator so we know when we have neutraized all the FC in the sample)
2)we then titrate with a reducing agent (Ferric Ammonium Sulfate--FAS) which 'neutalizes' the chorine in the sample. THIS is the quantitative part of the test. The amount of FAS used is important, not the amount of DPD. Once all the FC is neutralized and gone then there is non left to react with the DPD and the Wurster Dye is gone also--the sample turns colorless)
3) since this test is specific for hypochorous acid and hypochlorite ion (free chlorine or the equivalent acid and ion for bromine or iodine ) we can then determine the amount of other oxidizer present (usually our chloramines or bromamines but also this test will show MPS and ozone present) by adding a potassium iodide solution. This is easily oxidized by all the above oxidizing agents (chloramines, bromamines, MPS, ozone) and forms the iodine equivalents to hypochlorous acid and hypochlorite ions (Free Iodine) that once again form Wurster Dye and turn our sample pink again. If there is no oxidizer present the sample stays clear and we have no CC (or MPS or Ozone interference). If it turns pink we titrate again to reduce the free iodine back into iodide ions and the sample turns from pink to colorless and we have our CC reading.

So you see, the amount of DPD is not critical. You just need enough to determine if there is FC (or Free Bromine or Free Iodine) present in the sample and then you convert that into either chloride, bromide, or iodide by titrating witth a reducing agent (FAS). When all the FC (or other free halogen) is all converted there is no longer any left to react with the DPD and form Wurster Dye and the sample is colorless again. The amount of FAS needed to do this is what measures your FC (or Free Bromine)

To determine the CC you convert it into Free Iodine and titrate again. You do this by adding potassium iodide (which is very easily oxidzed into free iodine). The FAS reduces the free iodine back into iodide and, once again, the amount of FAS needed determines the concentration. The DPD is only a means to create the Wurster Dye, which is our indicator. The AMOUNT of Wurster dyd created is determined by the amount of Free halogen present and not by the amount of DPD, which is added in excess (even 1 scoop is excess as long as the pink color forms!)
Hope this explains it.

(Yeah, I'm the other chemistry geek in the forum!)

[chem geek]

Someone on another forum did a test using different scoops and they did get slightly different readings but generally only differing by 1 drop or sometimes 2 if the transition was on the edge between drops. The reason is mostly that with less dye the transition from light pink to clear is a little more subtle. There is an equilibrium between the chlorine in the water and that which has reacted with the dye (though the reaction is mostly towards reacting with dye) so with more dye the intensity of color at any given chlorine level (even a low level) is higher, though not proportionately. So it is potentially more likely for someone to miss the transition because it is so faint (i.e. they think something is clear when it is really just very faint pink). In other words, the issue isn't with the chemistry, but with your eyes and how clearly you can see more subtle transitions in color.

In practice, I use just one heaping scoop in a 25 ml sample and don't have a problem seeing the transition, but that's already a 0.2 ppm resolution so not a big deal. For those using a 10 ml sample, the transition is much stronger anyway since one drop represents 0.5 ppm so again, not a big deal.

[waterbear]

Someone on another forum did a test using different scoops and they did get slightly different readings but generally only differing by 1 drop or sometimes 2 if the transition was on the edge between drops. The reason is mostly that with less dye the transition from light pink to clear is a little more subtle.

And since the precision of this test is 1 drop this is not significant at all! People seem to loose sight (or don't know or understand) that ALL testing methods have an inherent margin of error. Drop counting tests are normally +/- 1 drop. With the Taylor K-2006 that would be +/- .5 or .2 ppm for the FAS DPD test, depending on whether the sample is 10 ml or 25 ml. and for the TA and CH tests it would be +/- 25 ppm or 10 ppm once again depending if the sample is 10 ml or 25 ml. On the pH test you can interpolate between 2 color blocks so you can get an accuracy of .1 unit. However, the CYA scale is not linear so you really cannot interpolate between 2 markings so your accuracy is +/- 10 ppm.

Many people confuse accuracy with precision. Accuracy means that the test is producing results that can be trusted and that they are correct. Precision is the amount of variation of results you get when you repeat the test on the same sample.

For example, let us say we have a standard solution for testing CH that contains 100 ppm. If we test it and always get 80 ppm from a particular test kit on 4 separate tests on our standard then that test kit precise (it give us the same result every time we test on the same sample) but it is not accurate (it is giving us an erroneous result).
Now let's test our standard with a different test kit. We get results of 70 ppm, 90 ppm, 100 ppm, and 120 ppm upon testing 4 times. Since we cannot get results that repeat at all this test kit is not precise. It might be accurate but have a very large "window" or resolution so we are not really getting any meaningful info (such as test strips that have as much as 40 ppm between color blocks). Actually, if we average our 4 tests we come up with 95 ppm so, in reality, this test kit is is fairly accurate but not very precise so the results from one individual test are not going to give us useful info but if we average many tests we can get useful info. This is time consuming so it is not a good solution.
Now, let's use a kit that has both good accuracy AND precision (such as a Taylor K-2006). Using a 25 ml sample (which should give us a precision of +/- 10 ppm because the test has an accuracy of +/- 1 drop) we get results of 90, 100, 100, and 110 ppm. If we average these results we get 100 ppm. In other words, we should always get 100 ppm but a result of 90 or 110 ppm are considered to be acceptable and in range and in the grand scheme of things with a pool or spa are not going to matter very much at all since the accuracy and precision of our water volume measurement usually have a much larger error when it comes to how much of a chemical to put in to adjust a water parameter. This is why we usually say to add less than what you think you need of a chemical and retest to "creep up to your goal" so you don't overdose. The only exception I can think of is when shocking a pool, where it is generally better to slightly overdose with chlorine than to slightly underdose.

To summarize, we just need to remember that this is a swimming pool, not a science experiment. We need a test kit that will give us reliable, repeatable results with small enough margin of error on any one test that we know that we are about where we need to be but we don't need to become obsessive that when you do a test one time you need 6 drops and when you repeat it you need 7 (or even 8!) so something must be wrong with the testkit. You are within the accepted margin of error for that test! If you consistently have an error greater than that then there is a problem somewhere and the first thing I would look at is your testing technique.

Also, for those of you using 'digital testers' be they strip readers or colorimeters with liquid or tablet reagents be aware that this does not change. Just because you get test result reading with 2 decimal places does not mean that it is accurate (repeatable) nor does it mean that you have two decimal points of precision. A perfect example is a respected company's pool store testing lab that uses little vials of dry reagent that have water added to them with a calibrated pipette and are then read in a colorimeter that costs over $1000. Lets use the TA test as an example It has a precision of + 10 ppm and - 20 ppm even though the readout gives you a number like 98 or 71, so one would assume the precision is 1 ppm, which is totally false. If one saw these two numbers on two readoutsand wanted a target TA of 80 ppm they would think that they would need to lower the TA in the first case since it is almost 20 ppm to high or bump it up a bit in the second case since it is almost 10 ppm too low However, when we look at the precision of this test we realize that if our TA is, in reality, 80 ppm then both test test results are within the precision of this testing system. The problem is that when you get your printout from the pool store it just says that your TA is 98 ppm or 71 ppm with instructions on how much "Akalinity UP" or "pH Down" that you need to bring the reading to the "ideal number". Just because a testing method is giving you a digital readout does not mean that is the precision of the test.

[JimK]

And since the precision of this test is 1 drop this is not significant at all! People seem to loose sight (or don't know or understand) that ALL testing methods have an inherent margin of error. Quote trimmed by mod

Great post! Thanks.