Extending the Range of Test Kits

[expose]

This is a way to extend the measurement range of your Test Kit.

While awaiting the arrival of my PS234, I needed a way to verify that I was maintaining a high enough shock level of bleach. So I took my $13 test kit from Wal-Mart and used the big cylinder to help me accurately dilute the pool water sample. I fill the cylinder to 5 ml with the pool sample water and then bring it up to the 25 ml level with tap water (distilled would be better). I swirl to mix and then test it for chlorine. If it measures 2 ppm, then that translates to 5 X 2 ppm = 10 ppm.

Use whatever ratio makes sense for the level you're trying to measure. For example if you dilute 1 unit of the sample water with 9 units of distilled water, you would multiply your reading by 10. 1 unit of sample with 14 units of distilled, you would multiply by 15, and so on.

Naturally, you loose accuracy, but at least it gets you in the ballpark.

This concept should work with any test that is looking for "parts per million" of something.

[CarlD]

Diluting your pool water 4:1 with tap water is likely to get you VERY spurious readings. Most tap water has some chlorine in it--prevents the town's water from spreading cholera, typhoid, etc.

I suggest you start with steam distilled water--every discount drug chain and most supermarkets stock it.

Use only as much dilution as you need. Start with 1 shot glass of distilled to 1 shot glass of pool water. Your WalMart cell goes to 5ppm--1:1 dilution brings it to 10ppm. If thats not enough, repeat with 2 shots of distilled to 1 pool--2:1 gives you up to 15ppm.

But by the time you reach 3:1, your accuracy is getting more and more difficult, and 4:1 is pretty much unreliable unless you are SUPER careful.

[cschnurr]

My Taylor test kit also reads to 5ppm for chlorine.
I was thinking wouldn't it be easier just to put in
2 drops and multiply by 2.5 (5/#drops)

I recognize that the accuracy would not be as good as if the scale went higher, but I would think it would be similiar to the errors from dilution.

[expose]

. . . it be easier just to put in
2 drops and multiply by 2.5 (5/#drops)
I don't know the mechanism by which this test works, but I do know that is does NOT behave the way you are suggesting. I experimented with the chlorine indicator in my test kit (hth OTO hydrochloric acid <10% Orthoiodine <1% apparently made by Arch Chemicals, Inc., Norwalk, CT.)

With a test sample containing a chlorine concentration of about 18 ppm, the sample was actually DARKER with 1 drop of the OTO solution than it was with the required 5 drops. That surprised me, so I tried it again and got the same results.

Then I tried it with a test sample containing a chlorine concentration of about 1 ppm. The sample color remained essentially the same regardless of the number of drops of the OTO solution (1 through 5 drops tested.)

On the other hand, the method that I suggested is definitely valid, at least in theory, although it is subject to loss of accuracy even when you carefully follow a procedure to minimize errors. The reason for it's validity is simple: by combining your test sample with distilled water you are DILUTING your sample - in other words reducing the concentration of chlorine. The amount of that reduction can be precisely calculated if you know the relative quantity of the distilled water to the quantity of the test sample.

Like I said, it's just a way to get a ballpark idea of high chlorine concentrations while waiting for a better test kit to arrive!