PH Testing & Chlorine Neutralizer

[Firefly2000]

I have the TF100 test kit and have been using it now for about 6 weeks. Here's my current readings..

FC: 10
CC: 0
TC: 10
TA: 60
CH: 260
CYA: 45
PH: 7.3 (using the Taylor K-1000 blue kit that comes with the TF100)

I also have a PH probe on my Rolachem, currently reading 7.5.

I would like to calibrate the PH probe, so I thought I would also test the PH using my old test kit, (a locally supplied Australian kit) for comparison purposes.

My old test kit read 7.7. The interesting thing is that the old kit requires that you add 1 drop of chlorine neutralizer before adding the 5 drops of dark pink PH reagent. (I read elsewhere on this forum that the better PH kits use chlorine neutralizer... Interestingly, the Taylor K-1000 does not)

So, I thought I would experiment further. I put on my put lab coat and protective glasses and did the following:

(1) Use the K-1000 test kit, but add one drop of the chlorine neutraliser from the old kit before adding the 5 drops of reagent. Result: PH = 7.7

(2) Use the old kit, but don't add the chlorine neutraliser. Result: PH = 7.3

So, in summary, both kits give the same reading with and without chlorine neutralizer! (7.3 vs 7.7)

So, question is which reading do I trust? How much affect does chlorine normally have on PH readings? (My FC is currently high at 10). I thought high FC tends to increase PH readings!

Some background:
I realize that most people here will say "trust the Taylor K-1000 - 7.3 is correct". And I would tend to agree...
My old test kit has reagents that are almost 2 years old. Is it likely that the chlorine neutralizer has gone "off" and is raising the PH reading?

 

[chem geek]

The Taylor pH tests have some chlorine neutralizer already in their pH Indicator drops so their comparator tube colors already account for the effect of the pH of the chlorine neutralizer solution itself and on normal amounts of chlorine. Kits that have you add separate drops may not be calibrated as well and may also not have as much pH stability in their test solutions. I quote below what Taylor says about false high pH readings in the presence of high chlorine levels.

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FALSE READINGS: high levels of chlorine (usually >10 ppm) will quickly and completely convert phenol red into another pH indicator (chlorphenol red). This new indicator is a dark purple when the water's pH is above 6.6. Unfortunately, some pool operators mistake the purple color for dark red and think the pool water is very alkaline and wrongly add acid to the pool.

When a sanitizer level is not extreme, only some of the phenol red may convert to chlorphenol red. However, purple+orange (for example, pH 7.4) = red. This error is more subtle as no purple color is observed and the operator does not suspect that a false high pH reading has been produced. Some operators neutralize the sanitizer first by adding a drop of chlorine neuralizer (i.e. sodium thiosulfate). However, thiosulfate solutions have a high pH and, if heavily used, may cause a false higher sample pH.
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It seems to me that you are seeing the effect mentioned in that last sentence.

Richard

 

[dschlic1]

I suggest that you purchase pH buffer calibration powders to calibrate your pH meter. Hach offers these in a convient powder form: pH 7.00 and pH 10.00