Strange PH readings

[chiefwej]

Bingo! Pool School says that correct pH readings are impossible at shock levels. Well with a CYA of 20, shock level is FC of 10.

 

[JoyfulNoise]

I don't recall it mentioned, but I wonder with little to no cya in the water, a FC of 5.5 is extremely harsh and could very well be causing a false high reading of the pH test. Is this a possibility Matt?

It is preached here that FC levels over 10 cause false high readings on the pH test but that should be much more dependent on the CYA level since a FC of 10 with 30ppm CYA is much more concentrated than 10 FC at 80ppm CYA.

This is not quite right. Let me explain.

First to the question of CYA testing and FC. It is possible for extraordinarily high FC to affect the formation of the melamine cyanurate precipitate but it actually causes a false LOW value not a false high value. If the FC level is high enough, then there will be more chlorinated cyanurate anions than pure cyanurate (hydrogen terminated) anions. Chlorine attached to the cyanurate anion will interfere with the hydrogen bonding that occurs between the cyanurate molecules and the melamine molecules (it's a complicated planar long-range 2D structure) and that will show up as less precipitate formed.

HOWEVER, the Taylor test gets around this problem by simply shifting the pH of the test solution to a lower value (pH ~ 5 or less). At low pH, almost all of the cyanurate is not chlorinated and therefore available to react with the melamine. So unless you have the conditions of very high FC AND very high pH (to the point where the R-0013 is unable to buffer the pH to a lower value), then the CYA test is valid even at shock levels.

Now, onto the second point. As far as FC is concerned with it's reaction toward phenol red, CYA has nothing to do with it. The R-0004 phenol red reagent (as well as the R-0014) has in it a mixture of dechlorinating chemicals to neutralize the FC in a pH neutral way. Peroxide (H2O2) and sodium thiosulfate (R-0007 reagent) will both dechlorinate water BUT they do so in a way that changes the pH. The R-0004 is formulated to avoid that kind of pH shift. However, there is only so much FC that can be removed by the addition of these buffering chemicals and so the stated limit is about 10ppm FC. The true limit is a little more complicated than that. Between 10ppm and 20ppm FC, the reaction between chlorine and phenol red starts to happen but does so at a slow rate. If your water had 15ppm FC in it and you added the pH reagent, the initial color will be very close to the actual pH and you would see the color become more purple as time goes on. At FC's higher than 20ppm, the reaction between chlorine and phenol red is really too fast to get an accurate read. Above 25ppm, the reaction happens almost instantly. There is a trick you can perform if your FC is between 10ppm and 20ppm - dilute your pool water sample 1:1 with DISTILLED water and you'll cut the FC in half without affecting pH much at all. This happens because the distilled water has a pH of 7.0 and almost no alkalinity, so it will not really affect the pH of the pool water mixed into it since the pool water has a MUCH higher TA. In other words, the TA of the pool water will buffer the diluted sample against pH change.

CYA has nothing to do with it because, even if it is present and buffering the chlorine, the reaction between chlorine and phenol red will still proceed and, as the active chlorine gets used up, the CYA will release more chlorine just as it would if it was oxidizing bather waste for fighting algae. Eventually all of the bound chlorine will be released as the reaction between active chlorine and phenol red proceeds to completion.