Can you "remove" fc so the pH test is accurate?

march2012

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Jan 21, 2012
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As some of you might remember, my condo complex had FC of 38 and CYA > 200 (leslies tested the water and said it was fine @ 45 CYA and 5+ FC).

I mentioned to the person maintaining the pool that pH could not be measured if FC is high. She said she adds a reagent that deactivates the chlorine in the sample so she can measure pH. Is this possible?
 

Bama Rambler

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The problem is the "stuff" (most likely sodium thiosulfate) that's added to reduce the FC, changes the pH of the sample so the test isn't accurate any more.
 

march2012

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Jan 21, 2012
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TX
Bama Rambler said:
The problem is the "stuff" (most likely sodium thiosulfate) that's added to reduce the FC, changes the pH of the sample so the test isn't accurate any more.

Yeah it was sodium thiosulfate

Here is what wikipedia says:

In pH testing of bleach substances. The universal indicator and any other liquid pH indicators are destroyed by bleach, rendering them useless for testing the pH. If one first adds sodium thiosulfate to such solutions, it will neutralize the color-removing effects of bleach and allow one to test the pH of bleach solutions with liquid indicators. The relevant reaction is akin to the iodine reaction: thiosulfate reduces the hypochlorite (active ingredient in bleach) and in so doing becomes oxidized to sulfate. The complete reaction is:
4 NaClO + Na2S2O3 + 2 NaOH ? 4 NaCl + 2 Na2SO4 + H2O

So the sodium thiosulfate will make a test read more acidic right?
 

JasonLion

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Adding sodium thiosulfate will raise the PH when chlorine is present. However, you have no useful way of knowing how much it will raise the PH, so this isn't a useful approach.
 

UnderWaterVanya

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Chem geek has said in another thread that you can dilute the water with distilled water (make sure it's distilled) which has 0 TA and it will allow you to lower FC without impact to the pH. Another option is to simply take the test FAST and hope for the best - the conversion to chlorphenol red isn't instant but it is quick. This is only really needed in situations like this where the FC has to be high due to high CYA - when shocking the FC levels are high and due to the bleach (per Chem geek) the pH is also elevated and you should not try to adjust it at that point because as the FC is consumed it will fall back to normal.

Again - in this case - HIGH CYA - there is a reason to do this.
 

X-PertPool

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what about running some water through a brita filter? Does the filter affect the pH, because I know it will reduce the chlorine
 

chem geek

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San Rafael, CA USA
Yes, activated carbon filters affect the pH, if for no other reason that the removal of hypochlorous acid is an acidic process:

C* + HOCl ---> C*O + H+ + Cl-
Activated Carbon + Hypochlorous Acid ---> Oxidized Carbon + Hydrogen Ion + Chloride Ion

In theory, one can calculate the original pH if one knows the original FC and the final pH, but there may be other pH effects with activated carbon beyond what is shown above.
 
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