CYA test - Just an FYI

[kevincad]

I don't believe an excess of the reagent is going to cause off results since the precipitaton is going to be in the reaction with the CYA in the water and once it has all reacted then an excess of reagent should not cause any problems. I could be wrong on this but I don't think so. The correct mix is equal parts of each.

I'd agree with you, if it weren't for the fact that when the poster used the marks, the CYA was 50, and when she used equal parts, the CYA was 30. That shows me that it would make a difference! Thanks for clearing it up. I'll make sure to use a 50/50 solution. Actually I'm going to try it both ways and see if I get different readings.

[chem geek]

There should be enough melamine in the reagent "half" to nearly completely consume the CYA with melamine leftover. So the precise amount of reagent added is not relevant. What IS relevant is the proportion (fraction) of pool water compared to the total volume of water after the reagent is added. The tubidity (cloudiness) is a function of how much CYA is in how much total water ("total" meaning the mix of pool water with reagent). [EDIT] So with CYA levels that are lower -- say less than 50 -- you should theoretically be able to add half the amount of melamine reagent and fill up the rest of the half with distilled or tap water and get the same result. The key is that the pool water should make up half of the total amount. [END-EDIT]

So, if the bottle markings are not truly 50/50, then this would influence the results of the test. If, for example, the markings led to an extreme of 25/75 with the 25% representing pool water and the 75% representing reagent, then you would get readings for CYA that were about half as large.

So yes, it is important that the markings be accurate from a fractional mixing point of view. The absolute total volume doesn't matter -- it is the ratio of pool water to total water ("total" being pool water plus reagent) that matters.

I checked my Taylor bottle Part #9191 and the markings do approximately give a 50/50 mix with the first marking at 7 ml and the second marking at 14 ml.

Note, however, that the difference in fraction required to get a 30 ppm reading compared to a 50 ppm reading is quite large. Assuming that a 50 ppm reading is from a true 50/50 mix, then a 30 ppm reading would require a 30/70 mix with 30 being the pool water portion. That should be quite a visible difference in the bottle unless the bottle has something very strange going on with it.

Richard

[waterbear]

Richard, thanks! Totally slipped my mind that the turbidity will be affected by the total amount of liquid in the mix! I was only thinking of the reaction itself!
That's what I get for trying to think after getting off of work! (I work with special needs kids and this year I have my hands full!)