More CYA questions and info

[otter86753]

I have been following with great interest the threads about testing CYA. I have been "vigorously" shaking my CYA test as opposed to "tilting" back and forth to mix. I used my usual method which was vigorously shaking and hit my target of 70 ppm of CYA. I then immediately repeated the test with a tilting back and forth to mix and tested at 60 ppm. I poured back and forth to test each method a total of three times and each time achieved the same test results of 70 and 60 respectively (no difference in a pour back and recheck). I know there has been considerable discussion of the accuracy of this test, and thought I would add my experience and see what others thought.

Here is my full set of numbers

CL 9
CC .5
PH 7.6
TA 80
CH 250
CYA 70 ( with vigorous shaking) 60 ( with tilting back and oath)
Borates 50

 

[Patrick_B]

The technical resolution is approximately 15 PPM (minimum of 10) so there really is no difference. Shake well to mix is more than adequate and changing shake methods is really just about like splitting hairs.

 

[otter86753]

So if there is a +/- of either 10 or 20 PPM accuracy margin per post #4 in this thread by JasonLion CYA Test Accuracy , I could have anywhere between 40 to 90 PPM CYA based on this 10 ppm differential in less than a two minute time span between the two tests? I do not have the problem I see reported by some people of not being able to see when the black dot disappears; I always see it disappear. I am going to get some of the standard R-7065 and test once there is a resolution with the 50 PPM issue.

 

[chem geek]

For lower CYA levels the margin is closer to +/- 10 ppm or even less while at higher CYA (near 100 ppm) it's closer to +/- 20 ppm though with care it again can be less. Basically you have multiple sources of error that can add together. The concentration of the melamine reagent is fairly tightly controlled by Taylor and should be within 5% or less in error (though we had a recent situation where there CYA standard was off by at least 10 ppm if not 20 ppm which obviously is not good). The measuring of the water volumes for the sample and reagent may be off and though usually this can be controlled to be within 5%, if sloppy the error could be 10% or more. The turbidity viewing part of the test is very much influenced by lighting conditions and one can easily get a 20 ppm swing if the wrong lighting is used, either indoor lighting (unless VERY strong indirect light is used) or direct sunlight can cause problems.

Personally, I wouldn't shake vigorously because we do not know if introducing lots of small bubbles of air into the mixture causes any problems with keeping the melamine cyanurate precipitate in suspension. The idea is simply to mix the sample water with the reagent and that is most easily done by just tilting the bottle to one end and then the other slowly back and forth during the 30 second interval before pouring into and viewing the CYA tube.