Alkalinity test question

[tcfarley]

I have the TFP standard solution for Total Alkalinity 100ppm, along with new TA reagents R-0007, R-0008 and R-0009 and a new test cylinder.

When I do the TA test with the standard solution (2 drops R-0007, 5 drops R-0008, and then counting the drops of R-0009), I get a TA measurement of 110ppm. That is, the 11th drop of R-0009 brings the color from a muddled green/gray/red to a full bright pink, and the 12th drop does not change the color. I have repeated the test two times (there's only enough standard solution for two tests), two days apart, with the same result each time.

What am I to make of this result? Does it mean the solution is not 100ppm? Should I use this result to calibrate my future readings (i.e., should I subtract 10ppm from all my future measurements, since my apparatus appears to read 110ppm for a solution that is actually 100ppm)?

My pH happens to be high right now (7.9)... could that be messing things up?

Thanks for your advice!
Todd

FC 3.0
pH 7.9
CYA 70
Salt 3500
CH 480

 

[ajw22]

Drop size matters for the TA test and most other tests. This says you are not being patient enough letting each drop form and fall off the tip of the bottle. Your technique for drops is 10% too small.

See...

Total Alkalinity

A drop-based test used to measure the alkalinity in water. Alkalinity helps serve as a … Read more…

www.troublefreepool.com

Notes

• Sometimes a static electric charge can build up on the R-0009 dropper bottle tip, causing the drops to be smaller than usual and making the test read higher than actual. You can prevent this by wipping the tip of the dropper bottle with a damp cloth or tissue before you start and after each drop.
• Hold the dropper bottles vertically and squeeze gently, so that drops come out slowly and seem to hang on the tip of the dropper bottle for a moment before falling.

There are many ways you can cause water testing errors - Water Testing Errors - Further Reading

A TA of 100 vs 100 makes no difference to anything. The TA test is the least important test in your kit. I wouldn't worry about it. But you can improve your drop technique.

 

[tcfarley]

Kind thanks for the insight and pointers, Allen. You were spot on!...

I remeasured my TA just now, this time being meticulous with my technique. Sure enough, my reading this time was 10ppm lower.

When I got the speedstir years ago, the drop tests kinda became like a contest -- "Look how fast I can do this now!" I've been plowing the drops into the cylinder and swiftly moving on to the next test. Whee!

Thanks for reminding me to slow down and smell the "doses," Allen.

Much appreciated,
Todd