Quote Originally Posted by dmagerl
I dont think its an issue of going bad. I think its an issue of getting more concentrated without adequate shaking.

If you look at the solution when you are adding drops to the test tube, the drops sink right to the bottom of the tube. They are much denser than water. It must do the same in the bottle and thus it will tend to get stronger as you get to the bottom of the bottle without adequate mixing/shakng.

So as you use up the bottle, the concentration changes and it appears you have more nitrate than you really do.

Thats my theory and I'm sticking to it :-)
Yes, thats why they tell you to shake the bottles. But i have two ph tests that have never been used that read the lowest reading everytime. I won them in an auction, unopened, but old. Explain that one ;)

They say the tests are good for a year or more. Of course, you have no clue how long one has been on the shelf and bad tests have been purchased in the past.

They do, for sure, expire with time.