Last fall, I got some of the 50ppM CYA standard to help me test the CYA. When I used it for a CYA test I saw that the black dot had to really disappear to find the correct reading.
Today I rechecked with the standard and the black dot disappears around 80-70, not 50. I did this at first outside in sunlight with my back to the sun, but even in full sunlight, the dot is still not visible at 70.
Is there an expiration on the standard? OR maybe I have done something else wrong. I filled the test bottle to the bottom of the label with the standard, and then to the top of the label with the reagent, I did overshot and went just above the label with the reagent. Shook it for 30 seconds and then tested. What am I missing?