Is it normal for the API Phosphate test kit to show bright blue when only K2S04 has been added?

Since my new planted tank shows bright blue with the test kit after dosing I decided to check somethings, clean tap water shows negative for phosphate but adding one drop of my mix of Potassium Sulphate(K2S04) and the test then shows bright blue.

I use the term bright blue since it is at or above the 10PPM range of phosphate.

Right now just dosing the tank with KN03 and Trace.

Well thanks for any help,,advice

I can't answer that question, but when I evaluated phosphate test kits 2 years ago, Seachem's kit was clearly more accurate than several others. They also included a test solution with it.

Bill