The results of two flame tests are shown in the following images. The first image shows the results of a flame test with potassium nitrate, and the second image shows the results of a flame test with potassium dihydrogen arsenate.

Why is potassium dihydrogen arsenate a duller color than potassium nitrate?

Potassium dihydrogen arsenate is often contaminated with sodium, which dulls the color of light emitted.

Potassium dihydrogen arsenate has a more stable excited state. so the emitted light is dimmer.

Potassium dihydrogen arsenate also contains arsenic, which is a dull blue color.

Potassium dihydrogen arsenate is heavier, so it does not excite as intensely