Testing for oxygen must be done immediately on the spot if reliable results are to be obtained. The sample bottle should be squeezed before sealing so as to ensure that no air is included in the brief time before the test is carried out.
My test kit is one supplied by WaterWatch, (Merck, Sauerstoff Test, part number 14662)
This kit contains three small dropper bottles, each containing a different reagent. 5ml of sample is placed in a tiny transparent plastic cup.
The chemicals are added and the resultant colour is compared by sliding the supplied colour card beneath the cup.