DPD
As a rule, liquid reagents do not consist of a single preparation but comprise several components that need to be added to the sample in a certain order.
As both the size and the number of drops have a decisive effect on the resultant colour complex, the reagents need to be added with a high degree of precision.
The shelf life of liquid reagents is reduced by temporary contact with oxygen in the air when the bottle is opened as well as by unsuitable storage environments (presence of sunlight or high temperatures).