Often, if an investigator needs to add a very small amount of a reagent to a chemical reaction, they will first do a serial dilution as opposed to measuring the chemical directly. Why

The chemical reagents may be defined as the substance necessary to carry out the chemical reactions. The reagents can be consumed fully or partially during the chemical reaction.

The chemical reagents are needed to be diluted serially if they are measured directly without any dilution because this might produce  the faulty colors  an can give incorrect result during the  spectrophotometer. The serial dilution is important to get the correct optical density.

the answer is:

carbon dioxide