Question

1. why must absorbance readings be taken shortly after adding the dye?

a. the dye evaporates over time
b. the color becomes unstable after 30 - 60 minutes
c. the protein precipitates immediately
d. the dye continues reacting with buffer salts

Explanation:

When dye is added, it forms a colored complex with the substance being measured (e.g., protein in some assays). Over time (usually after 30 - 60 minutes), the color of this complex can become unstable. This instability can lead to inaccurate absorbance readings. Option A is incorrect as dyes used in such assays (like Bradford dye) are not volatile and do not evaporate significantly in the short time frame. Option C is wrong because proteins do not precipitate immediately after dye addition in most standard assays. Option D is incorrect as the reaction between dye and buffer salts (if any) is usually not the reason for taking absorbance readings shortly after dye addition. The key factor is the stability of the colored complex formed by the dye - analyte interaction.

Answer:

B. The color becomes unstable after 30–60 minutes