OBJECTIVES/OVERVIEW

The primary objective of this experiment is to determine the concentration of an unknown nickel (II) sulfate solution. You will be using the Colorimeter shown in Figure 1. In this device, red light from the LED light source will pass through the solution and strike a photocell. The NiSO₄ solution used in this experiment has a deep green color. A higher concentration of the colored solution absorbs more light and transmits less light than a solution of lower concentration. The computer-interfaced colorimeter monitors the light received by the photocell as either an absorbance or a percent transmittance value.

Figure 1	Figure 2

You are to prepare five nickel sulfate solutions of known concentration (standard solutions). Each is transferred to a small, rectangular cuvette that is placed into the colorimeter. The amount of light that penetrates the solution and strikes the photocell is used to compute the absorbance of each solution. When a graph of absorbance vs. concentration is plotted for the standard solutions, a direct relationship should result, as shown in Figure 2. The direct relationship between absorbance and concentration for a solution is known as Beer’s Law. Beer's Law is valid only for dilute solutions.

The concentration of an unknown NiSO₄ solution is then determined by measuring its absorbance with the colorimeter. By locating the absorbance of the unknown on the vertical axis of the graph, the corresponding concentration can be found on the horizontal axis (follow the arrows in Figure 2).