Question

You have an unknown concentration of potassium permanganate. You have determined the wavelength at which this substance has the maximum absorbance. How would you generate a standard curve? How would you use this curve to determine the concentration of your unknown

Answer 1

1. Standard Curve: Standard Curves are the graphs that are created from the known sample values(Absorbance Vs Concentration).

To generate a standard curve for potassium permanganate (KMnO4): 1. Prepare a stock solution of KMnO4 in a volumetric flask.(Eg: 1M concentration). 2. Prepare different standard solutions from the stock solution.(Eg: 0.05M, 0.1M, 0.15M, 0.2M, 0.25M....) & a blank (mostly distilled water).     3. Now find the Absorbance values at the determined wavelength for different concentrations & note the OD values.(NOTE: While finding Absorbance in Spectrophotometer (or) Colorimeter use Blank to set the absorbance at "ZERO" OD at the determined wavelength before measuring Absorbance of known Concentrations). 4. Now draw the standard graph using OD values on y-axis for each known Concentrations on x-axis respectively.( I.e, absorbance Vs concentration). 5. The standard graph is a straight line.

2. Determination of concentration of unknown from standard graph:

First, find the Absorbance of unknown at the determined wavelength( same wavelength used for finding known samples) and not the OD values( here also blank is used).

Second, point out this unknown samples OD in the standard curve ( point out on the straight line of standard).

Generally we can find the concentration value by seeing the concentration on x-axis. (Note: the scale on x-axis is given after the concentration values of respective known sample & the scale on y-axis is given after absorbance values of respective known sample).