Straight Lines/Standard Curves

TOC

Standard curve–the wavelength

Choosing a wavelength of light

Before we use the spectro there is one other thing we need to know: the wavelength of light the compound (in this case the cordial) absorbs. In most experiments, this is known and is provided in the methods.  The wavelength can be different for different chemicals. Do you remember the spectrum of light??  Water doesn't absorb any light, so all wavelengths (colors) get through, and mixed together, they look like white (= colorless) light. Our cordial solutions look red because the solution absorbs more of the other colors of light. The more concentrated the drink, the more non-red light that gets absorbed–and correspondingly, what does get through looks redder. 

 

 

 

To test the concentration of our cordial, you would need to set your spectro to measure something other than a red wavelength. So in general, before you use a spectro, you have to figure out what wavelength works the best. If you remember from first year chemistry you could run an absorbance scan of the solution across all wavelengths to select the one that absorbs the best, or use a trial and error approach.  Remember, unlike our cordial solution, some chemicals absorb UV light, so by the naked eye solutions look clear, however they can be measured by selecting a wavelength in the UV range.  After you've figured out the best wavelength to use, you can proceed to the next step, actually gathering data to construct a standard curve.