Can spectrophotometry be used to determine the concentration of “colorless” solutes such as salt or sugar (or other soluble macromolecules like DNA or RNA)? Explain.

Answer 1

Absolutely. This is done all the time in research settings, especially for nucleic acids and proteins. No matter the application, they all work roughly the same way. Light of a specific wavelength is shined through a sample, and on the other side, a detector measures how much light reaches it. Depending on the settings, this could be an absorbance reading (how much light was absorbed by the sample) or a transmission reading (how much light made it through the sample). Nucleic acids and proteins have known wavelengths at which they absorb UV light. The amount of UV light absorbed is proportional to concentration of RNA/DNA or protein. You can make a standard curve of known concentrations, make a graph of that, and then plot your values on the curve to find unknown values, and this can be done with any solution with solutes that can absorb/scatter light. This is how it's typically done in a teaching lab environment, and is still common for concentration of bacteria or yeast in a sample. A more "rigorous" uses the Beer-Lambert Law, which describes how light intensity diminishes as it passes through media. You can find more information on spectrophotometry and Beer-Lambert Law at these links, if you're interested in the math behind it.https://en.wikipedia.org/wiki/Ultraviolet%E2%80%93visible_spectroscopy

and Beer-Lambert Law https://en.wikipedia.org/wiki/Beer%E2%80%93Lambert_law

Answer 2

In short, no.

Spectrophotometry relies on the absorbance of light by the solution. It works by sending a beam of light through a cuvette filled with the solution, and, depending on how much light passes through, the spectrophotometer can measure the amount of light/photons that was/were absorbed. If the solution is colorless, it absorbs no light. Think of it like shining light through a clear piece of glass as opposed to a colored sheet.

The Beer-Lambert (Beer's) law puts all these factors into 1 simple equation if you want to think of it mathematically:

A = ∈ x b x C

A - absorbance

∈ - extinction coefficient (specific to compound, you can look this up in a reference book or online)

b - pathlength (e.g., the width of the cuvette containing the solution)

C - concentration of the solution