Chemical and Biomolecular Engineering

Official blog of the Lehigh University Chemical Engineers

Spectrochemical Series — Chem Lab

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One of the most striking features of transition metal compounds is their wide range of colors, especially solutions of transition metals. The intensity and particular color of the solution varies depending on the identity of the ligand attached to it. A ligand is simply an ion or molecule that attaches to a central atom (eg. Cl-,NO2-). In the lab, we first sythesized (Cu(en)2(H2O)2)I2. The ligand in this case is en complex. The starting material was copper (II) acetate monohydrate. Ethylenediamine (en) is slowly added which acts as the ligand. Water and ethanol are added in several steps, along with the solution being filtrated using a Buchner funnel, to obtain the final crystal product.

(Cu(en)2(H2O)2)I2

(Cu(en)2(H2O)2)I2 Crystals

Our yield was much smaller than the picture (~.10 g). The next part of the lab allowed us to develop a spectrochemical series of copper. We made several different solutions containing copper and a ligand. The solutions all started with copper nitrate (Cu(NO3)2) which is light blue in color. We studied the effect of the ligands: ammonia, chloride, ethylenediamine (en), nitrite, and water. The en came from the crystals synthesized in the first part. Each of these ligands was added to copper nitrate and it changed the color of the solution. By determining the absorbance of each solution, we are able to develop a spectrochemical series of the ligands. This series ranks the ligands from high field to low field. The wavelength that each solution absorbs corresponds to the strength of its field.

Color Wheel with Wavelengths

Color Wheel with Wavelengths

We used a spectrometer to determine the absorbance of each solution. Briefly, when material emits a color, that means it is absorbing the opposite color. For example, if you are wearing a red shirt, it absorbs green light. We measured the maximum wavelength that each solution absorbed, which produced a scale for the series. Copper and nitrite produce a  yellow-green solution. When we found the maximum wavelength, it was 398 nm. So, the compound absorbed violet light, while emitting green-yellow light. I will post pictures of each color at the end of the post, see if you can estimate the wavelength that each absorbs.

In the end, we were able to develop a ranking of ligands and compare it to literature (known) values. This lab was very interesting because it dealt with inorganic material (copper metal). Just how different metals burn with different colors, they also produce different colors depending on what it attached to them. The semester is winding down, and I only have two chemistry labs left. It has been really helpful to physically perform what is learned in class through lab. I definitely think it allows me to learn material better and be able to apply it to other situations. Check out the pictures of various solutions and try to determine what wavelength they absorb. Remember, the complimentary color to the visible color is absorbed (across the color wheel).

Chloride

Chloride

Ammonia (left)

Ammonia (left)

en

en

Water

Water

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Author: Ben Dunmire

I am a sophomore Chemical Engineering major at Lehigh. On campus, I am the president of Club Baseball and a member of AIChE and NSCS. Outside of school, I enjoy the outdoors, fishing, and any type of athletic activity. You can reach me at bcd216@lehigh.edu.

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