9 Mar 2011

Pigment Syntheses and Qualitative Analysis

Lab Experiment

Submitted by Rebecca M. Jones, George Mason University
Categories
Description: 

This set of experiments provides an introduction to simple inorganic synthesis and qualitative analysis of inorganic pigments.  I have taught this series of experiments in my first semester junior level inorganic class for the past 5 years.  In part 1, students synthesize five inorganic pigments.  Part 2 involves identifying an unknown inorganic white pigment by chemical and physical tests.  These experiments are based upon those developed by Dr. Patricia Hill for the Chemistry in Art workshop in 2005 at Millersville University, in Millersville, PA; permission has been obtained from Dr. Hill to disseminate this adapted set of experiments on VIPEr.  The Chemistry in Art workshop is part of the Chemistry Collaborations, Workshops and Communities of Scholars (cCWCS) program.  This experiment was presented at the 20th Biennial Conference on Chemical Education in Bloomington, IN on July 31, 2008; the attached presentation contains images and additional information that may be helpful when implementing this experiment.

Learning Goals: 

The students taking this course have completed a year of general chemistry and at least one semester of organic.  As a result, most have adequate knowledge to thouroughly understand the chemistry observed.  At the end of part 1, a student should be able to write and balance the precipitation reactions for synthesis of synthetic malachite, barium white, and chrome yellow.  Students should also be able to qualitatively observe the macroscopic and microscopic syntheses. At the end of part 2, a student should be able to design a qualitative analysis experiment for inorganic pigments and interpret results from these experiments to identify an unknown.

 

Equipment needs: 

Please see the attached file entitled “Supplies and Implementation Notes for Pigment Syntheses”

 

Implementation Notes: 

Please see the attached file entitled “Supplies and Implementation Notes for Pigment Syntheses”

 

Time Required: 
Each part of the lab handout can be completed in one 3 hour lab period. I schedule 2 weeks for the complete set of experiments.
Evaluation
Evaluation Methods: 

I assess this group of experiments by pre-lab and discussion questions completed in the lab notebook and overall lab performance.  I also require a formal written report for the white pigment qualitative analysis (Part 2).  This report is in the form of a Journal of the American Chemical Society communication and is graded with a qualitative rubric.  I’ve attached the formal lab report instructions and grading rubric.

 

Evaluation Results: 

The students have enjoyed this set of experiments and showed a good understanding of the precipitation reactions.  As this experiment is near the beginning of the semester, many need to be reminded how to perform a simple stoichiometry calculation to determine percent yield.  The white pigment qualitative analysis experiment is the first of three open ended experiments I feature in this course.  Students have a varying degree of success with the process, but most correctly identify the unknown they were given.  Students have commented that this chemistry is easy to see and has a connection to chemistry in the real world. 

 

Creative Commons License: 
Creative Commons Licence

Comments

Rebecca,

Not that it is an easy pigment to make in the lab, but I added a link above under "Related Activities" to a VIPEr learning object based on a paper in JACS that describes a new blue pigment based on trigonal bipyramidal Mn3+.  

Thanks for posting these labs!

I used this experiment in my 200-level inorganic chemistry course this semester as a center point for our discussion of color generation in inorganic materials. Students loved the lab, and enjoyed creating all of the different colors.

The experiment itself is extremely simple, and gave very straightforward results. We had no problem completing both portions of the lab experiment in a single 4-hour lab, and it could probably be done in 3 h. In addition to getting students excited, the experiment gave us opportunities to discuss several important class topics:

- Changes in oxidation state (chrome green is made by burning)

- Solubility (or lack thereof) of inorganic salts

- The chromate solutions are great examples of the strong color caused by charge transfer (cannot have d-d transition because no d electrons).

I scaled up the synthesis of Prussian Blue (difficult to filter!) and left out the microscope component, because we do not have access to a good microscope in the chemistry lab. I purchased a jar of LiquiTex Gel medium from our local art store, and allowed students to mix their own paints using a mortar and pestle in the lab. This was much easier than I expected, and some students got very excited about this (others chose not to participate in the optional painting session - either due to desire for lunch, or a lack of artistic inclination). I have received requests from professors in the school art department to use the pigments; we're still working on safety concerns, but I'm considering changing out some of the pigments for less hazardous materials and building on this collaboration next time the course is offered. (Students also noted that it would be good to include a red pigment for art projects in future years.)

I found a paper on the structure and origin of color in Prussian Blue, one of the 5 pigments synthesized, and we used that as the basis for an in-class discussion about crystal structure, charge transfer, allowed and forbidden electronic transitions, and correlation diagrams (Robin, M. The Electronic Configuration of Prussian Blue, Inorg. Chem. 1(2) 1962 p 337-42, VIPEr object coming soon!). I assigned this paper along with the "New Blue Solid" paper that Maggie mentioned above, and this made for a very interesting class discussion (students read papers and answered a subset of reading q's in advance, we took a 1 h lecture period to discuss). Students were excited to understand color generation in a pigment that they had themselves synthesized. 

The lab itself is quite simple, but it offers ample opportunities to connect with other class ideas and was very effective in getting students excited about the topic.

 

 

I used this experiment in my second-year inorganic chemistry course. This was a very fun, quick lab for students, and some of my colleagues in the Art department got very excited about creating a collaborative project using the pigments synthesized in this lab.

The experiment itself is similar to precipitation and qualitative analysis labs from general chemistry, with dramatic changes when things react. It could be beneficial to include more advanced chemistry in the lab handout to increase its pedagogical value for a course at the second-year level. I would consider adding discussion of types of reaction, oxidation, solubility or metal salts, etc. in a future course. Could also discuss solid state structures and methods of color generation (CrO4- is d0 ⇒ must be CT color). 

I may also require students to seek out MSDS information for each chemical and to write their own safety section for this lab in future. There is little/no safety information given in the handout, and this would be a good opportunity for students to become familiar with locating that information themselves. 

The reactions in general were very straightforward and all students got good results. I was surprised that the malachite synthesis produced a pale blue rather than a green solid. The chrome green synthesis was exciting for students: the solid “catches fire” as it oxidizes. It doesn’t seem to matter whether or not crucible lid is used (make sure to do this in the hood, as there was a significant amount of yellowish gas evolved).  I scaled up the Prussian Blue synthesis, but the resulting pigment was very hard to filter, making a thick gel-like substance that clogs the filter paper. 

I purchased a jar of acrylic binder, paper, and paint brushes at the local art store, and allowed students to mix their own paints from the pigments and to create artwork at the end of the lab period. (I verified that these pigments are commercially available as art supplies, and that the binder should keep chemicals from being released. I also stressed to students the importance of keeping the finished paintings well segregated from food and other activities, as these pigments are toxic.) Students suggested including a red pigment for next year, to balance out the color palette.

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