Chemistry and Art

Laboratory: Pigments - Microchemical Tests

Microchemical Testing for Pigments—This will be done as demonstrations during the second week of working with the frescoes.

Groups use same pigments, except we will not test for TiO2.

A wide variety of chemical tests may be carried out to determine the presence of various elements and ions. By carrying them out on minute samples taken from cracks in a painting one may discover an artist's palette or find a modern pigment on an old painting or other interesting and useful information. In order to carry these tests out on very small samples a microscope with plenty of space for manipulation of the sample is used.

1. Use a pipet or a glass rod which has been given a narrow end to pick out a few particles of the pigment and place on the center of the microscope slide.

NOTE: Be extremely careful not to contaminate the pigments and other chemicals -be sure your glass rod is CLEAN.

2. Look at the sample under the microscope and describe in your laboratory notebook.

3. Work with the instructor. Use the flowcharts from Gettens and Stout and the notes below.

 

4. To do the various chemical tests, place a small drop of the reagent chemical next to the particles and allow it to run into the particles. Watch carefully under the microscope for any changes in color, bubbling, precipitation/ crystal formation. To mix 2 solutions (one containing dissolved pigment) place the drops next to each other (but not touching). Use the narrow end of the glass rod to join the drops with a narrow channel. Let diffusion of the solutions take place-and watch for a reaction.

BE SURE TO MAKE ALL OBSERVATIONS IN YOUR LABORATORY NOTEBOOK!!! Use a tabular rather than a paragraph form.

Some reactions to know:

1. Bubbling with the addition of acid indicates the presence of carbonate (an acid-base reaction):


CO32- + 2 H+ → H2O(l) + CO2(g)

2. Metal hydroxides such as Cu(OH)2 may be put into solution using acid for the dissolution, an acid-base reaction:

M(OH)x + xH+ → Mx+ + xH2O

 

 

3. Lead(II) reacts with iodide ion to precipitate the yellow lead iodide:

Pb2+ + I- → PbI2(s)

4. K4Fe(CN)6:

Copper(II) reacts with potassium ferrocyanide and acid (dilute HCl) to precipitate the pinkish-red copper ferrocyanide:

Cu2+ + K4Fe(CN)6 → Cu2[Fe(CN)6]

Fe(III) reacts to form a blue precipitate.

Fe3+ + K4Fe(CN)6 → Fe4[Fe(CN)6]3

5. Prussian blue plus dilute base (sodium hydroxide NaOH) turns brown:

Fe4[Fe(CN)6]3+ 12 NaOH → 4 Fe(OH)3 + 12 Na+ + 3 [Fe(CN)6]4-

Addition of acid will return a blue color, if enough pigment is present, through a two step process of dissolution of iron(III) hydroxide, followed by reformation of the ferric ferrocyanide:


1) Fe(OH)3 + 3 HCl → Fe3+ + 3 H2O

2) Fe3+ + [Fe(CN)6]4- → Fe4[Fe(CN)6]3

 

6. Addition of zinc metal to a copper(II) solution will cause the zinc to be oxidized and go into solution while the copper(II) is reduced to copper metal. This is an example of an oxidation-reduction reaction. Watch for the formation of dendrites of copper under the microscope. This may take some time, do not wash the slide off thinking nothing has happened.

Cu2+ + Zn(s) → Cu(s) + Zn2+

7. Potassium mercuric thiocyanate, K2Hg(SCN)4 reacts with several different cations to give unique tests. Notes from An Introduction to Microchemical Qualitative Analysis by Skip Palenik, McCrone Associates.

with Cu2+ → CuHg(SCN)4, yellow dendrites and/or yellow "propellers". Both are anisotropic with the polarization colors generally masked by the absorption colors; crystals show a characteristic color with top (reflected) light.

with Zn2+ → ZnHg(SCN)4, depending on concentration, feathery crosses (black by transmitted light, white by reflected light - anisotropic) and/or stubby prisms. The feathery cross actually consist of the prisms. May be useful to add a tiny drop of dilute nitric acid by touching the drop with a glass tip wet with the acid.

with Co2+ → CoHg(SCN)4, deep blue anisotropic prisms

with Fe3+ → [Fe(SCN)]2+, blood red solution

Some additional notes on microchemical tests:

The blue color in ultramarine is due to the S3- radical (contains an unpaired electron) anion (Huhey p.688). Addition of acid bleaches the color

Indigo is a blue dye. When it is reduced (as is necessary for dying) it is yellow-green. When oxidized (by hanging the dyed material in the air (oxygen)) it turns the familiar blue of bluejeans.

5. Try making mixtures of pigments (clean your glass rod between pigments) on your microscope slide. Can you identify both pigments?

6. Time permitting, identify one or more of the following: unknown white pigment, unknown blue pigment, unknown mixture of one blue and one white pigment.