LABORATORY EXPERIMENT pubs.acs.org/jchemeduc
Synthesis of Copper Pigments, Malachite and Verdigris: Making Tempera Paint Sally D. Solomon,* Susan A. Rutkowsky, Megan L. Mahon, and Erica M. Halpern Department of Chemistry, Drexel University, Philadelphia, Pennsylvania 19104, United States
bS Supporting Information ABSTRACT: Malachite and verdigris, two copper-based pigments, are synthesized in this experiment intended for use in a general chemistry laboratory. The preparation of egg tempera paint from malachite is also described. All procedures can be done with a magnetic stir plate, standard glassware present in any first-year laboratory, and household chemicals. Yields for the synthesis of malachite are about 95% making this an ideal activity for courses emphasizing green chemistry. Typical results from students enrolled in a general chemistry course are reported. While introducing the students to the composition of artists’ paints, performing this experiment provides students an opportunity to apply concepts in topics such as stoichiometry (excess reactants and yields), solubility, precipitation, and properties of emulsions. The importance of reading the experiment before coming to the laboratory is emphasized as students must come up with timesaving strategies to finish both pigment syntheses in the time allotted. KEYWORDS: First-Year Undergraduate/General, High School/Introductory Chemistry, Laboratory Instruction, Physical Chemistry, Dyes/Pigments, Green Chemistry, Precipitation/Solubility, Stoichiometry, Synthesis
A
ll paints contain pigment, which provides color, and a binder (or medium), which suspends the pigments and binds them to the surface of the object to be painted. Two paint pigments, malachite and verdigris, are synthesized in this general chemistry laboratory experiment and then a tempera paint is made using malachite. While introducing the students to the composition of artists’ paints, performing this experiment exposes students to chemical concepts such as stoichiometry (excess reactants and yields), solubility, precipitation, and properties of emulsions. Copper sulfate is the starting material for the one-step synthesis of malachite and the three-step synthesis of verdigris. Both syntheses can be done with a magnetic stir plate, standard glassware, and chemicals purchased from hardware stores and supermarkets. The products are dried and weighed to determine percent yields. Malachite pigment is combined with egg yolk binder to make tempera paint. A version of the student write-up for this experiment can be found in a laboratory manual that contains standard first-year general chemistry experiments, all of which can be done with household chemicals.1 The experiment was tested by students enrolled in a general chemistry course. Knowing they would be required to finish both syntheses in one lab period was a strong motivation for students to read the experiment before coming to class to devise timesaving strategies. Results obtained from the class include time required to finish both syntheses, yields of both pigments, as well as the ideas used by students to make best use of their time. This experiment can also be used to explore green chemistry. Among the principles to classify a reaction as “green” are the use of nonhazardous starting materials, their maximum incorporation into products, use of ambient temperature for energy efficiency, and production of nontoxic products. The malachite Copyright r 2011 American Chemical Society and Division of Chemical Education, Inc.
synthesis, done at room temperature with household materials and a yield of 95%, is suitable for a laboratory course emphasizing green chemistry. Although verdigris is also made at ambient temperature from similar household chemicals, their incorporation into the final product is reduced because the yields are much lower (3050%). Further, the copper acetate product is not as safe to handle, making this synthesis less “green”.
’ BACKGROUND Pigments: Malachite and Verdigris
Up until the 1800s when synthetic materials became available, pigments were made from natural minerals or from corrosion of metals. Copper-based pigments were used as early as the Egyptian fourth dynasty in 3000 B.C.E. when artifacts were shown to contain Egyptian blue (CaCuSi4O10). By this time, malachite was also in use, and by 1300 B.C.E., verdigris was on the list of pigments available to artists.2 Greenish malachite, copper(II) carbonate hydroxide, CuCO3Cu(OH)2, occurs with copper ore deposits in many parts of the world. Malachite is a secondary mineral, created by a chemical reaction with minerals that have already formed, for instance, by the action of water containing carbon dioxide or dissolved carbonate minerals on primary copper-containing rocks. Verdigris (“green of Greece”) can be collected by scraping the colored crust from sheets of copper exposed to vapors from vinegar, wine, or urine with addition of other substances such as NaCl, ammonium salts, and honey. If a copper penny (pre-1982) is placed in vinegar, a green solution of copper Published: July 12, 2011 1694
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Table 1. Chemicals for Synthesis of Pigments Chemical
Formula
Molar Mass/(g mol1)
Source
CAS No.
Copper(II) sulfate pentahydrate
CuSO4 3 5H2O
249.70
root killer
7758-99-8
Sodium carbonate
Na2CO3
105.99
Arm & Hammer washing soda
497-19-8
Sodium hydroxide(aq)
NaOH
39.99
lye; oven cleaner
1310-73-2
Acetic acid(aq)
C2H4O2
60.05
vinegar
64-19-7
17.03
Ammonia(aq)
NH3
ammonia cleaner 510%
1336-21-6
Malachite: copper(II) carbonate hydroxide
CuCO3Cu(OH)2
221.12
synthesis product
12069-69-1
Verdigris: copper(II) acetate monohydrate
Cu(CH3COO)2 3 H2O
199.65
synthesis product
6046-93-1
acetate is visible after a few weeks. However, the verdigris produced using methods similar to this can lead to a mixture of chemical compositions and different colors, particle sizes, and morpholigies.3,4 The synthetic methods described here for both coppercontaining pigments start with copper sulfate and use readily available materials and simple techniques. Methods used to prepare other pigments, including chrome yellow, PbCrO4,5,6 Prussian blue,5,7 thalo blue,5 iron(III) oxide,7 titanium dioxide,7,8 and charcoal5 have appeared in this Journal. Binders: Types of Paint
Paints are generally named from the binder used. Oil paint is made with linseed oil, egg tempera with egg yolk, and acrylic paint with acrylic polymers. Watercolor, however, is named for water, the diluent added to make the paint spreadable, rather than the binder, which is gum arabic or some other gum or starch. Although any pigment can be used with any binder, in some cases a particular binder may produce superior paint. For example, malachite gives brighter colors with egg tempera than with oil. In the second part of this laboratory exercise, the synthetic malachite pigment is dried, ground to remove lumps, then mixed with egg yolk/water emulsion to make egg tempera paint. A method used to prepare egg tempera paint has been described in a classroom activity on making artist’s paints.7
’ EXPERIMENTAL DETAILS Sources and CAS numbers for all chemicals involved are listed in Table 1. Synthesis of Malachite
Malachite is synthesized according to the reaction
(i) basic copper(II) sulfate, CuSO4 3 3Cu(OH)2, is formed upon addition of ammonia, (ii) followed by conversion to copper hydroxide using sodium hydroxide, and (iii) then addition of acetic acid to form the verdigris product.9 Vinegar (5% acetic acid) is substituted for pure glacial acetic acid, which has an overwhelmingly powerful pungent odor. This makes the isolation of the somewhat water-soluble copper acetate (7.2 g/100 mL cold and 20 g/100 mL hot) more difficult, but allows the reaction to be performed on the bench top. As in the synthesis of malachite, a magnetic stir plate and standard glassware will suffice for all steps. Step i: A precipitate of basic copper sulfate, CuSO4 3 3Cu(OH)2, is produced upon addition of ammonia solution: 4CuSO4 3 5H2 OðaqÞ þ 6NH 3 ðaqÞ f CuSO4 3 3CuðOHÞ2 ðsÞ þ 3ðNH 4 Þ2 SO4 ðaqÞ þ 14H2 OðlÞ Household ammonia (510% w/v) is added dropwise to a solution of 4.2 g of CuSO4 3 5H2O (0.017 mol) in 25 mL of distilled water with vigorous stirring. As each drop of NH3(aq) touches the liquid surface, the characteristic deep blue of the copper ammonium complex, [Cu(NH3)4]2+, is seen for an instant, then disappears. The dropwise addition of ammonia solution is continued until the blue color of the complex remains throughout the reaction mixture. This will require 2535 mL of NH3(aq). After the mixture is stirred for a few minutes, the solid precipitate of light blue CuSO4 3 3Cu(OH)2 is collected by gravity filtration and washed once with a small volume, 12 mL, of distilled water. The damp precipitate can be used for the next step. Step ii: In the second step, basic copper sulfate is converted to copper(II) hydroxide by reaction with sodium hydroxide:
2CuSO4 3 5H2 OðaqÞ þ 2Na2 CO3 ðaqÞ f CuCO3 CuðOHÞ2 ðsÞ þ 2Na2 SO4 ðaqÞ þ CO2 ðgÞ þ 9H2 OðlÞ in which hydrated copper(II) sulfate reacts with aqueous sodium carbonate to produce copper(II) carbonate hydroxide.9,10 In a 150 mL beaker, 6.2 g (0.025 mol) of CuSO4 3 5H2O is dissolved in 25 mL of distilled water. With vigorous stirring on a magnetic stir plate, a small excess of carbonate solution, 2.9 g (0.027 mol) of Na2CO3 in 25 mL distilled water, is added gradually to the copper solution accompanied by the evolution of carbon dioxide. After the reaction mixture is cooled in an ice bath for about 30 min, the precipitate of insoluble greenish-blue colored malachite is filtered, dried (about a week), and then stored in a covered vial until used to prepare paint. Synthesis of Verdigris
Copper(II) acetate monohydrate is synthesized starting with hydrated copper(II) sulfate according to a three-step procedure:11
CuSO4 3 3CuðOHÞ2 ðsÞ þ 2NaOHðaqÞ f 4CuðOHÞ2 ðsÞ þ ðNaÞ2 SO4 ðaqÞ The damp precipitate of basic copper sulfate removed from the filter paper is mixed with 40 mL of distilled water in a beaker and stirred vigorously while 8.5 mL of 2.0 M sodium hydroxide is added all at once. Once the resulting precipitate of bright blue copper hydroxide, Cu(OH)2, is filtered, a paper towel can be used to absorb as much excess liquid as possible. The copper hydroxide should be used for the next step as soon as possible, because when it is moist it can slowly turn from blue to black owing to the formation of black copper(II) oxide, CuO: CuðOHÞ2 ðsÞ f CuOðsÞ þ H2 OðlÞ However, the blue hydroxide is more stable when precipitated from a dilute solution of a basic copper salt12 as done in this procedure. No students who performed the reaction noticed any color change during the 20 min filtration. 1695
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Table 2. Yields of Synthetic Pigments
Figure 1. Malachite egg tempera paint applied to 6 in. 8 in. clay board.
Step iii: Finally, the blue copper(II) hydroxide is reacted with 40 mL of vinegar (5% acetic acid) to form the copper(II) acetate monohydrate product, verdigris: CuðOHÞ2 ðsÞ þ 2CH 3 COOHðaqÞ f CuðCH 3 COOÞ2 3 H2 OðaqÞ þ H2 OðlÞ The conversion takes place almost instantly forming a blue-green solution. The reaction is allowed to proceed with stirring for about 5 min or until all of the copper hydroxide comes into contact with the vinegar and disappears. The solution is evaporated to dryness, which takes about a week. After a day or two, dark green crystals begin to appear. When crushed, the color of the crystals is blue-green. From the mass of dry product, the percent yield is calculated given that acetic acid is the excess reactant. The pigment is stored in a covered vial. Making Paint
Oil paint can be made by mixing linseed oil with pigment, and egg tempera paint can be made using egg yolk/water emulsion. For this experiment, the decision to make egg tempera paint and to use malachite as the pigment was made for several reasons. Making tempera paint is easy to do and, unlike oil paint, can readily be cleaned using water. Malachite egg tempera produces vibrant color. No dust is produced from the minimal grinding needed to remove lumps from malachite. Making verdigris into a powder suitable for a pigment is more difficult and its dust is harmful upon inhalation (see Hazards). Unlike malachite, verdigris produces better color when mixed with oil binder rather than egg yolk. The malachite sample is ground using a mortar and pestle or a spatula until the visible lumps are removed, which takes 12 min. A fresh egg yolk is carefully separated from the egg white, then made as dry as possible by transferring from hand to hand or by using a paper towel. The yolk is poured into a small container. The malachite is first made into a paste by adding about 1 mL of water for every gram of pigment. To the malachite paste is added 11.5 mL of egg yolk for every milliliter of water that was used to make the paste. A spatula or glass rod is used to grind the pigment paste and yolk together to form paint, which should have the consistency of mayonnaise. Egg tempera paint must be tightly covered until used. Applied paint should dry within 12 min and once dried, should not flake off. A sample of malachite egg tempera paint produced from 5 g of pigment was used to create the image in Figure 1.
’ HAZARDS Dilute ammonia and all of the copper compounds are irritating to the eyes, respiratory tract, and the skin. Copper(II) acetate monohydrate (verdigris) is also harmful by inhalation. Dilute 2 M
a
Malachite (%)
Verdigris (%)
98.0a
74.0a
98.0
66.1
97.9
51.8
97.7
47.3
96.2
46.8
94.0
42.5
94.9
36.3
93.5 93.5
33.2 30.8
93.0
23.5
92.1
21.8
The values were obtained by an experienced chemist.
solutions of sodium hydroxide should be handled with care. Although the students wear gloves, it is still recommended that students who are allergic to eggs take special precautions to avoid letting egg touch their skin by allowing their lab partners to perform steps such as separating the egg, making the egg water emulsion, and mixing the emulsion with pigment.
’ CLASS TESTING Both the synthesis of malachite and verdigris were performed in a single 3-h laboratory period by 20 general chemistry students working in pairs. Time
Students had been advised to read the experiment carefully before lab to make best use of their time. They all noted that working on both experiments at once would minimize idle time during the filtering steps, each of which was expected to require about 1520 min to complete. Specific plans included measuring reagents, making solutions, and preparing an ice bath while waiting for copper sulfate to dissolve or for a filtering step to finish. The first pair of students was done in 1 h and 40 min followed by three more pairs of students 15 min later. Everyone else finished within 2 h and 15 min. Yield
Measuring yields was delayed for a week to allow the pigments to dry. The yield obtained for the one-step malachite synthesis was high for all students, averaging 95% (SD 2). An experienced chemist was able to obtain a yield of verdigris exceeding 70%; however, student yields for the multistep verdigris synthesis were lower, averaging 40% (SD 14). Values are listed from highest to lowest in Table 2. Yields for all three steps of the verdigris reaction were measured independently in the laboratory, to see which step or steps caused the yield to be lowered (for a detailed description of this study refer to the instructor’s guide in the Supporting Information). Paint
Rather than devoting a full two weeks for this experiment, the paint making could be eliminated or could be done in the first 30 min of a lab period before beginning another scheduled experiment. 1696
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’ SUMMARY Two artists’ pigments, malachite and verdigris, were synthesized in a 3-h general chemistry laboratory class using household chemicals. The students were able to make use of basic principles such as stoichiometry, precipitation, and solubility as well as time management skills. ’ ASSOCIATED CONTENT
bS
Supporting Information Two versions of the lab write-up for students are given, the step-by step version I as well as an alternative write-up, version II, similar to those used in organic laboratory manuals; a guide giving quantities of material needed as well as typical results for instructors. This material is available via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We wish to thank the Drexel University General Chemistry students who helped to determine the average yields for each pigment and who made many useful suggestions that improved the student laboratory write-ups. We also acknowledge Tina Lewinski for assisting with the photograph. ’ REFERENCES (1) Solomon, S.; Rutkowsky, S.; Boritz, C. Chemistry: An Everyday Approach to Chemical Investigation; Wiley: New York, 2009; pp 245 255. (2) Orna, M. V. J. Chem. Educ. 2001, 78, 1305–1311. (3) de la Roja, J. M.; Baonza, V. G.; San Andres, M. Spectrochim. Acta, Part A 2007, 68, 1120–1125. (4) San Andres, M.; de la Roja, J. M.; Baonza, V. G.; Sancho, N. J. Raman Spectrosc. 2010, 41, 1468–1476. (5) Orna, M. V. J. Chem. Educ. 1980, 57, 264–267. (6) Daines, T. L.; Morse, K. W. J. Chem. Educ. 1976, 53, 117–118. (7) Gettys, N. S. J. Chem. Educ. 2001, 78, 1320A–1320B. (8) Schueman, G.; Bruzan, R. J. Chem. Educ. 1989, 66, 327–328. (9) Pigments: Historical, Chemical, and Artistic. http://www. jcsparks.com/painted/pigment-chem.html#Mal (accessed Jun 2011). (10) Tanaka, H.; Yamane, M. J. Therm. Anal. 1992, 38, 627–634. (11) How Verdigris is made. http://webexhibits.org/pigments/ indiv/recipe/verdigris.html (accessed Jun 2011). (12) Neville, H. A.; Oswald, C. T. J. Phys. Chem. 1931, 35, 60–72.
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