Formal Lab Report Synthesis
A small amount of the unknown (approximately the tip of the spatula was used for all the pigments) was added into each clean dry test tube. For the first test 1 mL of 3 M of HNO3 was added. As soon as the acid came into with the solid, the pigment was observed to dissolve while also turning the solution into a pale clear colour. The same amount was used for the HCl reagent. As soon as the acid touched the pigment it yielded in a clear solution (solid dissolved after a few shakes). When the KI was added, the solution turned a pale yellow with white solid precipitate clumping together at the bottom of the test tube. See Table 2 for observations. The unknown had heat administered to it as well, in order to solidify the identity of the unknown;
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the colour was yellow.
In this experiment an inorganic pigment, chromium oxide green was synthesized, had its macroscopic reaction qualitatively observed and had a series of qualitative micro-chemical spot tests performed on it. In the second part of the experiment, a qualitative analysis experiment was designed for the purpose of identifying an unknown inorganic pigment. For the synthesis of chromic oxide green, a 11.50 % yield was observed. This low yield was due to sources of error in the performed experiment. One error that led to a loss of product was the filtering of the product with the Buchner funnel. A lot of product was lost as it was difficult to obtain the entire product from the filter paper and the sides of the funnel. Another source of error was the various transferring stages of the product. These errors can be minimized by exercising more caution when transferring the product to different places. This behaviour should increase the yield of the product.
The analysis showed that the unknown was ZnO.The unknown pigment was identified by analyzing Table 1 and Table 2. The pigment that had the most similarities to the unknown was considered the unknown, essentially process of elimination. After the addition of HNO3 the unknown solid dissolved and the solution turned pale clear. This crossed out the possibility of PbCO3•Pb(OH)2, CaO3 or TiO2 being identified as the unknown. The KI and HCl test also confirmed that the unknown could not be CaSO4 as it did not have the same results as the unknown (Table 2). Therefore ZnO, was the only pigment that was the logical choice for the identity as it had the same results under those conditions. After the analysis was complete, both the unknown and ZnO were heated with a flame. They both showed the same colour, yellow. This further confirmed that the unknown was ZnO as literature supports that under heat, it shows a yellow colour 1. These results are reliable as they were repeated four times in order to compensate for random error that might give false results. Such a result was observed when the ZnO showed a peach coloured solution after the addition of KI. When the reaction was repeated with new clean dry test tubes the colour was not observed. In the first part of this experiment, the pigments that used the macroscopic reactions were barium white, zinc yellow and chromium oxide green while only Prussian blue was synthesized using the microscopic reaction.
In the formation of barium white, the reaction involved was a double displacement reaction in which sodium in sodium sulfate being more reactive, was replaced by barium from barium chloride and was able to form barium sulfate, a white solid pigment and sodium chloride 2. As barium sulfate is insoluble in water it was relatively easy to observe and interpret what was happening as the chemicals mixed 2. The formation of zinc yellow also involved a double displacement reaction 3. The addition of zinc sulfate to potassium chromate formed a yellow coloured solution and when base was added, the solution turned into a bright yellow colour. This was difficult to observe as the colour change was very subtle, however, interpretation was manageable as pH paper was used to find when the addition of base should be stopped. Chromium oxide green was synthesized from the reaction between potassium dichromate and sulfur. The reaction involved heat thus, sulfur dioxide gas was released and colour change was observed from bright red to black. The final inorganic pigment was dark green. Due to the colour change it was easy to observe and interpret what was happening as the reactions progressed. The formation of Prussian blue used a microscopic reaction, that is the reactants were mixed together and their chemical reaction observed under a microscope. Prussian blue was produced by the oxidation of potassium ferrocyanide by ferric chloride and its product was observed to be insoluble in water 4. This was hard to observe as the reaction proceeded very quickly, as soon as the reactants touched each other the blue of the Prussian blue was observed, however the completion of the reaction was easy to observe due to the drastic colour difference. The macroscopic reactions
were generally easier to observe and interpret as the reactions were slow enough to be seen with the naked eye.
Lead white is a toxic pigment, compared to barium sulfate 5. Therefore an artist has a lot of benefit when deciding to use barium sulfate instead of lead white. Barium sulfate is also an extremely inert and transparent pigment that is often used as a protective material 2. It is also unchanged by high temperature, light or strong chemicals 2. This makes it ideal when dealing with environments that would be exposed to harsh conditions. This was observed during the micro-chemical spot plate tests. The barium white was observed to have not dissolved in both NaOH and HCl, only when H2SO4 was applied did the pigment dissolve. This was confirmed by literature 6. On the other hand, lead white, is chemically resistant to acids, alkalis and corrosive gases 5. However, in the presence of sulfides turns yellow or black and also can be oxidized by microorganisms to form brownish-black lead dioxide 5. Lead white, as a pigment has more fine qualities: good flow, low oil absorption and rapid drying and today it is still favored in oil paints for its opacity and warmth handling 5. One of the major disadvantages of lead, however, is the high toxicity level associated with it 5. Prussian blue cannot be used to paint walls, containing lime or alkali because it is alkali sensitive; it will form a brown colored ferric hydroxide 4. This was observed in the micro-chemical spot test with NaOH as the reagent, see Table 3. After the addition of the NaOH the Prussian blue was quickly degraded into a brown colour. For this reason a knowledgeable artist would not use calcium carbonate to lighten Prussian blue, due to the alkali nature of calcium carbonate 7. An artist would also not use lead white to lighten Prussian blue because a mixture of lead white and Prussian blue will change the color from blue to brown (the lead decays over time toward dark shades (the result of exposure to hydrogen sulfide), producing artwork which often looks very strange) 5. However if the artist had no choice they would use the lead white, as it changes the colour over a longer period of time than the calcium carbonate would 7. Lead white is the inorganic white pigment that darkens over time in the presence of air due to contact with sulfides (present in the air) or due to neighboring pigments which cause it to darken 5.
Zinc yellow and chrome yellow can be differentiated by the addition of acetic acid as chrome yellow is insoluble while zinc yellow is soluble, this was confirmed by literature 8. It is important to not add excess base during the synthesis of zinc yellow as too much base would have formed a zinc complex that is soluble in water which would have affected the pigment product 8. In the appropriate amount of base (see synthesis of zinc yellow in material and methods section) Zn(OH)2 would be the product, but in the case of excess base the complex ion, Zn(OH)42- would have formed (see appendix) 8.
the colour was yellow.
In this experiment an inorganic pigment, chromium oxide green was synthesized, had its macroscopic reaction qualitatively observed and had a series of qualitative micro-chemical spot tests performed on it. In the second part of the experiment, a qualitative analysis experiment was designed for the purpose of identifying an unknown inorganic pigment. For the synthesis of chromic oxide green, a 11.50 % yield was observed. This low yield was due to sources of error in the performed experiment. One error that led to a loss of product was the filtering of the product with the Buchner funnel. A lot of product was lost as it was difficult to obtain the entire product from the filter paper and the sides of the funnel. Another source of error was the various transferring stages of the product. These errors can be minimized by exercising more caution when transferring the product to different places. This behaviour should increase the yield of the product.
The analysis showed that the unknown was ZnO.The unknown pigment was identified by analyzing Table 1 and Table 2. The pigment that had the most similarities to the unknown was considered the unknown, essentially process of elimination. After the addition of HNO3 the unknown solid dissolved and the solution turned pale clear. This crossed out the possibility of PbCO3•Pb(OH)2, CaO3 or TiO2 being identified as the unknown. The KI and HCl test also confirmed that the unknown could not be CaSO4 as it did not have the same results as the unknown (Table 2). Therefore ZnO, was the only pigment that was the logical choice for the identity as it had the same results under those conditions. After the analysis was complete, both the unknown and ZnO were heated with a flame. They both showed the same colour, yellow. This further confirmed that the unknown was ZnO as literature supports that under heat, it shows a yellow colour 1. These results are reliable as they were repeated four times in order to compensate for random error that might give false results. Such a result was observed when the ZnO showed a peach coloured solution after the addition of KI. When the reaction was repeated with new clean dry test tubes the colour was not observed. In the first part of this experiment, the pigments that used the macroscopic reactions were barium white, zinc yellow and chromium oxide green while only Prussian blue was synthesized using the microscopic reaction.
In the formation of barium white, the reaction involved was a double displacement reaction in which sodium in sodium sulfate being more reactive, was replaced by barium from barium chloride and was able to form barium sulfate, a white solid pigment and sodium chloride 2. As barium sulfate is insoluble in water it was relatively easy to observe and interpret what was happening as the chemicals mixed 2. The formation of zinc yellow also involved a double displacement reaction 3. The addition of zinc sulfate to potassium chromate formed a yellow coloured solution and when base was added, the solution turned into a bright yellow colour. This was difficult to observe as the colour change was very subtle, however, interpretation was manageable as pH paper was used to find when the addition of base should be stopped. Chromium oxide green was synthesized from the reaction between potassium dichromate and sulfur. The reaction involved heat thus, sulfur dioxide gas was released and colour change was observed from bright red to black. The final inorganic pigment was dark green. Due to the colour change it was easy to observe and interpret what was happening as the reactions progressed. The formation of Prussian blue used a microscopic reaction, that is the reactants were mixed together and their chemical reaction observed under a microscope. Prussian blue was produced by the oxidation of potassium ferrocyanide by ferric chloride and its product was observed to be insoluble in water 4. This was hard to observe as the reaction proceeded very quickly, as soon as the reactants touched each other the blue of the Prussian blue was observed, however the completion of the reaction was easy to observe due to the drastic colour difference. The macroscopic reactions
were generally easier to observe and interpret as the reactions were slow enough to be seen with the naked eye.
Lead white is a toxic pigment, compared to barium sulfate 5. Therefore an artist has a lot of benefit when deciding to use barium sulfate instead of lead white. Barium sulfate is also an extremely inert and transparent pigment that is often used as a protective material 2. It is also unchanged by high temperature, light or strong chemicals 2. This makes it ideal when dealing with environments that would be exposed to harsh conditions. This was observed during the micro-chemical spot plate tests. The barium white was observed to have not dissolved in both NaOH and HCl, only when H2SO4 was applied did the pigment dissolve. This was confirmed by literature 6. On the other hand, lead white, is chemically resistant to acids, alkalis and corrosive gases 5. However, in the presence of sulfides turns yellow or black and also can be oxidized by microorganisms to form brownish-black lead dioxide 5. Lead white, as a pigment has more fine qualities: good flow, low oil absorption and rapid drying and today it is still favored in oil paints for its opacity and warmth handling 5. One of the major disadvantages of lead, however, is the high toxicity level associated with it 5. Prussian blue cannot be used to paint walls, containing lime or alkali because it is alkali sensitive; it will form a brown colored ferric hydroxide 4. This was observed in the micro-chemical spot test with NaOH as the reagent, see Table 3. After the addition of the NaOH the Prussian blue was quickly degraded into a brown colour. For this reason a knowledgeable artist would not use calcium carbonate to lighten Prussian blue, due to the alkali nature of calcium carbonate 7. An artist would also not use lead white to lighten Prussian blue because a mixture of lead white and Prussian blue will change the color from blue to brown (the lead decays over time toward dark shades (the result of exposure to hydrogen sulfide), producing artwork which often looks very strange) 5. However if the artist had no choice they would use the lead white, as it changes the colour over a longer period of time than the calcium carbonate would 7. Lead white is the inorganic white pigment that darkens over time in the presence of air due to contact with sulfides (present in the air) or due to neighboring pigments which cause it to darken 5.
Zinc yellow and chrome yellow can be differentiated by the addition of acetic acid as chrome yellow is insoluble while zinc yellow is soluble, this was confirmed by literature 8. It is important to not add excess base during the synthesis of zinc yellow as too much base would have formed a zinc complex that is soluble in water which would have affected the pigment product 8. In the appropriate amount of base (see synthesis of zinc yellow in material and methods section) Zn(OH)2 would be the product, but in the case of excess base the complex ion, Zn(OH)42- would have formed (see appendix) 8.