Blueprint paper can be made by soaking paper in a brown solution of Fe(CN)6^3- and iron (III) ammonium citrate. Wherever the paper is exposed to bright light, the paper turns blue. Explain why this happens.
I don't know how much detail you want but here is a fairly good explanation of what goes
on in the process.
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The process of turning blueprint paper blue involves a chemical reaction known as the Prussian blue reaction. Here's a step-by-step explanation:
Step 1: Preparation of the paper
The paper used for blueprint making is typically coated with a mixture of iron (III) ammonium citrate and a brown solution of potassium ferricyanide (Fe(CN)6^3-). This coating is initially colorless.
Step 2: Exposing to bright light
When blueprint paper is exposed to bright light, particularly UV light, certain chemical reactions occur in the paper's coating.
Step 3: Reduction of Fe(CN)6^3- ions
The bright light causes the reduction of Fe(CN)6^3- ions in the coating to Fe(CN)6^4- ions. As a result, the brown Fe(CN)6^3- complex starts to break apart, releasing Fe(CN)6^4- ions.
Step 4: Combining with Fe3+ ions
The Fe(CN)6^4- ions released from the Fe(CN)6^3- complex are then capable of combining with iron (III) (Fe3+) ions from the iron (III) ammonium citrate present in the coating.
Step 5: Formation of Prussian blue
The combination of Fe(CN)6^4- ions and Fe3+ ions results in the formation of a dark blue insoluble compound known as Prussian blue (Fe4[Fe(CN)6]3).
Step 6: Blue color development
As Prussian blue forms on the areas exposed to bright light, the presence of this blue compound gives the paper its characteristic blue color.
In summary, the exposure to bright light triggers a series of chemical reactions that lead to the formation of Prussian blue, resulting in the paper turning blue.
The blueprints are made using a chemical process known as the cyanotype process. This process involves two main components: a light-sensitive solution and paper. In this case, the light-sensitive solution is a mixture of ferric ammonium citrate and potassium ferricyanide (Fe(CN)6^3-), commonly referred to as the "brown solution."
Now, let's understand the process of how the blueprints are created:
1. Preparation: The paper is first soaked in the brown solution, which contains both the ferric ammonium citrate and potassium ferricyanide. These chemicals are not light-sensitive in their current form.
2. Exposing to Light: When the paper soaked in the brown solution is exposed to bright light or ultraviolet (UV) radiation, a chemical reaction occurs. The light reacts with the iron (III) ions in the ferric ammonium citrate, causing them to be reduced to iron (II) ions.
3. Formation of Prussian Blue: The reduced iron (II) ions in the presence of the ferricyanide ions (Fe(CN)6^3-) form a complex compound known as Prussian blue (Fe4[Fe(CN)6]3). This complex is responsible for the blue color that develops on the paper.
The reason why only the areas exposed to bright light turn blue is because the light initiates the reduction reaction of iron (III) to iron (II). The unexposed areas, shielded from the light, do not undergo this reduction process and, therefore, remain unchanged. As a result, upon rinsing the paper after exposure, the unexposed areas appear white or pale due to the removal of the excess chemicals from the surface.
Overall, the cyanotype process relies on photochemical reactions and the formation of Prussian blue to create blueprints. These blueprints are commonly associated with architectural drawings and old-fashioned photographic techniques.