Colorimetry (Colourimetry) Chemistry Tutorial

⚛ Colorimetry (colourimetry) is also referred to as colorimetric analysis (colourimetric analysis).

⚛ Colorimetry (colourimetry) is an analytical technique used by Chemists to determine the concentration of a coloured solution.

⚛ A solution is coloured because it absorbs different amounts of the different wavelengths that make up "white light".

⚛ A colorimeter (colourimeter) is an instrument used to measure the absorbance of light by a coloured solution.⁽¹⁾

⚛ The wavelength chosen for colorimetric analysis is that which gives maximum absorption for the coloured solution.

We can use a "colour wheel" to determine the colour of light to use.⁽²⁾

⚛ The intensity of the colour of a solution is related to the concentration of the solution.

The more concentrated the solution is, the more intense the colour will be.

⚛ To determine the concentration of a solution using colorimetry:

1. Step 1: Determine the wavelength (colour) of light to use for the colorimetric analysis.
2. Step 2: Prepare a set of standard solutions of known concentration.
3. Step 3: Measure the absorbance of each standard solution using the colorimeter.
4. Step 4: Plot the absorbance vs concentration for each standard solution on a graph.⁽³⁾
5. Step 5: Draw the line of best fit through the data points. This is the calibration curve.
6. Step 6: Measure the absorbance of the solution of unknown concentration using the colorimeter.
7. Step 7: Use the calibration curve to determine the concentration of this solution.

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Many solutions can be coloured.
You may already be familiar with some coloured aqueous solutions, for example:

• copper sulfate dissolved in water produces a blue solution (CuSO_4(aq))
• potassium permanganate dissolved in water produces a purple solution (KMnO_4(aq))
• iron(III) thiocyanate dissolved in water produces a red solution (FeSCN²⁺_(aq))
• bromine water is a red-brown solution (Br_2(aq))
• iodine dissolved in water is a yellow-brown solution
• the complex formed between starch and iodine is a deep blue colour
• acid-base indicator solutions come in a wide range of colours

The colour we see is the colour of the light that is transmitted through the solution.
"White light" is part of the electromagnetic spectrum and is made up of all the colours of the rainbow; red, orange, yellow, green, blue, indigo and violet (ROYGBIV).

If we shine white light through an aqueous solution of copper sulfate, the solution appears to be blue because it is transmitting the blue wavelengths of white light and absorbing other wavelengths such as red so that very little of these other wavelengths are transmitted through the solution to your eye.

The colour of the solution is due to the wavelengths of light that are not absorbed as "white light" passes through the solution.

Now, if we dilute the original copper sulfate solution by adding more water, the blue colour becomes less intense, it is a lighter blue colour because the solution is not absorbing as much of the other wavelengths as before:

If we were given an aqueous solution of copper sulfate of unknown concentration, solution (A), we might be able to take a guess at its concentration by looking at its colour:

The "blueness" of solution (A) seems to lie about half-way between that of the concentrated (0.50 mol L^-1) solution and the dilute (0.10 mol L^-1) solutions above, so we might guess that the concentration of solution (A) lies about halfway between these two solutions: (0.50 + 0.10) ÷ 2 = 0.30 mol L^-1.

Guessing the concentration of the solution in this way gives us a kind of "ball park" value for the concentration, but it is unlikely to result in us knowing the concentration of the solution accurately or precisely.
But there might be a better way.
We have already noted that there is a relationship between the intensity of the colour of solutions and their concentration.
And we have seen that a concentrated blue solution will absorb more of the "non-blue" wavelengths of light than a more dilute solution, and we have a machine, a colorimeter (colourimeter), that can measure the amount of absorbance for us!

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Colorimetry (colourimetry) is the name of the method we use to determine the concentration of a substance by measuring the relative absorption of light with respect to known concentrations of the substance.

A colorimeter (colourimeter) is literally a meter to measure colour. It is an instrument that measures the absorption of light of a specific wavelength (λ) by a solution.⁽⁴⁾

Therefore, we need to choose a wavelength of light to shine through our solution.
The solution must be able to absorb measurable amounts of this wavelength.
Different concentrations of the solution must absorb different amounts of this wavelength, and this difference must be significant (that is, much larger than the error inherent in the measurment).

If a solution is blue, like CuSO_4(aq), we would not use a wavelength corresponding to blue light because very little of this is absorbed (most is being transmitted).
Similarly, lots of the green and indigo wavelengths are not absorbed but are transmitted.
Therefore we would choose to shine light from the "red-orange-yellow" part of the spectrum at this solution because these are the wavelengths that are being absorbed.

We can use a "colour wheel" to decide which is most likely to be the best colour of light to shine on our solution:

If our solution is blue, we choose a light colour that is complementary to blue, that is, we choose the colour that is opposite this colour in the colour wheel, in this case we choose an orange light to shine on our blue solution.

Now, it may be possible to select an appropriate wavelength for the light our colorimeter uses, but it is quite likely that our colorimeter will use coloured filters in order to "filter out" (absorb) the wavelengths of light that we do not want from "white light".
In this case, if we want to determine the amount of orange light absorbed by our blue solutions, we would select an orange filter for our colorimeter.

A simplified schematic diagram of using an orange filter in a colorimeter to measure the concentration of a blue copper sulfate solution is shown below:

"white" light source		slit		orange filter		CuSO4(aq)		detector / recorder
	→		→		→		→	detector
		→

Light from the "white light source" passes through a slit then a filter to select the most appropriate wavelength of light. In this case, the orange filter absorbs all other wavelengths and allows the orange wavelengths to be transmitted through the filter.
Some of this "orange light" is then absorbed by the "blue solution" which is contained in a glass receptacle.⁽⁵⁾
The intensity of the "orange light" has decreased after it passes through the "blue solution" because the "blue solution" has absorbed a significant amount of "orange".

The amount of light absorbed is measured and recorded as the "absorbance".
The more concentrated the solution the greater the "absorbance" will be.

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A colorimeter (colourimeter) measures the absorption of light of a specific wavelength (λ) by a solution.
The colorimeter records this as the absorbance.

In order to determine the concentration of a solution, we will need to obtain the absorbances for a range of different concentrations of the same solution using the same wavelength of light (same filter).
We use the absorbance of each of these solutions of known concentration to draw a calibration curve.
We can then measure the absorbance of our solution of unknown concentration and use the calibration curve to determine its concentration.

If, for example, we have an aqueous solution of copper sulfate, CuSO_4(aq), and we want to determine its concentration colorimetrically, we would first prepare a stock solution of aqueous copper sulfate.
We then use this stock solution to prepare a number of dilute copper sulfate solutions, calculating the concentration of each solution.
Note that we should visually check the colour of our unknown solution against the colour of each of the prepared solutions in order to ensure that the colour of our solution lies between the colour of the most concentrated and most dilute solution we have prepared.
Next we use the colorimeter (colourimeter) to measure the absorbance of each of these solutions of known concentration.
The results of this experiment might look like those in the table below:

Next we plot these results on a graph and draw a "line of best fit" through the data points:

Now we measure the absorbance of our aqueous solution of copper sulfate of unknown concentration.

Example: Absorbance of unknown solution = 0.950

We plot the point (x) for this unknown solution on our calibration curve:

Reading off the graph we see that an absorbance of 0.950 corresponds to a concentration of 0.34 mol L^-1

Alternatively, if we know the mathematical equation for the straight line of the calibration curve, we can use that to determine the concentration of our unknown solution.
On the calibration graph we were given the expression:

Abs = 2.78 × [CuSO_4(aq)]

which is the mathematical equation for the calibration curve.
Substitute the value for the absorbance of our unknown solution into the equation:

0.950 = 2.78 × [CuSO_4(aq)]

Divide both sides of the equation by 2.78:

(0.950 ÷ 2.78) = (2.78 ÷ 2.78) × [CuSO_4(aq)]

Solve the equation to find the concentration of our unknown solution:

0.3417 mol L^-1 = [CuSO_4(aq)]

However, we are only justified in using 2 significant figures to express the concentration of our unknown solution, that is:

[CuSO_4(aq)] = 0.34 mol L^-1

In summary, to determine the concentration of a coloured solution using colorimetry:

1. Step 1: Determine the wavelength (colour) of light to use for the colorimetric analysis.
2. Step 2: Prepare a set of standard solutions of known concentration.
3. Step 3: Measure the absorbance of each standard solution using the colorimeter.
4. Step 4: Plot the absorbance vs concentration for each standard solution on a graph.
5. Step 5: Draw the line of best fit through the data points. This is the calibration curve.
6. Step 6: Measure the absorbance of the solution of unknown concentration using the colorimeter.
7. Step 7: Use the calibration curve to determine the concentration of this solution.

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Solving the Problem using the StoPGoPS model for problem solving:

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The amount of ethanol in a white wine can be determined by colorimetry.

First a 0.50 mL sample of white wine is diluted to 90 mL.

Then 10.0 mL of 0.100 mol L^-1 acidified potassium dichromate (K₂Cr₂O₇) is added.

The resulting test solution is warmed at 50°C for 15 minutes.

The balanced chemical equation for the reaction that is occurrring is given below:

2Cr₂O₇^2-_(aq) + 3C₂H₅OH_(aq) + 16H⁺_(aq) → 4Cr³⁺_(aq) + 3CH₃COOH_(aq) + 11H₂O_(l)

Aqueous solutions containing the dichromate ion are orange in colour so we can use a colorimeter fitted with a green light source to determine the amount of unreacted dichromate in the test solution.

Using the calibration curve given below, the concentration of dichromate in the test solution is found to be 0.0028 mol L^-1.

What was the absorbance of the test solution?

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