Unit 4 - Colorimetry

Question 1

What is colorimetric analysis

Answer 1

An instrumental method used to determine the concentration of a coloured analyte.

Question 2

What is colorimetery

Answer 2

Colorimetry is the measuring of colour intensity: it is used to determine the concentration of solutions that are coloured (have coloured substances) or that can be converted quantitatively into Colored species,eg manganese can be converted into purple permanganate ions.
- we work with solutions in colorimeter and so it is the concentration of the coloured species in the solution that we wish to determine.

Question 3

What is the beer lambert law and what does it state

Answer 3

The beer lambert law shows that the absorbance(intensity of light colour in solution) is directly proportional to the concentration of the coloured species , the law states the absorbance has to lie on the linear region of a calibration curve

Question 4

What is

• incident light
• transmitted light

Answer 4

• The light that passes into the sample is called the incident light.
• the light that passes through the sample is called transmitted light.
• The ratio of the incident light to the transmitted light gives an absorbance value

Question 5

Theory behind colour - how do we know a solution is coloured

Answer 5

will be coloured if it absorbs some (not all) parts of white light passing through it.
- the colour we see is not being absorbed (transmitted through the solution combining to give the colour we see)

Question 6

How can we see which colours are being absorbed

Answer 6

• the colour wheel (in the data booklet) can show which colours of Light are absorbed. The colour absorbed the most will be the complementary / opposite colour to what we see.

Question 7

How does a colorimeter work and what does it consist of

Answer 7

• a colorimeter works by passing a beam of light through the coloured sample.
• the light that passes into the sample is called the incident light and the light that passes through the sample is called the the transmitted light
• a colorimeter essentially consists of a light source , a coloured filter , a light detector and a recorder
• the light from the source passes through a coloured filter which changes the light to a specific wavelength. The incident light then passes into the sample

Question 8

Why is the filter important - what does it have to be

Answer 8

The filter chosen has to be the complementary colour to the solution as this will result in maximum absorbance. (Complementary colour would be opposite on the colour wheel)

Question 9

The more ________ the solution the greater the ________ of light

Answer 9

The more concentrated the solution the greater the absorbance of light

Question 10

Calibration graphs

Answer 10

• when absorbance is plotted against the concentration of the coloured standard solution , the resulting calibration graph should be a straight line indicating that the two are directly proportional.

Question 11

Calibration graphs must be drawn as _____ _____ ______ lines

Answer 11

Calibration graphs must be drawn as best fit straight lines

Question 12

How can the conc of the solution of unknown concentration be found using a calibration graph

Answer 12

The solution if the sample being analysed is placed in the colorimeter and the absorbance reading is noted. This value can be used to determine the concentration using a calibration graph as absorbance is directly proportional to concentration of coloured solution

Question 13

Points of good practice for calibration graphs

Answer 13

• absorbance values should be measured at least twice and an average taken at each concentration values which show anomaly (vary greatly from the line of best fit)can be removed
• the calibration graph should cover the dilution range likely to be used in the determination of the concentration of the unknown solution. If the solution of unknown concentration has an absorbance value that lies outwith those used in the calibration graph , another calibration graph should be prepared using more appropriate concentrations of the standard solutions
• If the data points on your calibration graph are close to the line of best fit, it can be assumed that the value obtained is more likely to be accurate/have a high degree of accuracy. If the data points are not lying close to the line of best fit than the value obtained is more likely to be inaccurate

Question 14

Describe fully the procedure required to obtain results that would allow a calibration graph to be drawn.

Answer 14

• several standard solutions of known concentration should be made
• the colorimter should be set to a suitable filter
• blank solution should be used in order to calibrate the colorimeter
• then the absorbance of each solution should be measured in order to create a calibration graph