Any colorimeter whether simple or complex consists of five basic components:
Light source
Wavelength selector (filter)
Sample container
Photo detector
Display
these are configured in the following fashion:
The Light Source
The light source to be used for successful colorimetric must produce energy at sufficient intensity throughout the whole visible spectrum e.g. 380 – 780nm. Fortunately, this is achieved readily by using a tungsten lamp. This as we all know (since we use it in the home) provides adequate light to enable us to see all the visible range.
Note that the energy decreases towards the near ultra violet but it is still sufficiently powerful to energies a photo detector down to around 350 nm.
To increase the intensity of light into the near UV range a tungsten halogen lamp may be used, this consists of a tungsten filament in a quartz envelope which also contains traces of a halogen such as iodine.
Tungsten lamps are of a type which use black body radiation e.g. dependent solely on its temperature. There is another type which uses radiation produced at specific energies e.g. the excitation of atoms such as a gas discharge lamp.
Here an electric current is passed through a gas at low pressure resulting in molecular excitation and the emission of light at very specific wavelengths. The mercury lamp is a good example of the gas discharge lamp which produces strong emission in the UV and blue parts of the spectrum.
The major disadvantage of the mercury lamp is that it can only be used at specific wavelengths. For continuous output in the ultra violet range deuterium lamps need to be used and though expensive and with a comparatively short life do have an effective output down to 190 nm.
The Wavelength Selection:
There are several options open to the manufacturer of a colorimeter when deciding how to select the wavelength i.e. produce monochromatic radiation (one wavelength band) from polychromatic radiation (white light).
These basic options are:
1. Gelatin filters
2. Interference filters
3. Grating monochromators
4. Prisms
The Sample Container
The colorimetric analysis is performed on solutions and as such the sample can only be analyzed within a container. The container must therefore be (or made to be) transparent at the wavelength at which the sample is being measured. Sample containers are offered in a variety of forms depending on whether the important factor is the sample size, speed of measurement, or accuracy.
For colorimetric work, all containers should be made of glass or clear plastic. However, if the analysis is to be made in the ultraviolet region of the spectrum then quartz vessels must be used as glass absorbs UV radiation.
The Photo Detectors
Detectors for colorimeters basically convert the resultant light beam once it has passed through the sample compartment into an electrical signal. There are several types and there’s nowhere better to start than the good old selenium photocell: 1. Selenium Photocell 2. The Photo Tube 3. The Silicon Photocell 4. Photomultiplier Tubes.
Display
Readings are presented in analog or digital form. Historically the analog display was most
important prior to the advent of the LCD (Liquid Crystal Display) or LED (Light Emitting Diode).
Advantages and disadvantages of analog and digital displays.
| Digital | Analogue |
| Advantages | |
| Easy to use | Easy to see the rate of change |
| Resolution is identical through the range | Shows the ABS / % transmittance relationship graphically |
| Choice of readout of ±0.01 ABU to±0.001ABU | Can read ABS and % transmittance simultaneously |
| Can be transferred to a printer | Traditional in education |
| The readout is objective i.e. not open to interpretation errors | Easy to read even in bright sunlight |
| Disadvantages | |
| Does not illustrate the relationship of %T and absorbance | The readout is subjective i.e. depends where you stand in relation to the needle |
| Difficult to see the rate of change | Scale can only be read to 1 ABU |
| Can only read absorbance or transmittance | The logarithmic display is difficult to understand |
| Can only read absorbance or transmittance | Not state of the art |
| Can only read absorbance or transmittance | Resolution is not the same over the scale |
| Can only read absorbance or transmittance | Cannot read in concentration |