Introduction:

Colorimetry is the analytical process (that is, spectroscopic method) employed to find out the concentrations of colored substances in solution. This relies on the fact that a colored substance absorbs light (that is, at the visible region) of a colour complementary to its own and the amount of light it absorbs (that is, absorbance) is proportional to its concentration. The colorimetric determinations are taken out by the utilization of an instrument known as colorimeter.

Principle of Colorimetry:

Colorimetric analysis is basically dependent on Beer-Lambert law. The concentration of the colored compound is associated to the amount of visible radiation absorbed via the colored compound. The colored compound absorbs white light at the visible region of a colour complementary to its own. Keep in mind that the white light is made up of various colours; red, orange, green, yellow, blue, indigo and violet. Such colours take place at particular wavelengths, example: blue takes place at the wavelength 435 to 480 nm, therefore, in Colorimetry the complimentary colour absorbed via a colored solution which takes place at specific wavelength.

The colour of the compound is generally because of the formation of a colored compound via the addition of a suitable reagent or it might be inherent in the desired constituent itself. The fundamental principle of most colorimetric measurements comprises in comparing, under well stated conditions, the colour generated by the substance in the unknown concentration by the similar colour produced via a known amount (that is, standard solution) of the similar substance being determined.

Colorimetric determinations:

The determination is identical to spectrophotometric determination. This will generally need:

1) A weighed quantity of the material beneath investigation in a suitable solvent.

2) A standard solution of the compound being found out in the similar solvent.

3) The requisite reagent.

4) Any ancillary reagents like buffers, acids or alkalis essential to establish the accurate conditions for format ion of the required colored product.

5) Preparation of the calibration curve.

6) Estimation of the unidentified concentration of the test solution from the calibration curve.

In observation of the sensitivity of colorimetric and spectrophotometric processes, the absorbance measurements are generally made up on very dilute solutions. In order to take adequate material for an accurate weight to be accomplished when preparing the original solution of analyte and the corresponding standard solution, it is generally essential to prepare solutions that are too concentrated for the absorbance measurements, and these should then be diluted precisely to the suitable strength.

The solutions of colour producing reagents are often unstable and generally should not be stored for more than a day or so. The colorimeter comprises of light source, Monochromator, slit, optical cell or cuvette, photoelectric cell and galvanometer.

Mode of operation:

The white light from a tungsten lamp passes via a condenser lens to provide a parallel beam that falls on the filter which is positioned to choose radiation of particular wavelength to impinge on a glass cuvette having the solution. As the light is passing via the solution, some part of it is absorbed via the sample component, whereas the part which is not absorbed is transmitted and detected through a photo electric cell (that is, detector). In order to evaluate the absorbance of a solution, the meter reading is initially adjusted to 100% transmittance (that is, zero absorbance) by a blank solution. The sample is then inserted in place of the blank and the absorbance is read directly. The concentration respective to the absorbance of the sample is then achieved from the standard or calibration curve. The filter is generally a complimentary colour of the test solution.

The filter is selected to choose the band of wavelengths that are most strongly absorbed via the colored solution example: this is described in the table below, by employing a yellow filter to use in measuring the concentration of a blue colored solution such as copper(II) sulphate or its ammine or amine complex.

Table: Band of wavelengths

Experiment: Colorimetric determination of manganese in steel

Purpose: To find out the concentration of manganese in steel

Discussion: Colorimetry is specifically suited to the determinant ion of manganese in steel since the manganese can be transformed to permanganate ions, which are colored. The conversion 'n' is accomplished in two phases. By using nitric acid, the manganese is first oxidized to manganese(II) ions, which are then oxidized to permanganate ions via the more powerful oxidising agent, potassium per iodate.

Equipment/Materials:

Table: Equipment of Colorimetric determination

CAUTION:

Wear the eye protection and if any chemical splashes on your skin wash it off instantly. The acidified 0.0010 mo l l^-1 potassium per manganate is injurious if ingested and irritates the skin and eyes. Wear gloves.

Both 2 mol l^-1 nitric acid and its vapor are toxic and corrosive, causing severe burns to the eyes, digestive and respirator y systems. Wear gloves.

85 percent phosphoric acid is corrosive: it irritates and burns the skin and eyes. This is a systemic irritant if inhaled and if swallowed causes severe internal injury. Wear gloves.

Acidified potassium per iodate solution is injurious if swallowed and is an irritant to the skin, eyes and respiratory system. This is as well corrosive. Wear gloves.

Potassium per sulphate is injurious if swallowed or inhaled as a dust. It irritates the skin, eyes and respiratory system, causing dermatitis and probable allergic reactions. Wear gloves.

Propanone is volatile and highly flammable, and is injurious if swallowed. The vapor irritates the skin, eyes and lungs, and is narcotic in high concentrations. Wear gloves.

Experimental Procedure:

Part A: Calibration graph

1) Rinse the burette, comprising the tip, by 0.0010 mol l^-1 acidified potassium per manganate and fill it by the similar solution.

2) Run 2cm³ of the per manganate solution into a 50cm³ standard flask and make up to the graduation mark having deionized water.

3) Stopper the flask and invert it sometimes to make sure the contents are fully mixed.

4) Rinse a cuvette by some of the solution and fill it.

5) By using a colorimeter (fitted by a green filter) assess the absorbance of the solution in the cuvette. If you encompass more than one green filter, select the one that provides maximum absorbance.

6) Repeat steps (2 to 5) by 4, 6, 8, 10, 12 and 14 cm³ of the per manganate stock solution in the burette.

7) Make use of dilution for mular to compute the concentration of the 2cm³ to 14cm³ of the diluted stock solution.

8) Plot a calibration graph of 'absorbance' against the concentration of potassium per manganate.

Part B: Conversion of manganese to permanganate

1) Degrease a steel paper clip by swirling it by a little propanone in the beaker. By using tweezers eliminate the paper clip and leave it to dry for a minute or so forth a paper towel.

2) Cut the paper clip to small pieces.

3) Weigh precisely around 0.2 g of the paper clip pieces and transfer them to a 250cm³ glass beaker.

4) Add around 40cm³ of 2 mol l^-1 nitric acid to the beaker and cover it by a clock glass.

5) Heat the mixture carefully, in a fume cupboard, till the reaction begins. Carry on heating gently to maintain the reaction, however remove the source of heat if the reaction becomes excessively vigorous.

6) Once the steel has reacted, let the solution to cool a little. Add a couple of anti-bumping granules and then boil the solution till no more brown fumes are given off.

7) Once this solution has cooled considerably - no more than 'hand hot'- adds around 5cm³ of 85 percent phosphoric acid, around 0.2 g of potassium per sulphate and a couple of fresh anti- bumping granules. Boil the mixture for around 5 minutes.

8) To this solution, add around 15 cm³ of the acidified potassium per iodate solution plus a couple of fresh anti-bumping granules and then smoothly boil the mixture. The solution will begin to turn pink. Continue gently boiling till the intensity of the pink colour remains constant. This must take around 5 minutes. 

9) Let the pink solution to cool to room temperature and then transfer it to a 100 cm³ standard flask, leaving the anti-bumping granules in the beaker.

10) Rinse the beaker many times by a little deionized water and add the rinsings (however not the anti-bumping granules) to the flask.

11) Make up the solution to the graduation mark by deionized water.

12) Stopper the flask and invert it many times to make sure the contents are fully mixed.

13) By using a colorimeter fitted by the suitable green filter, assess the absorbance of the solution.

14) Make use of your calibration graph to transform the absorbance to a permanganate concentration and then compute the percentage by mass of manganese in the steel paper clip.

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