Colorimetry is used to measure the intensity of absorption of coloured compounds over a narrow range of frequencies. It is a useful way of determining the concentration of a chemical in a coloured solution. Beers law states that the amount of light absorbed by a solution is directly proportional to path length and concentration. A = abc This graph shows the relationship between absorbance and concentration that they are proportional to each other. In a colorimeter a narrow beam of light passed through a filter towards a tube containing the coloured solution.
Any light that passes through the solution is detected by a photocell and a reading of absorbance is displayed on the meter. Before you start to measure the absorbance of any sample you firstly have to calibrate or reference the colorimeter with distilled water so that the baseline will be placed at 0. 00Abs or 100% T. Also appropriate wavelengths have to be selected before starting to measure the absorbance of your sample.
How much manganese is there in a paperclip? Method 1. Cut up a paper clip in to small pieces and place in a weighing boat to weigh accurately about 0.
2g using a weighing balance. 2. Measure approximately 70cm3 nitric acid using a measuring cylinder and pour it into a beaker. Then add the small pieces of paperclip to it. 3. Place the beaker with the nitric acid and paperclip under a Bunsen burner to warm up but make sure to not let it boil. The nitric acid oxidises the Manganese to Mn 2+ (aq) ions 4.
Add about 10 cm3 of phosphoric(v) acid to the beaker followed by 10 cm3 of potassium iodate (VII) solution. Boil the solution carefully for 10 minutes and then allow to cool.
(The phosphoric(v) acid prevents the precipitating of insoluble iron (III) salts). 5. Pour the solution the solution into 100cm3 volumetric flask using a small funnel. Rinse the remaining solution from the beaker and funnel into the flask with distilled water and add further more distilled water to the flask till the graduation mark. 6. Stopper the flask and shake it to ensure that the solution is uniform and has been mixed thoroughly. All the Manganese in the 0. 2g of paperclip is now in the purple solution as the Manganate. Using colorimetry to find the % in a paperclip
1. Pipette 10cm3 of the prepared potassium Manganate (VII) solution into a 100cm3 volumetric flask and make up to the graduation mark with distilled water. This corresponds to the solution which would be obtained from the paperclip containing 2. 0% Mn (2. 0% Mn solution). Set aside a sample of this solution to use in the colorimeter. 2. Take two burettes and fill one with water and the other with 2. 0% Mn solution. Make up 20cm3 samples of solutions to correspond to 1%, 0. 8%, 0. 6%, 0. 4% and 0. 2% solutions by using suitable volumes from the two burettes. 3.
Draw a table and take a note of the colorimeter readings given by each of the diluted potassium Manganate solutions together with that produced by the paperclip solutions. 4. Draw a graph to show the % of Manganese in the paperclip against colorimeter reading. Dilution table Results table % Mn Cm3 of 2% Mn solution Cm3 water Abs of sample = 0. 08 Conclusion.
My graph shows a clear positive correlation between the absorbance and the concentration of Mn solution. Therefore this graph agrees with Beers law which states hat the amount of light absorbed by a solution is directly proportional to path length and concentration. This can be written as A C; A is the amount of light absorbed and C is the concentration of the Mn solution. You can work out the concentration of the Manganese by using the line of best fit from the concentration against the absorbance knowing that the absorbance of the Manganese sample is 0. 08. Therefore the concentration is 2% of paperclip, we could end up with 0. 0115g in 100cm3. However this standard solution is too hard to make up because the amount is too small to weigh.
Therefore we can do 1. 15g instead in 1 dm3 and then dilute it. This will be equivalent to a solution from a paperclip with 20% of Mn. This solution can be diluted to provide 1:10 to give 2% solution to make series of dilutions for the calibration curve. Evaluation Experimental errors-when any measurements are involved in an experiment there is always uncertainty or errors in the experiment. The source of this uncertainty is the precision of the equipments used. The percentage error can be worked by the equation: Error x 100 Reading The percentage error of the balance = 0. 005 x 100 = 2.
5% 0. 20 The percentage error of the smallest value measured from the burette: 0. 05 x100 = 2 2. 5% The percentage error of the largest value measured from the burette: 0. 05 x100 = 0. 28 18 The percentage error of the pipette: 0. 06 x 100 = 0. 06% 10 The percentage error of the volumetric flask: 0. 2 x 100 = 0. 2% 100 From looking at the percentage errors that have been calculated we can calculate which is a significant error (if error is more than 1%) and which is not a significant error. We can see that the percentage error of the balance is 2. 5% therefore it is a significant error.
Also the percentage error of the smallest value measured using a pipette is 2. 5% aswell and therefore it is also a significant error. Procedural errors- they are errors made when carrying out the method. These errors can efect the reliability and accuracy of the results. The steps that are taken to ensure an accurate method are: – Repeating the reading 3 times and then taking an average. – Before measuring the absorbance of the sample, firstly reference/ calibrate with water. – Started with the lowest concentration to avoid contamination by more concentration solution. – Rinse the cuvette with the solution you will be testing.
Also rinse the burette with the solution you will use. – When making up the standard solution in the volumetric flask fill up to the graduation mark using a dropping pipette and also invert and mix thoroughly. Risk assessment Chemical Hazard Precaution Nitric acid (2 mol dm-3) Irritant Eye protection and lab coat must be worn at all times. If solution is splashed in the eye flood the eye with gently running tap water for 10 minutes. If spilt on skin or clothes flood the effected area with large quantities of water and remove contaminated clothing. Finally, if spilt in laboratory wash the area of the spill thoroughly.
Phosphoric acid Corrosive Eye protection and lab coat must be worn at all times. If splashed in the eye flood the eye with gently running tap water for 10 minutes. If spilt on skin or clothing remove contaminated clothing and quickly wipe as much liquid as possible off the skin with a dry cloth before flooding the area with a large excess of water. If spilt in laboratory rinse the area of spill. Potassium Iodate (2. 0 mol dm-3) Harmful Oxidising Eye protection and lab coat must be worn at all times. If solution gets into the eye flood the eye with gently running tap water.
If spilt on skin or clothing remove contaminated clothing and soak well with water and wash off the skin with gently running tap water. If spilt in laboratory rinse the area of spill. Potassium Manganate Harmful Danger to the environment Oxidising Eye protection, lab coat and gloves must be worn at all times as it stains the hands. If solution gets into the eye flood the eye with gently running tap water. If spilt on skin or clothing remove contaminated clothing and soak well with water and wash off the skin with gently running tap water. If spilt in laboratory rinse the area of spill.
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