The Determination of the Quinine Content of Water- Lab Report

Transcription

1 The Determination of the Quinine Content of Water- Lab Report Introduction This experiment aims to use a UV-Vis spectrophotometer to analyse the amount of quinine in tonic water, and study the effects of different sample preparation techniques on the accuracy of measurement of the concentration of quinine. Besides quinine, tonic water contains citric acid along with sugars and saccharin, of which saccharin absorbs in the same UV-Vis spectral range used in this experiment. This means that the saccharin acts as an interferent, and must be purified. A further limitation is the presence of citric acid, which lowers the ph of the tonic water. The effect of this purification can be assessed using spiking and recovery. Method A blank solution along with a set of standard solutions were prepared in test tubes using a separate graduated pipette to measure out each of the solutions displayed in the Calibration Solutions table. Sample 1 contained 0.0cm 3 of 50ppm quinine, and each subsequent sample contained 0.5cm 3 more than the previous sample. Sample 1 also contained 2.5cm 3 of 0.05M H 2SO 4, with each subsequent sample containing 0.5cm 3 less than the previous sample. The absorbance of these samples were measured using the spectrophotometer at 350nm, and recorded. A graph of absorbance vs concentration was then plotted using this data, and an equation of the line of best fit was obtained. Six diluted tonic water solutions were then prepared in test tubes, again using a separate graduated pipette for each solution. Three of these solutions were spiked with 1.00cm 3 quinine (50ppm), the other three were not. These measurements were recorded and are displayed in the table titled Diluted samples of tonic water. The absorbance of these samples were then recorded and the apparent concentration was calculated using the equation obtained from a graph based on our calibration solutions. Six more diluted tonic solutions were prepared using separate graduated pipettes for each solution. Three of these were spiked with 1.0cm 3 50ppm of quinine, the other three were not. These measurements can be found in the table labelled Liquid/Liquid extraction. These solutions were then brought to ph 10 using 4M ammonium hydroxide solution. 5.00cm 3 of 2:1 Chloroform: Isopropanol solution was then added, and the solutions were transferred to separate centrifuge tubes and shaken each for 20 seconds, and they were then left to separate. Exactly 3.00cm 3 of the organic layer (at the bottom) was then extracted using a graduated pipette to four new sample tubes. 3.00cm 3 of 0.05M H 2SO 4 was added to each tube and again shaken for 20 seconds and allowed to separate. 2.5ml of the aqueous layer from each sample was then transferred to four cuvettes. The absorbance of these solutions at 350nm were then recorded, and from this the apparent concentrations of the samples could be calculated. Results are displayed on the next page.

2 Calibration Solutions Sample Number 50ppm Quinine 0.05M H 2SO 4 Absorbance Solution /45-5 = =0.017*30 + c (0.017*30) = Equation of line: y= 0.017x Diluted samples of tonic water Sample 50ppm Number Quinine 0.05 H 2SO 4 Tonic Water Absorbance Concentration Solution DU DU DU DS DS DS

3 Mean concentration of DU samples: ( )/3= Mean concentration of DS samples: ( )/3= Standard Deviation of DU values: DU DU DU Total= Standard deviation of DU values= sqrt(7.451/2) = Standard Deviation of DS values: DS DS DS Total= Standard deviation of DS values= sqrt(0.298/2) = Liquid/Liquid extraction Sample 50ppm Number Quinine 0.05 H 2SO 4 Tonic Water Absorbance Concentration Solution EU EU EU ES ES ES Mean concentration of EU samples: ( )/3 = Mean concentration of ES samples: ( )/3 = Standard deviation of ES samples: ES ES E Total= Standard deviation of ES samples: sqrt(17.400/2) = Standard deviation of EU samples: EU EU EU Total= Standard deviation of EU samples: sqrt(11.006/2) = 2.346

4 Calculation of quinine in original tonic water in unspiked samples (displayed in the above tables) Method Equation line: y= 0.017x (y = absorption, x= concentration) x= y+0.005/0.017 DU calculations /0.017 = /0.017= /0.017= EU calculations /0.017= /0.017= /0.017= Weight of quinine in DU samples: ((15.647/1000)*5)= mg, therefore concentration = 78.2mg/dm 3 ((12.117/1000)*5)= mg, therefore concentration = 60.6mg/dm 3 ((12.529/1000)*5)= mg, therefore concentration = 62.6mg/dm 3 Weight of quinine in EU samples: ((27.824/1000)*4)= mg, therefore concentration = 111.3mg/dm 3 ((24.235/1000)*4)= mg, therefore concentration = 96.9mg/dm 3 ((28.647/1000)*4)= mg, therefore concentration = 114.6mg/dm 3 Measured increase in concentration due to spiking: DS-DU = Quinine added to DS: 1*50mg dm 3 = 50mg dm 3 Percentage recovery for DS: (10.374/50)*100= 20.75% ES-EU = Quinine added to EU: (1/1000)*50mg dm -3 = 0.05 (0.05*3/5)/3=0.01 (0.01*2.5)/0.0025= 10ppm Percentage recovery for EU: (15.059/10)*100= % The discussion is on the following page.

5 Discussion The quality of the data can be questioned to some extent because of the large standard deviation values for the ES and EU samples. This calls into question the quality of data because the value of concentrations of quinine that were calculated lie within a larger range of the mean and therefore make them marginally less reliable. The interferents, including saccharin and citric acid, were limited by performing a liquid-liquid extraction of the quinine. This involved ionising the quinine with the addition of H 2SO 4 to make the molecule exist in the aqueous phase from which it could be extracted. Knowing what phase the quinine and the interferents were present in means that the interferents can be removed without losing the quinine. The liquid-liquid extraction was performed using a graduated pipette, however it may be wiser to use a separating funnel if this experiment was to be performed again, as it will reduce the risk of contamination. A further discussion point concerns various transferring errors that were made over the course of the experiment. Several separate graduated pipettes were used, each one adding a degree of uncertainty to the values we recorded. There was additional error introduced when the samples were transferred from the test tubes to the centrifuge tubes, as traces of the sample were left behind. The results obtained differ quite substantially as a result of different preparation strategies. The extraction method removed more interferents and as a result of this had a higher percentage recovery. The dilution method involved more measurements and introduced more error, so as expected it had a lower percentage recovery. The accuracy of the dilution method could be considered more reliable because it had low standard deviation values compared to the extraction method, suggesting the mean concentration value is more representative of the sample as a whole. Conversely, the extraction method could be concluded as the more accurate method because it removes the interferents so we can be sure that it was just the quinine that was measured and nothing else. The larger standard deviation values for this method was more down to individual human error as opposed to a flaw with the method itself, as the measurements were not made by the same person who made the measurements for the dilution method, suggesting a further source of error. Based on the data obtained from the experiment, the extraction method appears to be the more accurate form of measurement, for reasons discussed above concerning the removal of interferents and percentage recovery rates.