Determination of Copper by Complexation Essay
Determination of Copper by Complexation
To determine the concentration of copper in an unknown solution by using copper complexation, solvent extraction and spectrophotometry. Standards are used to create a calibration curve and the unknown concentration of copper is then calculated by using the linear equation from the calibration curve. The concentration of copper in the unknown solution 201 was found to be 12.57± 0.25 μg/mL.
Copper is an essential mineral in everyday life, it is in necessary for the production of red blood and it also keeps the immune systems, nervous system ad hones healthy. Too much copper in a diet can possible health risks. There are several different ways to determine the concentration of copper in the unknown solution such as colourimetric titration, microwave digestions and spectrophotometry however using the solvent-extraction provides the most accurate measurements of solvent and using the spectrophotometry provides good accuracy at low concentrations of 10-6 g/m (ppm).
Solvent Extraction is the transfer of a solute from one phase to another. This process allows to isolate or concentrate the desired analyte or to separate it from species that would interfere in the analysis. The most used process is the extraction of an aqueous solution with an organic solvent. Diethylether, toluene, and hexane are common solvents that are immiscible with and less dense than water. They form a separate phase that ﬂoats on top of the aqueous phase. Chloroform and carbon tetrachloride are common solvents that are denser than water. In the two-phase mixture, one phase is mostly water and the other phase is mostly organic because they are immiscible liquids. Two liquids are miscible if they form a single phase when they are mixed in any ratio. However organic solvents with low polarity are generally immiscible with water. 
Although some literature states that the solubility of copper(II) diethyldithiocarbamate complex is much better in carbon tetrachloride than in chloroform. Scientist have performed a series of tests where both chloroform and carbon tetrachloride were used as solvents. The results indicated that the type of extraction solvent does not have a significant influence on the accuracy of determination which is why the less toxic chloroform is recommended.
Diethyldithiocarbamate acid is very unstable even in weakly acidic mediums and has a limited value in acid solution. The rate of decomposition is directly proportional to the hydrogen ion concentration where the half-lives of the reagent at room temperature are listed in table 1.
Unfortunately, many other heavy metals also five coloured products that is extractable into the organic phase. Hence the selectivity method is often improved by chemical masking interferences. Where ammonium citrate buffers in order to eliminate common containment interference like iron. 
Suppose that solute S is partitioned between phases 1 and 2. The partition coefﬁcient, K, is the equilibrium constant for the reaction. Partition coefﬁcient: K = AS2/AS1 = [S]2 / [S]1. Where K refers to the activity of solute in phase 1. 
The effect of pH on the solute is dependent on if it’s an acid or base, its charge changes as the pH is changed. Usually, a neutral species is more soluble in an organic solvent and a charged species is more soluble in aqueous solution. Charged species tend to be more soluble in water than in organic solvent shown in figure 1. 
Figure 1. Effect of pH on the distribution coefﬁcient for the extraction of a base into an organic solvent. In this example, K 3.0 and pKa for BHis 9.00
Figure 2. Copper diethyldithiocarbamate complex
The complexing agent, diethyldithiocarbamate acid reacts with copper ions in slightly acidic or ammonium solution to form a brown soluble copper complex. The resulting aqueous suspension is often turbid and not suitable for absorption spectrophotometry. However the colloidal suspension can be extracted into organic solvents with chloroform) and the Yellow organic solution can be used for spectrophotometry. 
Spectrophotometry is a tool used to measure the amount of light that a sample absorbs. There are several different instruments all with different functions. The spectrophotometer is an instrument that operates by passing a beam of light through a sample and measuring the intensity of light reaching the detector. The beam of light consists of a stream of photons, when the photons encounter an analyte molecule, there is a chance the analyte will absorb the photon. This absorption reduces the number of photons in the beam of light, thereby reducing the intensity of the light beam. 
The instrument has a blank that is the same as the standards and unknown solutions except that the blank does not contain the solute that absorbs light. This measurement is necessary, because the cuvette and other solutions scatter some of the light.
The experimental data is used to calculate two quantities: the transmittance (_T_) and the absorbance (_A_). Where Transmittance = I/I0 and absorbance = _A_ = – log10 _T_ 
The transmittance is simply the fraction of light in the original beam that passes through the sample and reaches the detector. The remainder of the light, 1 – _T_, is the fraction of the light absorbed by the sample. If no light is absorbed, the absorbance is zero and there is 100% transmittance. Each unit in absorbance corresponds with an order of magnitude in the fraction of light transmitted. 
This experiment requires a specific wavelength of 436 nm for the absorbance due to this wavelength being the maximum wavelength for copper (λmax = 436 nm).
Copper is an essential mineral in everyday life, it is in necessary for the production of red blood and it also keeps the immune systems, nervous system ad hones healthy. Too much copper in a diet can possible health risks however there have been no useful studies for upper level intakes of copper, and the daily recommended does is 14-18 years 1,300 μg/day of copper, 19-30 years 1,300 μg/day of copper, 31-50 years 1,300 μg/day of copper 
1. A sodium diethyldithiocarbanate solution was prepared by dissolving 1.0 g of sodium diethyldithiocarbanate into 50 mL of deionized water in a beaker. An ammonium citrate solution was already prepared by dissolving 20 g of ammonium citrate in 100 mL of water.
2. The standards were prepared by using a copper stock solution that weighs 0.1000 g of copper. Which was dissolved slightly in a 8 M nitric Acid and diluted four times with deionized water, and boiled to remove nitrogen oxides and diluted to the 1 liter mark in a volumetric flask.
3. 25.0 ml of the copper stock solution was then transferred to a 250.0 mL volumetric flask and diluted to the mark. After which 5 standard solutions were made by transferring 5.0, 10.0, 20.0, 30.0 and 40.0 mL portions of the diluted copper solution and diluted to the mark with deionized water and mixed thoroughly.
4. An unknown solution was prepared by pipetting 10.00 mL of the unknown aqueous sample into a 500 mL volumetric flask and diluted to the mark with deionized water and mix thoroughly.
5. They then preformed an extraction by pipetting 50.0 mL of each standard solution prepared into 5 different 125 mL separatory funnels. A blank was created by transferring 50 mL of water into a 6th 125 mL separatory funnel. Then 50 mL of the diluted unknown was transferred into 3 separate separatory funnels.
6. They then added 5 mL of ammonium citrate solution and 2 mL of 15 M ammonium hydroxide into each of the 9 separatory funnels.
7. They then added 1 mL of the sodium carbonate solution to the first of the nine separatory funnels and mixed thoroughly, after which they pipetted into the separatory funnel exactly 10.00 mL of the CHCl3. Then they mixed thoroughly for 1 at least one minute with frequent venting of the funnel. (NOTE: to vent into the fume hood slowly and away from human face). After which the CHCl3 layer was drained into a cuvette, stoppered. They measured the cuvettes % transmittance using a spectrophotometry set at the 436 nm range.
8. This step was then preformed again with the nearest accuracy of time and venting to ensure same parameters for the other 8 separatory funnels.
The results of the solvent extraction and spectrophotometry can be found in Table 3, as well as the calibration curve used to obtain the calibration equation are shown in Graph 1. The experiment preformed fairly well. The final unknown concentration of copper in the unknown solution 406 was found to be 12.57 ± 0.25 μg/mL.
The daily intake of this unknown solution should be approximately no more than 100 mL before any possible health risks can occur. Since there have been no beneficial studies for high levels of copper in daily intake it can be expected that higher levels of copper in a daily dose could be higher without severe implications. The daily recommended does for copper is 1,300 μg/day, and to exceed this amount more than 100 mL of the unknown would have to be consumed.
All of the experimental procedures were followed, and during step 7 of the experimental, there was a leaky plastic cap for the 125 mL separator flask and a small amount of liquid was lost, however this lost did not affect our results enough to repeat the step again.
It is important that exactly 10.00 mL of Chloroform (CHCl3) was used for each other the 125 mL separatory funnels as this is the organic solvent and if the concentration of the organic solvent is changed from the other funnels then the results will vary as they do not have the same concentration and therefore produce different results.
Ammonium citrate in the solution eliminates chemical interference from other metals by maintaining the solution at a high pH that does not allow the other metals to form complexes with the Chloroform therefore preventing them from interfering with the copper during the experiment.
Chloroform is preferred compared carbon tetrachloride since it is less toxic and preforms almost equally. Scientist preformed a series of tests where chloroform and carbon tetrachloride were used as solvents. The results indicate that the type of extraction solvent does not have a significant influence on the accuracy of determination which is why the less toxic chloroform is recommended.
The transmittance is measured at 436 nm due to the copper having its maximum wavelength at 436 nm, which allows for the best absorbance and the highest accuracy of the compound as low as 10-6 g/mL (ppm).
The largest advantage for solvent extraction is that it gives an absolute measurement. This allows a direct measurement of the each phase for a given composition for complete mixture.  Where as other methods have blends of several compositions that do not allow for complete mixture. Even though it is widely used it is also specific to certain solvents for both phases in the mixture. Another challenge is the sample size, where samples that are large surface area to volume ratios may lead to inaccurate measurements. 
There are several different ways to determine the concentration of copper in the unknown solution such as colourimetric titration, microwave digestions and spectrophotometry however using the solvent-extraction provides with the most accurate solutions and using the spectrophotometry provides with the great accuracy at low concentrations.
An improvement for this experiment would be to have an improved precision instrument for measuring the 10.00 mL of chloroform then a small graduated cylinder for measurements. Also the 125 mL separatory funnels were fairly loose in the holders and require close attention to make sure they did not fall over. As well as one of the plastic lids on the separatory funnel was leaky and caused a small spill when shaking which could of offset the results and this could be a large source of error had some of the solvent or chloroform escaped.
The Calibration curve shows a R2 value of 0.998 which can be considered a fairly accurate linear trendline. This small variation is due to either the slow value of the standard 2. at 0.0001 g/mL or the high value of standard 3. at 0.0002 g/mL in Table 2.
The two sources of error that could have been possible within the experiment include the inaccuracy of the 10.00 mL graduated cylinder of chloroform, and the leaking of the separatory funnel. Other sources of error would be cuvettes are not completely clean and cause interference with the light source, the ammonium citrate buffer reached its half-life and the pH increased and some unwanted metals are not causing interference. The same parameters of 1 min shaking and venting are not the same for all trials causing variations in the colloidal suspension yellow organic solution used for spectrophotometry.
Solvent extraction and spectrophotometry were useful for determining the concentration of copper in the solution. Some of the experimental procedures should be altered in order to improve accuracy. The concentration of copper in the unknown solution 201 was found to be 12.57± 0.25 μg/mL. The daily intake of this unknown solution should be approximately no more than 100 mL before any possible health risks can occur.
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