[Hello dear readers! The reference list has not been added yet. I will add it as soon as possible. Thank you!!]

1 Aim

1. To synthesise crude CuI via redox reaction of Cu²⁺ and I^– and the subsequent precipitation.

2. To purify crude CuI via recrystalization in KI solution;

3. To determine the purity of the purified sample by UV-vis spectroscopy, using the standard solutions of different concentrations as reference.

2 Introduction

Copper is one of the earliest elements known to human civilisations. However, for thousands of years the application of copper had been largely confined to its metallic form and Cu(II). It is only in the recent centuries that Cu(I) chemistry has attracted special interest among chemists. Though Cu⁺ disproportionates readily in acidic solutions, most solid Cu(I) compounds, and many Cu(I) centred complex ions are relatively stable. Cu(I) forms various structurally or synthetically significant compounds like [Cu₄Br₆]^3- and CuCN. Among them, copper(I) iodide is one with wide application. It is used as a catalyst in organic reactions, especially in electrophilic substitution on aryl groups. It forms brown Cu₂HgI₄ with mercury vapour so as to perform mercury detection. Furthermore, CuI is used as a cloud seeding agent.

The industrial synthesis of CuI involves heating Cu and I₂ in concentrated HI solution. However, this is inconvenient for laboratory purposes. In labotatories CuI is commonly made via the reduction of CuSO₄ by KI in an aqueous solution. Cu⁺ then immediately reacts with I^– to form a white CuI precipitate. The redox reaction is forbidden by a positive free energy, but a very low k_sp of CuI (1.27×10^-12) drives the total reaction forward. The crude CuI formed may be purified by dissolving in hot concentrated KI solution and recrystallise upon cooling and dilution.

UV-vis spectroscopy can be used to determine the purity of the sample obtained. The Cu(I) in the sample, converted to a Cu(III)-cuprizone solution at pH 8.0-9.0, has a characteristic absorbance peak at 600nm. It is then straightforward to apply Beer-Lambert’s law and compare this absorbance with those of standard Cu-cuprizone solutions to analyse the purity of the sample.

3 Experimental

3.1 Synthesis of CuI

A solution with 1.32g KI and 2.29g Na₂S₂O₃·5H₂O dissolved in 10mL of deionised water was added dropwise to a solution of 2.01g CuSO₄·5H₂O in 10mL of deionised water with constant stirring. 20mL of deionised water was then added into the resulting suspension. The solid was allowed to settle for 5 minutes and the supernatant was decanted. Another 20mL deionised water was added, followed by decantation again. The suspension was then transferred into a centrifuge tube with 12mL ethanol, and centrifugation was performed at 6000rpm for 1 minute. The supernatant was decanted. Repeat the centrifugation once with 14mL ethanol, and another twice with 8mL diethyl ether. The supernatant was decanted after each centrifugation. The crude CuI obtained was air dried inside the fumehood.

3.2 Purification of crude CuI

1.000g crude CuI and 17.98g KI were dissolved in 20mL of deionised water at 70°C with constant stirring. After all the solid was dissolved, stirring was stopped and 0.49g activated char coal was added to the solution.  Suction filtrations were performed twice to the mixture. A hot KI solution (15.4g in 15mL of water at 70°C) was used to wash the solid residue after the first suction filtration. The clear filtrate was diluted with 200mL deionised water with constant stirring, and cooled in an ice bath for 45 minutes until most of the precipitate was settled. The supernatant was decanted and four centrifugations were performed on the solid product at 6000 rpm for 1 minute, the first two with 12mL ethanol and the later two with 8mL diethyl ether. The resulting purified CuI was dried in a water bath of 60-70°C for 45 minutes.

3.3 Preparation of standard solutions for calibration graph

0, 3.0, 6.0, and 8.0mL of a 0.0100g/L CuSO₄·5H₂O stock solution were dispensed into four 25mL beakers respectively. To each beaker was added 2mL 10% ammonium citration solution (pH pre-adjusted to 8-9). The pH of the solutions was adjusted to 8-9 by 5M aqueous ammonia. 3mL of 0.1% cuprizone stock solution was then added to each beaker. The solutions were transferred into four 25-mL volumetric flasks, and diluted to 25mL by deionised water. The UV-vis spectroscopy of the standard solutions was performed on a Shimadzu UV-1800 spectrophotometer at 250-800nm. Their absorbances at 600nm were recorded and used to plot the calibration curve.

3.4 Spectrophotometric analysis of purified CuI sample

40mg of the purified CuI sample was dissolved with 6mL of 5M aqueous ammonia solution and 200mL of deionised water. The solution was transferred into a 250mL volumetric flask and diluted to 250mL with deionised water. 1mL of this solution was dispensed into a 25-mL beaker and diluted with 10mL of deionised water. After adding 2mL of 10% ammonium citrate solution (pH pre-adjusted to 8-9), 8 drops of 5M aqueous ammonia, and 3mL of 0.1% curprizone stock solution, the solution was transferred into a 25-mL volumetric flask and diluted to 25mL. Spectroscopy of the sample solution was run at 400-800nm on a Shimadzu UV-1800 spectrophotometer, and the absorbance at 600nm was recorded for analysis of the purified CuI sample.

4 Results and discussion

4.1 Synthesis of crude CuI

The reactions of the synthesis is Cu²⁺(aq)+4I^–(aq)→2CuI(s)+I₂(s) and I₂(s)+2Na₂S₂O₃(aq)→2NaI(aq)+Na₂S₄O₆(aq). Upon addition of the colourless solution KI- Na₂S₂O₃ solution into the blue CuSO₄ solution, a white CuI precipitate formed immediately, together with a trace of purple I2 on the surface of the mixture, but the purple colour disappeared within seconds after its formation. The blue colour of the reaction mixture gradually faints, and the final suspension was white with a light brown due to unreduced I₂ trapped in the CuI precipitate.

The precipitate was a white, fine powder, and hard to settle in water. However, it settled faster in ethanol, and even faster in diethyl ether. While its suspension in water was always milky, CuI quickly forms a flocculent precipitate in diethyl ether. This is due to the fact that CuI is an ionic compound and interacts more strongly with polar solvents (like H₂O) and less strongly with nonpolar solvents (like diethyl ether).

The crude CuI was difficult to dry. After 2 hours of air drying the sample still had a smell of diethyl ether. The white powder turned light brown during the drying as it underwent photochemical decomposition into Cu and I₂. The mass of the crude CuI obtained was 1.84g, which was greater than the theoretical value if all the I- in KI (the limiting reagent) were converted to CuI. This shows that there are impurities in the sample, most possibly the trapped I₂ and remaining diethyl ether.

4.2 Purification of CuI

During the purification, KI dissolves quickly in water upon heating and stirring. As the temperature reached 65°C, the crude CuI began to dissolve slowly, forming a brown solution. The filtrate obtained by the first suction filtration was greyish, with some white CuI recrystallised. After reheating the filtrate to 70°C (to re-dissolve the CuI) and performing a second suction filtration, the filtrate was colourless and clear. It, however, took an unusually long time for the CuI to precipitate from the diluted filtrate: after 20 minutes of ice bath, the solution remained perfectly clear, but at the 25^th minute a fair amount of white solid slowly began to appear at the lower half of the beaker. The CuI precipitate was in the form of very fine white particles, and took about 40 minutes to settle. Again, it was difficult to dry this purified CuI; after 45 minutes of water bath at 60-70°C, vapours of diethyl ether could still be seen rising from the sample. The obtained purified sample of CuI was purely white. It did not turn brown as the crude sample, because the drying did not last as long as 2 hours. The mass of the purified CuI was 0.144g.

If the crude sample were all purified in this manner, the mass of of purfied CuI would be 0.144g/1g×1.84g=0.265g

As KI is the limiting reagent, the theoretical mass of CuI is m(KI) / M(KI) × M(CuI) = 1.51g.

The percentage yield is therefore 0.265 / 1.51 = 17.5%

It is noted that the percentage yield is dissatisfactory, and the main problem is apparently that the mass of the purified sample was much less than the mass of the crude CuI (0.144g) used for purification (1.000g). It is unlikely that the impurities (I2, Cu, diethyl ether, some K+, Na+, S₂O₃^2-, S₄O₆^2-) would take up to more than 80% of the crude sample, and so it is highly probable that a large proportion of CuI was lost during the purification.

The fact that it took a very long time for CuI to appear in the diluted solution in the ice bath and that CuI appeared only at the lower half of the solution seems to suggest that most of the CuI may be lost during the suction filtrations, so that the solution did not become saturated with CuI until the temperature had dropped sufficiently low. As activated char coal adsorbs impurities as well as CuI and [CuI₂]^–, some potential product was expected to be trapped by activated char coal and not recovered. In theory, the adsorbed product could be washed off by the hot KI solution, however, during the experiment, the strength of the vacuum pump was not controlled well, and the suction went so strong that the hot KI solution flowed into the filter flask immediately, and the washing was therefore very ineffective. In addition, due to ignorance of the important of washing, in the second suction I did not wash the solid. Later in the experiment, white crystals were found on the filter papers of the second filtration, which shows that some product was indeed lost in the filtrations. Therefore, the experiment could be improved, if instead of activated char coal, chemical methods to remove the impurities while retaining the product were employed. For example, as the main impurity was I2, Na₂S₂O₃ could be added dropwise until the solution is colouless.

Another possibility is that a fair amount of CuI was still in the form of [CuI₂]^– and other Cu(I)-iodide ions, and was decanted with the supernatant. This may also be true because in preparation of the hot KI solution, I dissolved 15.4g KI in 15mL of water instead of the recommended 10g. This means more water was needed to separate CuI from the Cu(I)-iodide ions, and dilution with 200mL of water may not be enough.

Which factor was more dominant could only be determined by futher experiments.

4.3 Spectrophotometric analysis of the purified sample

The concentrations and absorbances at 600nm of the standard solutions is given in Table 1.

Table 1 The concentrations of the standard solutions and their respective absorbances at 600nm

Concentration/(10-6 g Cu/mL)	0	1.20	2.40	3.20
Absorbance	0.007	0.288	0.629	0.785

From the data the calibration graph can be drawn, as shown in Figure 1. The curve exhibits good linearity, as is predicted by Beer-Lambert’s law.

Figure 1 The calibration curve of Cu-cuprizone solutions. Absorbance was measured at 600nm.

The purified CuI sample dissolved readily in aqueous ammonia and forms a light blue [Cu(NH₃)₂]⁺ solution. Upon addtion of cuprizone, it turned into a deep blue Cu-cuprizone solution.

The absorbance of the sample solution at 600nm was 0.349. From the regression function of the calibratioin curve y = 0.2486x + 0.0046, (where y is the absorbance and x is the concentration,) this absorbance value corresponds to a concentration of 1.39×10^-6 g Cu/mL. The mass of Cu in the 25-mL sample solution is therefore 1.39×10^-6 g/mL × 25 mL =3.48×10^-5 g.

The sample solution was made from 1mL of the 250-mL [Cu(NH₃)₂]⁺ solution. So the mass of Cu in that solution is 3.48×10^-5 g × 250 = 8.69 mg.

However, the 250-mL solution was made from 40mg of purified CuI. Therefore, the mass of CuI in the 40mg sample is 8.69mg/M(Cu)×M(CuI)=26.0mg.

The purity of the purfied CuI sample is therefore 26.0/40.0=65%.

A purity of 65% is clearly dissatisfactory, and it implies that the percentage yield of 17.5% is not reliable, as a large portion (1/3) of the purified sample was impurities, according to the spectrophotometric results. I was greatly confused about what the impurities might be. As the sample was purely white, it is unlikely that there were a considerable amount of Cu and I2, or copper oxides. It is also impossible that species in the supernatant, mainly KI, would be present in the sample, because the literature solubility of KI is 128g/mL, and there is no reason for it to precipitate together with CuI in the 200mL solution. The only possible impurity seems to be the solvents, i.e. water, ethanol, and diethyl ether. However, the literature densities at 25°C of water, ethanol, diethyl ether are 0.997, 0.785, and 0.713g/mL, respectively. The literature density of CuI is 5.67g/cm³. If 1/3 of the mass were solvents, then the volume of the solvents would be at least 2.83 times that of CuI, and at most 3.98 times. This is unreasonable, because the CuI obtained was apparently a solid, and appeared to be dry, as pressing the sample against a filter paper did not result in a wet area on the paper. There may be some residual diethyl ether, as vapours was still emerging from the sample at the end of the drying, but it should not take up a high percentage. Moreover, the drying lasted 45 minutes, which is much longer than the suggested 15 minutes.

The above discussion suggests that a purity of only 65% is somewhat inexpliable. The problem may lie in the procedure of making the sample solution instead of the impurities. Upon careful recollection of the procedure in 3.4 I found that a very possible explanation is that the pH went so high after the addtion of 5M aqueous ammonia that the Cu-cuprizone complex decomposed due to its instability at high pH. In the preparation of standard Cu-cuprizone solutions, pH papers was given, and I was able to carefully control the amount of aqueous ammonia added so that the pH could lie precisely between 8 and 9. However, while preparing the sample solution, pH papers were not given and I decided the amount of aqueous ammonia added (8 drops) based on a rough guess. However, I later remembered that in the preparation of the standard solutions, 5 drops was enough to reach a pH of 9. Therefore, the excessive 3 drops may well have driven the pH out of the buffer range of ammonium citrate, and destroy the complex, so not all Cu was detected by UV-vis spectroscopy.

Errors in the data may be caused by using different spectrophotometers for the standard solutions and the sample solution, and the dates of the two performances were one week apart. Air disturbance can cause inaccuracies in weighing. Parallexes and poor timing were responsible for errors in determining volumes in the burette.

5 Conclusion

In conclusion, a brownish crude CuI was successfully synthesised from the reaction of Cu2+ and I-, and was purified into a white sample. The percentage yield was 17.5% due to adsorbance of the potential product on activated char coal, or perhaps inadequate dilution. The purity of the white sample was found by UV-vis spectroscopy to be 65%, but this was most likely to be attributed to a mistake of adding an excessive amount of aqueous ammonia in preparation of the sample solution, which decomposed the Cu-cuprizone complex, so this purity value is not convincing. The actual purity can be believed to be much higher than 65%, since the physical properties of the purified CuI sample denies substantial existence of all possible impurities. If this is indeed the case, then the 17.5% percentage yield is reliable, but it is a pity that the exact purity remains unknown. For a better, quicker synthesis of CuI, a chemical method that replaces activated char coal to remove impurities in the [CuI₂]^– solution needs to be explored further, as well as a more efficient way to evaporate residual diethyl ether from the sample.