I. Design

Background Information

The coordination compound (central metal atom) in this investigation is copper (II) (Cu²⁺ ) and is surrounded by the nonmetal nitrogen atoms that donate a lone pair of electrons, called ligands. When forming the copper-ammonia complex ion, the nitrogen ligands first form coordination bonds to the copper (II) metal ion by donating a lone pair of electrons. As the electrons donated by the nitrogen approach the copper (II) ion, the electrons receding in the d-orbital repel the additional electrons, and this causes the d-sublevel to split into energy levels where the sublevel closer to the ligands becomes higher in energy due to the repulsion, while the other sublevel is lower in energy. 

When the d-sublevel splits into separate energies, the electrons occupying d-orbitals in the lower level absorb energy and transition to a high energy level, causing an excited state and producing blue photons of light. The specific cyan blue color occurs because of the copper (II) unique difference in energy between the d-sublevels, the +2 oxidation state, the nitrogen ligands involved, and the arrangement of the ligands around the central ion. The cyan blue color perceived by the eye is the transmitted color, while the absorbed color is the opposite red color (Halpern & Kauffman, 2018).

When NH₃ donates its lone pair to Cu²⁺, a coordinate covalent bond is formed, resulting in the formation of a complex ion. When the complex ion is formed, stability is achieved (Clark, n.d).

In this investigation, the spectrophotometer is used to measure the number of photons (the intensity of light) absorbed after a specific wavelength of light passes through the varying copper-ammonia complex solutions in a cuvette. Since the formation of the copper-ammonia complex results in a unique blue color, the spectrophotometer indirectly quantitatively measures the concentration of the complex in the solution by the color intensity/absorbance (V ollbrecht, n.d.).

The Beer-Lambert law shows the relationship between absorbance and concentration:

In the context of this experiment, the absorption values obtained from the spectrophotometer are plotted against varying ammonia concentration on a calibration to obtain the slope that represents the molar absorption coefficient. The optical path length is the width of the cuvette (usually 1 cm). Then, after inputting all known values into the Beer-Lambert law, the unknown concentration of the ammonia-comminia complex can be determined (“Beer-Lambert Law”, n.d.).

Hypothesis

If the concentration of ammonia increases in the copper (II) ion solution, then the concentration of copper-ammonia complex ions will increase.

Variables

Materials

• Copper(II) sulfate (CuSO₄) anhydrous
• 2.64 M ± 0.05 𝑀 Ammonia (NH₃) solution
• Distilled water
• Vernier Go Direct UV-VIS Spectrophotometer resolution of ± 0 . 001
• 1 cm Cuvettes – Assumed to be a defined constant for Beer-Lamber Law and do not affect significant sigures in calculations.
• 100.0 mL Volumetric Flask ± 0.01 𝑚𝐿
• 300 mL Beaker ± 5 𝑚𝐿
• Test Tubes
• Test Tube Rack
• Test Tube Stopper
• 10 mL Volumetric Pipette ± 0.02 𝑚𝐿
• 1 mL Volumetric Pipette ± 0.01 𝑚𝐿
• Protective gear (gloves, goggles, lab coat)

Risk Assessment

Note: The formation of the copper-ammonia complex does not introduce new risks and hazards than those of its reactants. Standard precautions listed for ammonia and copper (II) sulfate should be followed.

Safety & Ethical Concerns

Ammonia discharged into the water can be detrimental to fish and other aquatic life, even in very small concentrations. When exposed to ammonia, aquatic species struggle to excrete the toxins from their bodies (Peacock, 2023). Hence, before disposing in the drain, neutralize the solutions with water.

Copper (II) sulfate is a heavy metal that can cause water contamination, brain damage, and liver failure in aquatic animals. Hence, disposal of copper (II) sulfate should be done in hazardous waste disposal (Cleveland Clinic, 2022).

Overall, the experiment adheres to principles of green chemistry by minimizing waste through microscale experimentation. For example, less concentrated ammonia solutions (less than 0.150 mol dm-3) are prepared to minimize waste while still allowing for observable complex formation. Also, 30 mL total solution is used to reduce overall waste. All waste is disposed of in an environmentally considerate manner.

Procedure

Preliminary Research: The preliminary lab was performed to ensure an appropriate range of ammonia volume/concentration was used in the overall solutions, guaranteeing that the final experiment yielded distinct and accurate results applicable for analysis. This was achieved by ensuring the complex ion formation did not reach a limit in the absorbance readings of the spectrophotometer. 

Additionally, this lab was conducted to determine the wavelength on the spectrophotometer that yielded the highest absorbance values. The procedure for the preliminary lab was similar to that of the final experiment, but the preliminary lab had several trials with experimentation of varying ammonia concentration intervals (ex. 0.1 mol dm-3, 0.05 mol dm-3, 0.025 mol dm-3) and collected absorbance on a variety of wavelengths (ex.650, 630, 600). The findings consisted of controlling ammonia concentration to range of 0.025 mol dm-3, and using a 600 nm wavelength on the spectrophotometer.

The procedure was designed with the aid of ChatGPT (Sept. 10, 2024 version) to ensure clarity and specificity of the experimental setup.

1. Prepared 100 mL 0.050 M copper(II) sulfate solution using the copper (II) sulfate powder in a beaker and transferred to a volumetric flask. 0.0500 M to prevent high absorbance values that exceed the spectrophotometer’s range.
2. Used a pipette to add 2 mL of 0.050 M copper(II) sulfate solution into each test tube on the test tube rack (one flask for each concentration of ammonia).
3. Prepared 10 varying ammonia concentrations in the flasks using a pipette and labeled them with the concentration. Reference the table below to create the ammonia concentrations. Note:
   • 1 mol-3 dm = 1M = 1 mol/L
   • 1mL = 1cm3, hence, instead of mL (although the markings in the pipette), is used as it is the SI unit for volume and aligns better with cm3 mol dm used in calculations.
   • 1 mL = 1𝑐𝑚 3, hence, instead of mL (although the markings on the pipette), is used as it is the SI unit for volume and aligns better with 𝑐𝑚 3 𝑚𝑜𝑙 𝑑𝑚 used in calculations.
4. Turned on the spectrophotometer to allow it to warm for 200 seconds.
5. Set the wavelength to 600 nm because the visually observed/transmitted color of copper (II) sulfate solution is blue which means the color absorbed is orange. Since orange falls in the range of 585 and 647 nm, this range was considered for the wavelength. Also, after experimenting with several wavelengths in the preliminary trial, it was proven that the ideal wavelength with maximum absorbance.
6. Calibrated the spectrophotometer using distilled water in a cuvette (blank solution).
7. Inserted the test tube stopper in the test tube and inverted the test tube ~5 times to mix the solution thoroughly.
8. Poured the first concentration of ammonia into a cuvette to conduct the trial.
9. Measured and recorded the absorbance value in 0.0001 resolution in a spreadsheet.
10. Repeated steps 7 to 9 again for the rest of the test tubes/concentrations.
11. Repeated steps 2 to 11 for 2 more trials to prevent random errors and have more consistent data.

II. Data Collecting and Processing

Raw Data

Data/ Calculations & Analysis

To determine the coefficient, the average absorbance from all three trials was used, and the absorbance versus concentration of ammonia was plotted on a graph in Figure 3. The slope is the value of the molar absorption coefficient ( ε).

A linear regression is added because it was initially hypothesized that, as ammonia concentration increases, absorbance would also increase, as greater copper-ammonia complexes would form. The low R 2 value of 0.518 does not fully support the initial hypothesis.

Ammonia concentrations were used in the calibration curve as the changes in ammonia directly influenced the extent and type of complex formation. This approach allows for determining the molar absorption coefficient by analyzing changes in ammonia concentration and its impact on absorption, hence, complex ion formation.

Slope = Molar Absorbance Coefficient = 1.34 L·mol⁻¹·cm⁻¹

Next, the Beer-Lambert Law (A= εlc) was used to determine the concentration of the colored complex for all the trials by inputting the experimental value of absorbance (A) , graphed slope (ε), and optical path length (1 cm).

Finally, analyzed the graph to determine the relationship between ammonia concentration and the intensity of the color, indicated by absorbance. Added an appropriate regression line and obtained the coefficient of determination or R2, to determine the fit of the data to the regression line/model.

Initially, it was hypothesized that as the ammonia concentration increases, the concentration of the copper ammonia complex would increase linearly, and only monoamines would be formed. However, when the data was graphed in Figure 7, a low R squared value of 0.521 and the inability of the line of best fit to pass the origin showed a weak linear correlation and three distinct relationships. The data was separated into three graphs demonstrating how changes in ammonia concentration created different complexes.

The first region in Figure 8 shows that as the ammonia concentration increases, the copper-ammonia concentration increases with a high relative gradient of 2.48 and an R-squared value of 0.999. The high increases in complex concentration indicate that the first complex formation has a 1:1 stoichiometric ratio of ammonia to copper, resulting in the formation of the [Cu(NH₃)]²⁺ complex. As shown in Fig. 9, this demonstrates a first ligand substitution where one water molecule is replaced by ammonia, as ammonia forms stronger bonds than water (LibreTexts, 2023).

[Cu(H2 O)6 ] 2++ NH3⇌ [Cu(NH3 )] 2++ H2 O

The second region in Figure 10 shows that as the ammonia concentration increases, the copper-ammonia concentration increases slowly with a lower relative gradient of 2.2 and an R-squared value of 0.966. This shows how the second complex formation has a 2:1 stoichiometric ratio of ammonia to copper, formation of [Cu(NH₃)₂]²⁺ complex where the second ammonia molecule coordinates with the complex (Fig. 11). The slower increase is explained by the decreasing availability of free copper ions and the shift of equilibrium toward the second complex formation (LibreTexts, 2023).

[Cu(H2 O)6 ] 2++ 2NH3⇌ [Cu(NH3 )2 ] 2++ 2H2 O

The third region in Figure 12 shows that as the ammonia concentration increases, the copper-ammonia concentration decreases with a negative gradient of -1.7 and an R-squared value of 0.597. The low R-squared value is a result of the first data point being from complex [Cu(NH3 )2 ] 2+, which was employed to show the change in stoichiometric ratio of ammonia to copper and illustrate a notable change. As shown in Fig. 13, this shift in equilibrium shows the formation of higher-order complexes such as [Cu(NH₃)₄]²⁺ (LibreTexts, 2023).

[Cu(H2 O)6 ] 2++ 4NH3⇌ [Cu(NH3 )4 ] 2++ 4H2 O

However, the reduction in copper-ammonia concentration may be due to reaching a saturation point because of the limited availability of copper ions, or the high ammonia concentration may cause the dissociation of previously formed complexes. Since the process is dynamic, the inability of [Cu((NH3)4]2+ to be stable results in an equilibrium shift, where the ligands separate from the metal ion and the complex dissociates into its original or simple components, thereby reducing the complex concentration (LibreTexts, 2024). For example, as [Cu(NH3 )4 ] 2+form, they may also dissociate:

[Cu((NH3 )4 ] 2+⇌ [Cu(NH3 )2 ] 2++ 2 NH3

As the complex formed changed from monoammine to tetraammine, each step in ligand substitution altered the d-orbital splitting, which in turn impacted the color of the solution and, consequently, the spectrophotometer’s absorption, as both the wavelength and absorption intensity were affected. Since ammonia is a stronger ligand than water, it increases the crystal field splitting energy (Δ), and the d-orbitals split further. This results in the complex absorbing higher-energy light (shorter wavelengths) as the electron transitions from a lower to higher energy level (“Crystal Field Theory”, 2023).

However, the drop in substitution and copper-ammonia complex ion with greater/excess ammonia concentration demonstrated that although the d-orbital split and crystal splitting energy was greater, there were fewer complexes being formed that resulted in a lower concentration/absorption (“Crystal Field Theory”, 2023).

As the crystal splitting energy varied across the different complex formations and higher energy light was absorbed as there was greater ligand substitution, the wavelength of light would have also decreased. This would mean that the different complexes have a maximum peak absorption at contrasting wavelengths (“Crystal Field Theory”, 2023). 

III. Conclusion, Evaluation, & Applicability in the Paint Industry

Conclusion

This investigation aimed to determine the relationship between ammonia concentration and the concentration of copper-ammonia complexes; however, the data suggested that as the ammonia concentration increased, more than one copper-ammonia complex was formed, as increasing amounts of ammonia caused variations in the d-sublevel split and ligand substitution. 

The initial hypothesis was refuted because the additions of ammonia concentrations did not lead solely to increases in [Cu(NH3)]2+ due to the limited copper sulfate present in the solution; the greater formation of [Cu(NH3 )] 2+ was not possible. 

Rather, the results of the investigation showed that greater ammonia concentration also leads to the formation of [Cu(NH3 )2 ] 2+ and [Cu(NH3 )4 ] 2+ complexes (as shown in Fig. 8, 10, 12). 

Also, greater ammonia concentration did not necessarily lead to higher copper-ammonia complex concentrations as complex concentrations decreased after 0.100 mol dm-3 ammonia due to ligand substitution limitations and equilibrium shifts (Fig. 7).

To assess the investigation, a comparison was made with a study from the Journal of Physics: Conference Series. The study focused on optimizing the complexation of ammonia with copper (II) ions. The ammonia concentration in this investigation and the study by Guspita and Ulianas overlapped slightly, as the study’s concentrations ranged from 0.003 M to 0.08 M. 

Also, the study used a lower Cu²⁺ concentration (0.01 M). Despite the differences, the overall results of Guspita and Ulianas’ research support and explain that carrying ammonia concentrations leads to the formation of distinct complexes. Additionally, in the study, increasing ammonia concentrations initially led to increases in complex formation and absorbance, but decreased at higher ammonia concentrations of 0.075 mol dm-3 due to equilibrium and ligand substitution limitations in the complex ion (Guspita and Uliannas, 2020).

Evaluation

Strengths of the Investigation

Firstly, uncertainty propagation was performed throughout data collection and calculations to assess the validity of the data; it was found that the uncertainties in the results were relatively low at 1.52% on average, showing the data’s precision. Also, when the overall data in Fig. 7 was split into regions and represented in Fig. 8,9, and 10, the trends were more linear with higher coefficient of determination values (except Fig. 10 as the trend was decreasing).

Additionally, when the investigation is compared to another similar reputable study, there is similarity in the comparable findings, showing that the data collected is accurate. Finally, the trends deduced from the results align with the foundational chemical principles of ligand substitution and colored-complex formation.

Limitation & Improvements

Applicability in the Paint Industry

Copper complexes have historically played a significant role in the development of pigments and coatings. For instance, copper ammonia complexes can affect the color, stability, and chemical characteristics of specific paints and coatings. They are associated with copper-based pigments. Manufacturers can learn more about the following by comprehending how different ammonia concentrations impact the formation of the copper-ammonia complex:

• Color intensity and homogeneity: The saturation and hue of pigments are influenced by the concentration of copper complexes, which is essential for constant paint quality.
• Chemical stability: Ammonia can act as a stabilizing agent for copper ions in solutions, preventing premature precipitation or degradation in paints and coatings.
• Application optimization: By modifying formulations for solvent- or water-based paints, an understanding of complex formation can guarantee the best possible performance and durability.

This study presents a model for predicting how varying additive concentrations may impact pigment behavior in actual paint formulations by measuring the copper-ammonia complex using spectrophotometry and the Beer-Lambert Law. This method connects basic chemical concepts with real-world applications in the paint industry.

Future Investigation

To expand the study, an investigation into how varying temperatures affect complex formation when both ammonia and copper sulfate concentrations are held constant. This study can analyze the impact of temperature on d-orbital splitting, absorption, and kinetics by using concepts of thermodynamics, enthalpy, and entropy.

Mariyah Matawala is a young professional and scholar combining business, science, and sustainability. She bridges the industrial world of coatings and paint with academic study in global business, chemistry, and writing composition. She is the director of the Paint Foundation and Regent Paints Inc. Her work lies at an interplay of environmental responsibility by contributing to initiatives that promote a circular economy with zero-waste, technical/material industries, and corporate business structures that concern manufacturing and supply-chain innovations, with an overall broader connection to social and philosophical questions.

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