Determination of the composition of a complex using

continuous variation method

Student’s ID number: 20212197

Class ID number: 151539

Instructor: Ph.D Nguyen Van Nghia

Experiment date: 10/03/2025

I. Introduction
The composition of the copper (II) –nitroso complex was determined by
identifying the maximum absorbance point on the graph using the
continuous variation method (also known as the Job plot method). This
method is employed to determine the stoichiometric coefficient in complex
formation reactions and is only applicable when a single complex species is
[2]
formed, with no hydrolysis, polymerization, or dissociation taking place . In
this method, the total concentration of the reactants is kept constant, whereas
their molar ratios are systematically varied to determine the composition of the
complex [1]. The solution exhibiting the maximum complex formation indicates
the stoichiometric ratio of metal ion Me n+ to ligand R present in the resulting
[1]
complex . In this experiment, copper metal ions are complexed with the
reagent, nitroso-R salt [2].

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 Figure 1. The chemical structure of the nitroso- R salt [1]
Depending on environmental conditions and concentration levels, the
metal ion Mⁿ⁺ interacts with the reagent HR to form various complexes . In
an acidic medium, the complex predominantly exists in the form of MeR; in a
weakly acidic or neutral medium, it mainly takes the form of MeR₂; in an basic
[2]
medium, it exists primarily as MeR₃ . Therefore, special attention must be
paid to the reaction medium in which the composition of the complex is
determined. In this case, the reaction between Cu²⁺ and HR occurs as follows
[1]
.
In acid medium: Cu2+ + HR ↔ [CuR]+ + H+ (1)
In weakly acidic or neutral conditions: Cu2+ + 2HR ↔ [CuR2] + 2H+ (2)
In basic medium: Cu2+ + 3HR ↔ [CuR3]- + 3H+ (3)
This experiment aims to determine the composition of the copper (II)–
[2]
nitroso-R complex . The absorbance measurements of the solutions were
utilized to construct a graph illustrating the relationship between absorbance
(A) and the volume of CuSO₄ solution. From this graph, the value of n in the
CuRn complex was determined by applying the following formula:
VR
n=
V CuSO ₄
II. Methodology

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The chemicals utilized in the experiment consist of a 1.00 x 10 ⁻³ M CuSO₄
solution, a 1.00 x 10⁻³ M nitroso-R salt, a 0.05 M Na₂SO₄ solution, and
H₂SO₄ with a pH of 4. All of these chemicals were readily available in the
laboratory.

The absorption spectrum was measured using a spectrophotometer. The

A series of solutions were prepared in nine 25.0 mL volumetric flasks,

Table 1. Composition of solutions used in the Job plot method.

1 2 3 4 5 6 7 8 9
Nitroso-R 10-3M
9.00 8.00 7.00 6.00 5.00 4.00 3.00 2.00 1.00
(ml)
CuSO4 10-3 M
1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00
(ml)
Na2SO4 0.05M
5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0
(ml)
H2SO4 pH=4 The solution was standardized to the mark using a burette and
mixed thoroughly.

Distilled water was used as the reference solution. The absorbance of each
solution was recorded at a wavelength of 490 nm on a spectrophotometer,
employing a cuvette with a path length of 1 cm.

The obtained measurements were utilized to construct the absorbance (A) with
the volume of CuSO₄ solution graph. From this, the maximum point was
identified, and the composition of the formed complex can be determined.
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III. Results
The absorbance measurements for each flask are shown in Figure 2 is the
A/The volume of CuSO₄ solution plot derived from the above results.

Figure 2. Job’s plot showing the formation of the copper (II)- nitroso-R salt
complex based on continuous variation.
The linear y= 0.176x+ 0.0857 was drawn by connecting three points. The
linear y= -0.1682x+ 1.7135 was drawn by connecting four points. These lines
intersect at x= 4.70mL, indicating the maximum absorbance. The
corresponding volume of CuSO₄ at this point is 4.70 mL.. At this point, the
CuRn complex is determined:
VR 10−4.7
n= = =1.12 1
V CuSO ₄ 4.7
IV. Discussion
The composition of the CuRn complex depends on the environment. In this
case, the complex is formed in an H₂SO₄ acidic environment with a pH of 4.
Therefore, the resulting complex is in the form of CuR, which is entirely

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 consistent with the results obtained from the experiment. Similarly, in other
environments, the shape of the graph will resemble that shown in Figure 1.
The value of n is determined from the resulting graph. Based on the results
and the graph, it can be concluded that as the concentration of the reagent
increases, the amount of the complex also increases. In this series of solutions,
although the relative concentration of each component varies, the total amount
remains constant. There is one solution where the concentration of the
complexes reaches its maximum. This is the solution where the ratio between
Cu²⁺ and R is equal. Therefore, the experimental results support the validity of
the hypothesis.
The calculated value of n is not exactly 1 due to errors in the experimental
process. These errors may include improper sample preparation, the use of an
inaccurate reference sample, or insufficient washing of the apparatus, which
can lead to contamination of the solutions. In addition, inaccuracies during
titration caused by equipment malfunctions may also occur. All of these factors
contribute to a decrease in the sensitivity of the method, resulting in less
accurate measurements.
V. References

Bo Mon Hoa Phan Tich, Tai lieu huong dan Thi Nghiem Phan Tich Bang Cong
[1]
cu, Ha Noi: HUST, 2020.

[2] T. T. Thuy, Phan tich cong cu, Ha Noi: HUST, 2016.

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 APPENDICES
PARAMETERS IN FIGURE 2

Volumetric flasks A
1 0.261
2 0.439
3 0.613
4 0.749
5 0.792
6 0.696
7 0.548
8 0.369
9 0.195

LIST OF ABBREVIATIONS

VR Volume of Nitroso-R salt