Synthesis and Chemistry of K2S2O8

Topics: Redox, Chemistry, Electrochemistry Pages: 9 (2136 words) Published: April 1, 2014


In this experiment, a sample of K2S2O8 was prepared by the electrolysis of an aqueous solution of H2SO4 and K2SO4. The peroxodisulfate anion, S2O82-, was also observed for its ability to serve as a counterion for precipitation by preparing a copper (II) complex by reacting hydrated copper (II) sulfate with ammonium peroxodisulfate in the presence of pyridine. This same ability, coupled with its strong oxidizing ability allowed for stabilization of the unusual oxidation state of 2+ for silver which was observed by preparing an analogous silver (II) complex by reacting silver (I) nitrate with ammonium peroxodisulfate in the presence of pyridine. IR spectra for the three products were obtained, as well as qualitative tests for Product A (K2S2O8) in comparison with H2O2, confirm the presence of the peroxodisulfate anion and the identity of the individual yields.


Electrolysis is a widely used technique in the large-scale preparation of several industrially important inorganic chemicals, one of which is potassium peroxodisufate. The S2O82- ion is one of the strongest known oxidizing agents, even stronger than H2O2, and will oxidize many elements to their highest oxidation states.

The amount of product generated depended on the total number of electrons passed through the solution. Because of the product of the current I (in amps) and the time of electrolysis t (in seconds) gives coulombs (C) of electricity and 96,500 C oxidized (or reduced) one equivalent of reactant, the theoretical yield of product will be:

Theoretical yield = (coulombs passed)/(96,485 coulombs/mol) (molar mass/e- transferred per ion) = (I t/ 96,485) (molar mass/e- transferred per ion)

This equation is a summary of Faraday’s 1st Law of Electrolysis wherein the mass of a substance altered at an electrode during electrolysis is directly proportional to the quantity of electricity transferred at that electrode. Quantity of electricity refers to the quantity of electrical charge, typically measured in coulombs.

The actual yields are frequently less than this number of grams because of side reactions, and so percentage yields are usually evaluated. In electrochemistry, percentage yield is called current efficiency.

Percentage yield = current efficiency = (actual yield/theoretical yield) x 100

The peroxodisulfate anion, S2O82-, can serve as a counterion to form stable and highly insoluble salt, observed in the reaction of hydrated copper (II) sulfate with ammonium peroxodisulfate in the presence of pyridine to form a copper (II) complex:

CuSO4H2O + (NH4)2S2O8 + 4py  [Cu(py)4]S2O8 + (NH4)2SO4 + 5H2O

The strong oxidizing powers of S2O82- have also permitted the syntheses of coordination complexes of silver in the unusual oxidation state of the complex [Ag(py)4]S2O8. Silver nitrate was made to react with ammonium peroxodisulfate in the presence of pyridine:

2AgNO3+ 3(NH4)2S2O8 + 8py  2[Ag(py)4]S2O8 + 2NH4NO3 + 2(NH4)2SO4

The cation Ag(py)42+ has a square planar geometry, analogous to that of Cu(py)42+. In this, reaction, the S2O82- serves both as an oxidant and as a counterion to precipitate Ag(py)42+ ion.

Infrared (IR) spectroscopy is one of the most common spectroscopic techniques used by organic and inorganic chemists wherein it is simply the absorption measurement of different IR frequencies by a sample positioned in the path of an IR beam. A criterion for IR absorption is a net change in dipole moment of a molecule as it vibrates or rotates. As the molecule vibrates, there is a fluctuation in its dipole moment, causes a filed that interacts with the electric field associated with radiation. This also occurs when asymmetric molecules around their centers results in a dipole moment change, which permits interaction with the radiation field. The main goal of IR spectroscopic analysis is to determine the chemical functional groups on the sample as they...

References: 1. Girolmi, G.S.; Rauchfuss, T.B.; Angelici, R.J. Synthesis and Technique in Inorganic Chemistry: A Laboratory Manual. University Science Books, 1999.
2. Miller, F.; Wilkins, C. Infrared Spectra and Characteristic Frequencies of Inorganic Ions. Ph.D. Dissertation, Mellon Institute, Pittsburgh, PA, 1952.
3. Szafran, Z.; Pike, R. M.; Singh, M. M. Microscale Inorganic Chemistry - A Comprehensive Laboratory Experience. Wiley, 1991.
4. Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, Parts A and B. Wiley, 1997.
5. University of Calgary, Department of Chemistry, Chemistry 331, Inorganic Chemistry: Main Group Elements, Online Lab Manual, Fall 2013, Project #2 pp 33-36.
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