SINTESIS

1. Synthesis and characterization of Potassium tris Oxalato Chromate (III)
Trihydrate (K3[Cr(C2O4)3].3 H2O )
Objective
To synthesis and characterize the potassium tris oxalato chromate (III) trihydrate
Theory
The experiment is based on the following equation:
K2Cr2O7 + 7H2C2O4 + K2C2O4 H2O K3[Cr(C2O4)3] + 6CO2 +H2O
Chemicals
1. Potassium dichromate 2. Sodium hydroxide 3. Sulphuric acid
4. Potassium oxalate monohydrate 5. Potassium permanganate 6.Silver nitrate
7. Oxalic acid dihydrate
8. Potassium iodide 9. ammonium Persulfate
1

2. A. Method (Preparation)
1. Make a solution of potassium oxalate monohydrate (2.3 g) and oxalic acid
dihydrate (5.5 g) in water (110-120 ml).
2. To this solution add solid potassium dichromate (1.9 g) in small portions
with constant stirring for about 45 min or until it becomes a dark olive
solution with no undissolved solids. Continue stirring and add an equal
volume (~20 mL) of ethanol. When the complex has oiled out, decant off the
mostly colorless solution and discard. Triturate the oily solid with ether to
isolate a powdery dark blue-green solid.
2

3. 3. Collect the complex by suction filtration and wash with three
10-mL portions of ether.
4. Allow the solid to dry under vacuum (deep green shiny crystals
of potassium tris-oxalate chromium (III) trihydrate will be formed).
5.Determine the percentage yield, and transfer to a labeled vial.
3

4. Tasks
1. Calculate percent yield
2. % H2O in the green crystals
3. Estimate chromium
4. Estimate oxalate
5. Record the uv-visible spectra and interpret
6. Record IR spectra and interpret
7. Record thermogram data and interpet
4

5. B. Analysis
i. % H2O in the green crystals
1. Weigh 0.7-1.0 g (±0.0001g) of the complex salt into a pre-weighed
evaporating dish.
2. Label or mark the dish so you can identify it and place the
evaporating dish and complex salt in an oven set at 110°C for
approximately 45 minutes.
3. Cool the dish and contents and weigh to the nearest ±0.1 mg.
4. Calculate the % H2O in the crystals.
5

6. ii. Estimation of chromium
1. Dissolve an accurately weighed sample (in the range 0.10 – 0.15 g)
of the complex in distilled water (100 ml).
2. To this add sulphuric acid (5 N, 5 ml), ammonium persulfate (~1 g)
and silver nitrate solution (0.1 M, 0.1 ml).
3. Heat the solution under boiling condition for half an hour and the
color of the solution is orange yellow at this stage.
4. Cool the solution to room temperature and estimate the chromium
iodometrically.
(A duplicate run should be performed simultaneously)
6

7. iii. Estimation of oxalate
MnO4
– + C2O4
2- + H+ → Mn2+ + CO2 + H2O
1. Dissolve an accurately weighed sample (in the range 0.10 – 0.15 g)
of the complex in 50-70 ml of distilled water.
2. Warm the solution after adding sulphuric acid (5 N, 3 ml) and then
carefully add dilute sodium hydroxide solution (in the initial stage use
a concentrated solution and at the near neutral point use a dilute
solution) until no more precipitation of the hydrated chromium oxide
is observed (excess use of alkali should be avoided).
7

8. 3. Filter, and wash thoroughly with enough water.
4. Combine the filtrate and washing, acidify with sulphuric acid (5 N, 15
ml),
5. Warm to 70°C and then titrate with standard potassium
permanganate solution until the first appearance of a faint permanent
pink color occurs.
6. Calculate the percent of oxalate in the complex salt.
(Two sets of experiments should be conducted simultaneously).
8

9. C. Characterization
Spectroscopy of the Cr(III) Complex
i. UV-Visible (MeOH) spectra of potassium tris oxalato chromate
(III) trihydrate
- Prepare aqueous solution of the complex
- Obtain the absorbance spectrum for each complex
- Determine the longest wavelength maximum in units of
nanometers.
ii. FT-IR spectra (KBr, cm-1) of potassium tris oxalato chromate (III)
trihydrate
iii. Thermogram (10°C / min) of potassium tris oxalato chromate
(III) trihydrate
9