INTRODUCTION:
Volumetric estimation of tin by iodimetric titration has some disadvantages because throughout the entire process CO2atmosphere to be maintained strictly to avoid aerial oxidation of Sn2+ to Sn4+. Moreover std. iodine solution is volatile to some extent. Standard iodine solution as well as distilled water used in the analysis should be entirely free from dissolved oxygen. Gravimetric estimation of tin is time consuming; moreover co-precipitation of other metal ions may take place which requires the removal of other interfering metal ions to achieve actual tin content. During gravimetric estimation of tin, the sample is treated with HNO3, when a precipitate of hydrous stannic oxide is formed, the formula for this may be written as H2SnO3, Metastannic acid:
3Sn+H2O+4HNO3= 3H2SnO3+ 4NO
But the precipitate exhibits powerful adsorption properties for certain ions, for example, iron, lead, copper, nickel, zinc and antimony. The amount of impurity may be determined by adding to the impure weighed oxide fifteen times its weight of pure ammonium iodide and heating for 15 minutes at 425-475OC or until no further fumes are evolved. The tin is volatilized as stannic iodide:
SnO2+4NH4I= SnI4 +4NH3 +2H2O
Antimony, if present, will be volatilized as antimonious iodide. The residue is treated with 2-3 ml of conc. HNO3, evaporated, ignited and the residual metallic oxides weighed: the loss in weight results the weight of pure stannic oxide in the precipitate. On the other hand, complexometric EDTA titration for the determination of tin is rapid, accurate and selective as well as inexpensive.
PRINCIPLE OF EDTA METHOD:
A mixture of Sn4+ and Pb2+ ions are complexed by adding an excess of std. EDTA solution, the excess being determined by titration with std Pb(NO3)2 solution; the total lead plus tin is thus determined. Sodium fluoride is added and this displaces EDTA from Sn(IV)-EDTA complex; the liberated EDTA is determined by titration with a std. soln. of Pb(NO3)2. Both Sn4+ and Pb2+ will react with EDTA as follows:
Sn4+ + H2Y2- = SnY + 2 H+
Pb2+ + H2 Y2- = PbY2- + 2H+
In this experiment, Sn4+ and Pb2+ ions are complexed by excess EDTA and the excess is then back titrated with std. Pb(NO3)2 solution using xylenol orange indicator. While conducting the titration, 30% hexamine soln. is added to maintain the pH around 6. When a fixed amount of NaF is added to the solution, the fluoride ions form a stable complex with Sn4+ but do not react with Pb2+ at pH 6. Thus, in presence of fluoride ion, EDTA from Sn-EDTA complex is liberated which can be titrated again with std. Pb(NO3)2 soln.
EXPERIMENTAL:
The sample is taken in 250 ml beaker. 2-3 ml of conc. HNO3 & 10 ml conc. HCl added and the soln. is kept at room temp. for 15 minutes with occasional stirring. The beaker with the sample then placed on the hot plate or an water bath at low heat (50-60degree centigrade) for complete dissolution. After complete dissolution, the soln. is boiled for a few minutes to expel nitric acid. The solution is pale yellow in colour. To this solution, 25 ml of 0.2M EDTA solution is added; the solution is boiled for a few minutes. The solution is cooled and the volume made to 250 ml in a volumetric flask. This is the stock solution. 25 ml of sample soln. is then pipette out to a 250 ml conical flask. 15 ml of hexamine solution, 100 ml distilled water and a few drops of xylenol orange indicator is added. The solution is then titrated with 0.01 M standard Pb(NO3)2 solution until the color changes from yellow to red. This end point corresponds to total lead and tin content. Then 2 gm NaF is added to the same flask. The solution returns to yellow color and then titrated again with std. Pb(NO3)2 soln. till the color changes from yellow to red. The final burette reading is noted. Always a blank test is carried out using all the prescribed reagents as stated above but without adding the sample.
RESULTS:
BCS CRM sample No. 178/2 of tin base white metal has been analyzed for which the results are as follows:
|
No. of tests carried out |
Results Obtained in % |
Declared Sn Content in % |
Recovery % of Sn |
Standard Deviation |
Relative Standard Deviation in % |
|
1. |
82.32 |
82.20 |
100.14 |
0.13 |
0.16 |
|
2. |
82.08 |
99.85 |
|||
|
3. |
82.15 |
99.94 |
|||
|
4. |
82.25 |
100.06 |
|||
|
5. |
81.99 |
99.74 |
CONCLUSION:
EDTA Complexometric method for the estimation of tin in tin base white metal has been studied. The method is less time consuming, less hazardous and quite accurate. Moreover reagents used in this method are not expensive and are easily available. So this method can be used to replace classical iodimetric method and gravimetric method for estimation of tin.