Experiment: Analysis of Solder

To determine the percentage of tin and lead present in the given amount of solder we have conducted the following test in our Lab.

Principle:
Solder is an alloy of tin and lead but may sometimes contain 1-3% of Sb also. The method used to determine the percentage of tin and lead in the alloy is EDTA back titration. Many metals cannot be titrated directly they may precipitate from the solution in the ph range necessary for the titration, or they may form inert complexes or a suitable indicator is not available. Then an excess standard EDTA is added ,the resulting solution is buffered to the desired ph and the excess EDTA is back titrated with a standard solution of metal ion solution. The end point is detected with metal indicator.

In this experiment tin and lead is complexed by excess of standard EDTA and excess EDTA is found out by titration with standard lead nitrate solution. The indicator used is xylenol orange. Control of ph makes use of different stabilities of metal EDTA complex. Total lead plus tin content is thus determined. Sodium fluoride is added and this displaces the EDTA from the tin-EDTA complex , the liberated EDTA is determined by titration with std. Lead nitrate solution. The equations involved could be written as follows,

EDTA(excess) + Pb2+       PbEDTA

EDTA + Pb2+ +Sn2+        PbEDTA + SnEDTA+ EDTA(excess)

NaF + SnEDTA          SnF2 + EDTA(released)

Preparation of standard solution of lead nitrate:
About 1.656 g of laed nitrate crystals is weighed accurately and dissolved in 5 ml dil HNO3 in 100 ml standard flask and made it up-to the mark.

Standardization of EDTA:
10 ml of EDTA solution is taken in a 100 ml std flask and made it upto the mark. 10 ml of this solution is taken in a conical flask . add 1-2 spatula of hexamine buffer and 2 drops of xylenol orange indicator. Titrate against std. Lead nitrate solution taken in burette. Repeat the titration for concordant readings.

The  following 8 steps  describe the procedure  for analysing a  sample of an alloy that contains tin and lead.

• A 0.4062 g sample of alloy was dissolved by  heating it in a small beaker with a mixture of conc. hydrochloric and conc. nitric acid.  The beaker was heated until all the alloy dissolved.  In this procedure, lead is oxidised to Pb(II)  and tin becomes Sn(IV)
• After 5 minutes of heating to expel oxides of nitrogen and chlorine, some acid remained.  The solution was cooled then a precipitate of some tin compounds and a lead compound appeared.
• A 25.00 mL aliquot of EDTA solution was added.  The precipitate dissolved and a clear, colourless solution was obtained.
• This solution was quantitatively transferred to a 250.0 mL volumetric flask and made up to the mark with distilled water.
• A 25.00 mL aliquot of this solution was taken in a conical flask and added hexamine (hexamethylenetetramine), and two drops of Xylenol Orange solution.The pH of the solution maintained at 6.
• The clear, yellow solution from Step 5 was titrated with standard lead nitrate solution until the colour  just changed from yellow to  red.
• 2.0g of solid NaF was added to the titration flask. The solution immediately turned back to yellow.
• The solution was titrated with more standard lead nitrate solution until the colour changed to red again.

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Simultaneous Estimation of Mn and Cr In a solution of KMnO4 and K2Cr2O7

AIM

Estimation of Mn and Cr in a solution of KMnO4 and K2Cr2O7.

APPARATUS REQUIRED

Clorimetry,100cc standard flasks, beakers, pipette and cuvette.

PRINCIPLE

Optical density of the mixture containing m compound is measured at  M suitable choosen  λ and n simultaneously.Equations are set suppose Ca and Cb are the concentrations of two compounds of a binary mixture and Dab1 and Dab2 are its optical density at 1 and 2.

If Da1 and Da2 are the optical densities of pure A of concentration Ca1 and Cb1 those of pure B of concentration Ca1 and Cb1 those of pure B of concentration Cb1 then assuming  Beers law to be valid ,we have optical densities of mixture to be

Dab(1) =  Da(1) Ca/C’a  +  Db(1)Cb/C’b………………………….(1)

Dab(2) = Da(2) Ca/C’a   + Db(2)Cb/C’b………………………….(2)

Thus the measurement of O.D (optical density) of the mixture and the solution of pure component of KMnO4 concentration at suitable choosen λ enable to calculate Ca and Cb.

PROCEDURE

Prepare 100ml each of 0.001N K2Cr2O7 and 0.001N KMnO4 in 0.5ml H2SO4.Find the optical density of the solution .Da and Db for  the available λ .Then find the ratio of Da/Db.

Select the optimum λ corresponding to minimum and maximum of Da/Db. Measure the O.D(optical density) of given concentration of KMnO4 and K2Cr2O7 using equation 1 and 2.

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