Tag: Lab

Estimation of Copper as Copper (1) thiocyanate Gravimetry

Estimation of Copper as Copper(1)Thiocyanate Gravimetry

Estimation of Copper as Copper (1) Thiocyanate Gravimetry

Determination of the amount of copper in a solution of Copper(II)sulphate.

This can be achieved by precipitating copper as copper(I)thiocyanate and massing the precipitated copper(I)thiocyanate, hence this is a gravimetric estimation.

2CuSO4 + 2NH4CNS + 2H2O + SO2 => 2CuCNS + (NH4) 2SO4 + H2SO4

Transfer the given copper(II)sulphate solution to a 100 mL volumetric flask, 20 mL of this solution is pipetted into a 250 mL Erlenmeyer flask. Add about 5 mL of 1 M H2SO4, followed by about 25 to 30 mL of H2SO3 which has been freshly prepared, by passing SO2 though distilled water. This solution is diluted to about 150 mL and heated to boiling. Add about 10 mL of 10 % NH4CNS in small quantities to the warm solution with constant stirring. The NH4CNS should be in excess. As the limiting reagent is the Copper(II) ions.

When the copper(I)thiocyanate is precipitated the solution should be colourless, if not use more ammonium thiocyanate, at this time the solution still should have the odour of sulfur dioxide.

After 24 hours the solution is filtered through a previously washed cleaned, dried and weighed sintered glass crucible (IG-4). The precipitate is washed 10 to 15 times. Add 1 mL of 10 % ammoniumsulphocyanide (NH4CNS) and 5 drops of saturated sulphurous acid to 100 mL of distilled water, cool it and use this liquid for washing the precipitate. Once this is done the final washing is done several times using 20% ethanol to remove the ammonium thiocyanate. The precipitate is dried at 110 to 120 oC for an hour, the precipitate is cooled and massed as CuCNS. The process of heating cooling and weighing until the mass remains constant.

Calculation

Mass of CuCNS = x grams

Mass 1 mole of CuCNS = 121.54g/mol

1 Mole of CuCNS contains 63.54 grams of copper

‘y’ = Mass of copper = 63.54/121.54 * (x mass of CuCNS)

Since we had taken 20 mL of the copper ion solution the total mass of copper(II) present in the solution provided is y*5

Things to note

H2SO3 is used to reduce Cu2+ to Cu+

Keep the pH low to reduce the solubility of the precipitate

Excess ammonium thiocyanate would help in complexing copper ions so should be avoided

Estimation of Magnesium ions in water using EDTA

Estimation of Magnesium ions in water using EDTA

The principle on which the titration lab is based:

The reaction between Mg2+ ions and EDTA can be represented like this.

Mg2+ + H2Y2- → MgY2- + 2H+

Prepare a standard solution of magnesium sulfate and titrate it against the given EDTA solution using Eriochrome Black T as the indicator. A buffer solution is prepared for maintaining the pH of about 10. When the reaction is complete all the magnesium ions would have been complexed with EDTA and the free indicator would impart a blue color to the solution. Estimation of magnesium ions using edta.

Procedure:

Preparation of 0.025M MgSO4.7H2O:  Dissolve 0.616 grams of analytic grade magnesium sulfate into a 100 mL volumetric flask. Dissolve the salt completely using distilled or de-ionized water.

Standardization of EDTA: 20 mL of the standard magnesium sulfate solution is pipetted out into a 250 mL Erlenmeyer flask and diluted to 100 mL . Add 2 mL of a buffer solution of pH 10. Add 4 drops of Eriochrome Black T to the solution. The solution is warmed to 40 degrees C and titrated against EDTA taken in the burette. At the end point the color changes from wine red to blue. Repeat titrations for concordant values.

Estimation of magnesium ions in the given sample: 20 mL of the given sample of solution containing magnesium ions is pipetted into a 250 Erlenmeyer flask, the solution is diluted to 100 mL, warmed to 40 degrees C, 2 mL of a buffer solution of pH 10 is added followed by 4 drops of Eriochrome black T solution. The solution is titrated against the standardized EDTA solution. Repeat the titrations to obtain concordant values. The end point is the color change from red to blue.

Using the volumes of solutions used, their determined molarity, you will be able to calculate the amount of magnesium in the given sample of water.

This is how you can perform an estimation of magnesium using edta.

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