1. Determination of Cu in Brass
By: Prakruti Modi

2. STANDARDIZATION OF 0.1 N Na2S2O3
Purpose of Standardization
Sodium thiosulfate is a secondary standard
It is difficult to keep the sample pure due its nature of decomposition

3. Procedure
Boil 1 L of distilled water for several minutes, and then let it cool
To get rid of CO2
You want to get rid of CO2 because it can also react the sodium thiosulfate or titrated with sulfite
Add about 25g of Sodium thiosulfate and 0.1g of Na2CO3
You do not want to let it oxidize by air
Stir until all the crystals are dissolved and transfer to a clean bottle
To make a homogenous solution
Store in dark
WHY?
To avoid dissociation from light. Thiosufate is photo sensitive chemical.

4. Cont…
Transfer 0.6 to 0.7 g of primary standard KIO3 to a weighting bottle, and dry for 1 or 2 hrs at 160 degree Celsius and then transfer the KIO3 to a 250-mL volumetric flask
It is important to dry the KIO3 to ensure there is no water in the sample
Dissolve in 100 ml of water, and then dilute to the mark
To make sure that all the sample dissolved properly and also if the volumetric flask is filled all the way there is no room for the solution to move
With a volumetric pipet, transfer 50-ml of aliquot to the KIO3 solution to 250-mL Erlenmeyer flasks
It is better to use volumetric pipet instead of using a graduated cylinder to transfer the desired amount because pipet has better accuracy in comparison with graduated cylinder

5. CARRY ON EACH REPLICATE TRIAL ONE AT A TIME FROM THIS POINT FORWARD
Add 2g of reagent grade KI (iodate free) to each sample, and swirl gently until dissolved.
CARRY ON EACH REPLICATE TRIAL ONE AT A TIME FROM THIS POINT FORWARD
Because the reaction taking place is very reactive and to avoid loosing iodide

6. Add 10 ml of 1 F HCl, and titrate with the Sodium thiosulfate solution until the color becomes light yellow
with added HCl that has just been served with an excess of potassium iodide. A disproportionation has occurred in the oxidation states of the iodine in iodate and iodide to produce elemental iodine complexed with the excess iodide to produce the triodide ion. A deep red-brown solution results (almost black), as on the right

7. Add 5 ml of starch indicator solution, continue titration
Add 5 ml of starch indicator solution, continue titration. The endpoint signal is the disappearance of the blue starch-iodine complex color
We added the starch solution after titrating up to a certain point because starch is a organic molecule containing long chains which can trap the iodide and not let it react with sodium thiosulfate

8. Determination of Cu in Brass
Weight out 0.3 samples, and transfer to 250ml Erlenmeyer flasks
Weighing has to be done on the analytical balance in order to get the correct results while calculating for molarity
Add 5ml of 6 F HNO3, and warm in a fume hood until the sample is completely decomposed
Add 10ml of concentrated H2SO4, and evaporate in a fume hood until copious fumes of SO3 are observed
It is important to carry out the above two steps in the fume hood because the reaction produces NO2 and SO3 gas which can harm if inhaled
Cool, and then carefully add 20 ml of H2O. Boil 1or 2 min, and cool
To dilute

9. CARRY ON EACH TRIAL ONE AT A TIME FROM THIS POINT FORWARD
Add concentrated ammonia DROPWISE while swirling until the blue copper ammonia complex forms
It is important to perform the above step carefully and dropwise because addition in large amounts can create bumping and also it produces lot of heat as it is a exothermic reaction between concentrated acid and concentrated base
Add 3 F H2SO4 dropwise until the blue color just disappears and then 2 ml of 85% H3PO4, and cool to room temperature.
Concentrated phosphoric acid is added to bring the pH to a point at which the titration can be ideally performed
To free Cu from complex
CARRY ON EACH TRIAL ONE AT A TIME FROM THIS POINT FORWARD

10. Add solution containing 4g of KI in 10 mL of H2O to the sample, and titrate immediately with 0.1 N Sodium thiosulfate until the color becomes light yellow
It needs to be done immediately because Iodide is photo or light sensitive therefore it can escape easily
Add 5mL starch indicator solution, and continue titrating until the blue color is nearly gone
To generate the dark iodine/beta-amylose complex assumes homogeneity throughout the solution, as on the right

11. Add 2 g of KSCN and complete the titration
The reduction of Cu2+ to Cu+ occurs as the result of the oxidation of I- to I2. The I2 combines with iodide ion to produce the dark brown triodide ion, I3-. The excess iodide ion also causes the reduced copper to precipitate as white cuprous iodide, CuI. I2 and I3- in solution tend to adsorb on the surface of CuI thus becoming unavailable for rapid reduction by the thiosulfate. As a result, iodometric titrations involving reduced copper tend to yield lower results unless the adsorbed I2 can be liberated by adding thiocyanate ion, SCN-, which competes with the adsorbed iodine molecules on the surface of solid particles of CuI
The endpoint signal is the disappearance of the blue starch-iodine complex color

12. Questions What is the purpose of Standardization?
Why do we store the Sodium Thiosulfate solution in dark?

13. References http://www.csudh.edu/oliver/demos/hh-cubr/hh-cubr.htm
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