The Determination of Copper in Brass - Lu Le Laboratory

Discussion

Thiosulfate can also be standardized against pure copper wire or foil. This procedure is advantageous when the solution is to be used for the determination of copper because any systematic error in the method tends to be canceled.

    Copper(II) is reduced quantitatively to copper(I) by iodate ion:

2Cu²⁺ + 4I^- → 2CuI_(s) + I₂

The importance of CuI formation in the forcing this reaction to completion can be seen from the following standard electrode potentials:

Cu²⁺ + e^- ↹ Cu⁺         E⁰ = 0.15 V

I₂ + 2e^- ↹ 2I^-            E⁰ = 0.54 V

Cu²⁺ + I^- + e^- ↹ CuI_(s)     E⁰ = 0.86 V

The first two potentials suggest that iodide should have no tendency to reduce copper(II); the formation of CuI, however, favors the reduction. The solution must contain at least 4% excess iodide to force the reaction to completion. Moreover, the pH must be less than 4 to prevent the formation of basic copper species that react slowly and incompletely with iodide ion. The acidity of the solution cannot be greater than about 0.3M, however, because of the tendency of iodide ion to undergo air oxidation, a process catalyzes by copper salts. Nitrogen oxides also catalyze the air oxidation of iodide ion. A common source of these oxides is the nitric acid ordinarily used to dissolve metallic copper and other copper-containing solids. Urea is used to scavenge nitrogen oxides from solutions:

(NH₂)₂CO + 2HNO₂ → 2N_{2 (g)} + CO_{2 (g)} + 3H₂O

The titration of iodine by thiosulfate tends to yield slightly low results owing to the adsorption of small but measurable quantities of iodine on solid CuI. The adsorbed iodine is released only slowly, even when thiosulfate is in excess; transient and premature end points result. This difficulty is largely overcome by the addition of thiocyanate ion. The sparingly soluble copper(I) thiocyanate replaces part of the copper iodide at the surface of the solid:

CuI_(s) + SCN^- → CuSCN_(s) +I^-

Accompanying this reaction is the release of the adsorbed iodine, which thus becomes available for titrated to prevent interference from a slow reaction between the two species, possibly

2SCN^- + I₂ → (SCN)₂

The standardization procedure is readily adapted to the determination of copper in the copper alloy that also contains appreciable amounts of tin, lead, and zinc (and perhaps minor amounts of nickel and iron). The method is relatively simple and applicable to brasses with less than 2% iron. A weighted sample is treated with nitric acid, which causes the tin to precipitate as a hydrates oxide of uncertain composition. Evaporation with sulfuric acid to the appearance of sulfur trioxide eliminates the excess nitrate, redissolves the tin compound, and possibly causes the formation of lead sulfate. The pH is adjusted through the addition of ammonia, followed by acidification with a measure amount of phosphoric acid. An excess of potassium iodide is added, and the liberated iodine is titrated with standard thiosulfate.

Procedure

1.    Weight (to the nearest 0.1mg) 0.3g samples into 250 mL conical flasks, and introduce 5 mL of 6 M HNO₃ into each; warm (use the hood) until solution is complete.

Powder of Copper Sample (Pure Copper)  

Nitrogen dioxide is evaporating

2.    Add 10 mL of concentrated H₂SO₄, and evaporate (use the hood) until copious white fumes of SO₃ are given off. Allow the mixture to coll.

 

3.    Cautiously add 30 mL of distilled water, boil for 1 to 2 min, and again cool.

4.    Add concentrated NH₃ dropwise and with thorough mixing to produce the intensely blue Cu(NH₃)₄²⁺ ; the solution should smell faintly of ammonia[1].

 

 Some Cu(OH)2 forms at the initial of adding aqueous ammonia

 

5.    Make dropwise additions of 3 M H₂SO₄ until the color of the complex just disappears, and then add 2.0 mL of 85% H₃PO₄ [2]. Cool to room temperature.

 

6.    Treat each sample individually from this point on to minimize the air oxidation of iodine ion. Add 4.0 g of KI to the sample, and titrate immediately with Na₂S₂O₃ until the solution becomes pale yellow.

 

7.    Add 5 mL of starch indicator, and continue the titration until the blue color becomes fanit.

 

8.    Add 2 g of KSCN; swirl vigorously for 30 s.

9.    Complete the titration, using the disappearance of the blue starch/I₂ color as the end point.

 

10. Report the percentage of Cu in the sample.

Notes

[1] Do not sniff vapors directly from the flask; instead, waft them toward your nose with a waving motion of your hand.

[2] The H₃PO₄ can form precipitate with some impurities such as iron and nickel.

   

Experimental Record

Weight of Copper Sample	0.300 g
	= 4.721 mmol
Molarity of Na2S2O3	0.100 M
Consumption of Na2S2O3	10.80 mL
	11.40 mL
Average	11.10 mL
Weight of Copper	11.10x0.100x(42.5/10.0)
	= 4.718 mmol
Percentage of Copper in the sample	(4.718 / 4.721) x 100%
	= 99.94 %