Purpose: to Determine the Composition of a Brass Sample

Experiment # 6.2 Analysis of Brass

Purpose: To determine the composition of a brass sample

Objectives: 1. Properly calibrate and use a Spectronic 20 spectrophotometer 2. Determine the relationship between concentration and absorbance from data obtained from a Spect 20. 3. Construct a standard curve (calibration curve) from solutions of known concentration. 4. Use the calibration curve to estimate concentration of an unknown solution. 5. Convert the molar concentration to a mass percent value.

Background: Brass is an alloy of copper and zinc metals with tin and lead sometimes added to alter its physical properties. It is widely used in jewelry, electrical fixtures, musical instruments, pipe fittings, and bullet casings. Brass with a high copper content is easy to form and bend and can be polished to a high luster. Naval brass is a particular mixture of copper and zinc, with a small amount of tin added, which is commonly used for hardware components. Many standard alloys of this metal are available and determining the composition can allow a particular sample to be traced to its manufacturer. Bullet casings , for example, are generally C260 brass which is 70% copper and 30% zinc although the exact proportions may vary slightly. We can take advantage of the fact that most metals are susceptible to oxidation. Solubilization of metal samples. In order to carry out solution analysis of solid samples, it is first necessary to render them in a soluble form. For metallic samples, this generally involves oxidation of constituent elementsto their corresponding cations. Oxidation (corrosion) of metals is generally promoted by acidic conditions. The nature of the specific reaction that occurs depends upon both the chemical nature of the metal (specifically, its ease of oxidation or activity) and that of the acid employed. For some particularly active metals (e.g., Zn or Sn), H+ ions in acidic solution can act directly as the electron acceptor (or oxidizing agent), + +2 resulting in the formation of hydrogen gas: Zn(s) + 2 H --> Zn (aq) + H2 (g) Less active metals (e.g., Cu) may require the presence of a more potent oxidizing agent. For example, the - nitrate (NO3 ) ions present in nitric acid are strong electron-acceptors owing to the high positive oxidation number of nitrogen: Cu(s) + 4 HNO3(aq) --> Cu(NO3)2(aq) + 2 NO2(g) + 2 H2O(l)

Some metal ions have spectral properties and these can be used to spectrophotometrically identify and quantitate presence in solution. Note how the zinc has virtually no spectral properties, but the copper ion does. If the copper ion can be complexed with ammonium ion, the resulting spectral signal can be quite large.

The nature of metal ions in solution. While a full discussion of the chemistry of metal ions in aqueous solution is beyond the scope of this course, a few pertinent points that are essential to understanding the present lab procedure are presented here. First, recall that the "molecular" formula for an ionic compound (e.g., ZnCl2(aq)) commonly used in balanced chemical equations is a type of shorthand notation that may be expanded into the "ionic" form: Zn2 +(aq) + 2 Cl¯(aq). The latter representation emphasizes the fact that the anions and cations of the compound behave as independent particles in solution, and each has its own particular association with water molecules from the solvent, denoted by the suffix (aq). In the case of metal cations, this association may be strong enough to be regarded as a type of chemical bond, known as a coordinate covalent bond. The chemical species bonded to the metal ion (H2O molecules in the present example) are known as ligands, and the resulting aggregates (typically containing from two to six ligands bonded to a single metal ion) are known as coordination complexes. Copper(II) and zinc(II) ions in aqueous solution are both known to bind four water ligands, 2 + 2 + yielding complexes which may be formulated as [Cu(H2O)4] and [Zn(H2O)4] , respectively. 2 + 2 + In other words, Cu (aq) is really [Cu(H2O)4] . Since these metal ions bind more strongly 2 + 2 + to ammonia molecules than they do to H2O, addition of aqueous ammonia solution to Cu (aq) or Zn (aq) results in ligand substitution reactions: 2 + 2 + [M(H2O)4] + 4 NH3(aq) --> [M(NH3)4] + 4 H2O(l) (M = Cu or Zn)

In this experiment, we will solubilize a brass sample in nitric acid, then neutralize the acid with ammonia to create a copper complex and then evaluate the concentration of copper ion spectrophotometrically. Materials: Brass wire sample, 2 cuvettes, 7 M HNO3, Cu(NH3)4 complex (132 g/mole) + − 2M NH3 ([NH4 ][OH ]), hot plate, small beaker, test tubes, rack. 100 ml volumetric flask. 5 ml graduated pipeete, ddH2O, plastic dropper, pH paper. watch glass,

Procedure: Part A preparation of brass sample 1) Obtain a brass wire sample from instructor of 0.1 to 0.15 g 2) Weigh it on the top load balance, and on the analytical balance. record mass. 3) Place it into a small beaker and add 2.5 mls of concentrated nitric acid (goggles!!!!!!!). Allow to decompose in the fume hood on a hotplate. 4) While the brass is dissolving, go to part B.

Part B preparation of a standard curve 1) Make 10 mls of a 0.1 M Cu(NH3)4 solution. 2) Make dilutions such that you have at least 5 mls of 0.02 M, 0.01 M., 0.005 M , 0.0025 M, 0.00125 M, and 0.000625M . I will let you figure out the most efficient way to do this. 3) Read the absorbance of each dilution at 625 nm and record and graph the results. 4) Draw a best fit straight line for the points.

Note: we could have made out standard curve by preparing a copper sample in a similar way to the brass sample. Having the pre-made complex simply saves time. In a real lab situation, a control copper sample would be used to make your standard curve.

Part C analysis of the unknown brass sample 1) Make sure your sample from part A is completely dissolved. 2) To the sample of dissolved brass add 20 ml of 2M. NH3 3) Carefully transfer this to a 100 ml volumetric flask and rinse with 2M NH3. Add the rinses to the flask and fill to the calibration line with 2M NH3. 4) Measure the absorbance of the resulting solution at 625 nm. Use the 2M NH3 solution as a blank.

Data: mass of brass sample ______

molarity of brass sample/ 100 mls assuming it is all copper ______Cu = 63.5 g/mol

absorbance of brass sample in NH3 = ______

find the point on your standard curve that matches the abs of the brass sample

What is that concentration? ______

What is the molar extinction coefficient for copper ammonium complex based on your standard curve?

How many moles of copper were in your sample? ______

How much would this amount of copper have weighed? What is the percent composition of the ______wire? ______