Overview - Metals in plastic (milk jug)
A variety of plastics (organic polymers) are useful materials which are ubiquitous in everyday life in an industrial society. One such polymer is polyethylene, a material you can find in such diverse settings as food wrappers, pick-up truch bed liners, and milk jugs. Polyethylene is prepared by the polymerization of ethylene (C2H4) to form CH3(CH2)nCH3, where n is generally on the order of 10,000 or so. Two broad classes of polyethylenes are formed from the "homopolymerization" of ethylene. Low density polyethylene (LDPE) is made by the polymerization of ethylene using a free radical process. High density polyethylene (HDPE) is made by the polymerization of ethylene using a metal catalyst. LDPE is less crystalline than HDPE so materials derived from LDPE are generally clearer but not as strong as HDPE.
Milk jugs are not very clear by design. Very little light should be able to get through the container to minimize photodecomposition of vitamins in milk. Strength is also important in milk jugs as they are often stored on top of one another and should withstand rough handling. Milk jugs are made of HDPE using a blow molding process. The HDPE which gives the best properties for making a good polymer (resin in the vernacular) is made from a chromium supported on silica catalyst.
A catalyst is a material or compound which acts as a facilitator to a chemical reaction to allow it to proceed faster than it otherwise would. While the chemical reaction rate increases, the catalyst by definition is not used up. In most cases once the catalyst has done its job it is separated from the products and reused. It is an unusual and often overlooked fact that in the manufacture of of HDPE that the catalyst is not separated from the product. Thus milk jugs have small amounts of catalyst residue, chromium, embedded within the material. Chromium is not an element one generally wants around. In higher oxidation states it is a known carcinogen. In high enough concentrations, greater than about 10 ppm, it causes localized oxidation of the HDPE resulting in ugly and poorly performing milk jugs. Not to mention the alarming fact that a high concentration of chromium is in contact with your milk! Monitoring of chromium from catalyst residue in milk jugs is thus an important and practical analytical problem.
In addition to chromium in HDPE, oftentimes other metals may be present in your supposedly pure organic polymer. Additives are often added to polyethylene during processing to impart desirable properties such as opaqueness, color and resistance to oxidation. In this project you will be asked first to qualitatively determine which elements above Z=11 in the periodic table are present in a milk jug, which either you can bring in yourself or we can provide. The second part of the project will entail a quantitative analysis of one of the elements the milk jug is qualitatively found to contain. Assuming chromium is found in your milk jug, and the chromium concentration is important, you should try to find its concentration.
So how to go about solving such a problem? There are a variety of instrumental techniques available to the analyst which will perform elemental analyses. Three instrumental methods of analysis come immediately to mind; X-ray fluorescence, atomic absorption, and atomic emission. We have at our disposal all three methods, though atomic emission capability of only a select few elements not including chromium. You will need to use a combination of at least two of these methods to qualitatively determine what elements are present in the milk jug, and quantitatively determine the content of one of those metals. Read about these methods and apply them appropriately to devise an analytical plan of attack.
There are a few things which may not be obvious at first but must be considered. It was mentioned that a chromium concentration greater than about 10 ppm results in a poorly performing polymer. Assume for the moment that the chromium concentration in your milk jug is about 5 ppm. This means that there are 5 micrograms of chromium per gram of polymer. This is probably within the detection limits of the EDXRF, which can be used to analyze the solid. What does this concentration mean when the sample is dissolved in an aqueous solution? If the density of water is 1 g/mL, then 1 ppm = 1 microgram/mL. To dissolve (digest) the polyethylene and obtain an aqueous solution of Cr, you must perform a microwave digestion. You can digest no more than about 2.4 g of polyethylene at a time in the microwave, in a total volume of about 50 mL. Calculate the concentration (in ppm) of chromium in the resulting solution. The detection limit for Cr in the AA is about 0.1 ppm. Before doing an atomic absorption analysis you will need to determine a Cr detection limit yourself. Is the chromium in the resulting solution detectable? If not, how do you make the solution more concentrated? The act of making a solution more concentrated prior to analysis is termed preconcentration.