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Experiment 2 2/1 Environmental Chemistry II

Analysis of Copper and Arsenic in Treated Wood Using

Atomic Absorption Spectrophotometry1 Preliminary Questions Your answers must be approved by a demonstrator before you make up solutions. 1. Cu(NO3)2.3H2O (FW 241.58) will be used to prepare your copper stock solution.

How much will you need to make up 250 ml of stock solution containing 250 g/ml (= 250 ppm) of copper?

2. Your stock solution will be diluted to prepare standard solutions for calibration of

the photometer. Calculate the volumes of 250 g/ml stock solution required to prepare 50 ml each of solutions with concentrations of 5, 10, 15, 20 and 25 g/ml.

3. Arsenious oxide, As2O3, was used to prepare the arsenic stock solution. Using

the Material Safety Data Sheet for As2O3, list all hazards associated with and precautions that should be taken when using this compound. What first aid measures should be taken in the event of contact?

Introduction Chemicals or mixtures of chemicals have long been used to treat wood in an effort to extend the lifetime of the wood under conditions which would normally accelerate deterioration. Examples of treated wood include fence posts, plywood used for wood foundations and permapine used to build children's playgrounds. In recent years, wood has been pressure treated with a green-coloured aqueous solution containing copper, chromium and arsenic. This compound, known as "copper chromated arsenic" is toxic to insects, bacteria and small mammals and also retards the growth of mildew and fungus. The drawback to this treatment is that it can slowly leach out of treated lumber due to its water solubility. Copper, arsenic and chromium can also get into groundwater if spent solutions used to treat lumber are improperly disposed of. In addition, burning wood treated with this substance can contaminate a large area around the burn site with toxic metal compounds. The purpose of this experiment is to use atomic absorption spectrophotometry to determine the amount of copper and arsenic in a sample of copper chromated arsenic-treated wood. Atomic line absorption spectra are used for analysing various metals. Atomic absorption spectrophotometry ("AA") offers sensitivity, selectivity and simplicity and was developed by CSIRO Melbourne scientists around 1955. "AA" is an important tool in analytical laboratories.

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Experiment 2 2/2 Environmental Chemistry II Experimental Note: do not dispose of any solutions before reading the section on waste disposal at the end of the experimental section. Extraction with Nitric Acid Carry out this procedure on two samples so that duplicate results may be obtained and averaged. Weigh accurately into a 150 ml beaker (to the nearest 0.1 mg) approximately 1 g of small particles from a pressure treated wood sample. Add 50 ml of 4M nitric acid, cover the beaker with a watch glass. Leave on the steambath (fumehood) for 20 minutes, keeping a close watch on the samples to ensure they do not froth over. Allow the two samples to cool. Filter off any insoluble material using vacuum filtration with a buchner funnel. Rinse the beaker and the watch glass four times each with portions of deionised water, each time pouring the rinse water through the funnel so that it is added to the 50 ml of filtrate. Transfer the filtrate combined with rinses to a clean 250 ml volumetric flask and make up to the mark with deionised water. Ensure thorough mixing by inverting the flask at least ten times. Preparation of standard solutions AA is very sensitive and low concentrations are used, so all glassware must be clean! Wash well with hot tap water then rinse thoroughly with deionised water. A 250 g/ml arsenic stock solution has been provided for you, but you must prepare the 250 g/ml copper stock solution (250 ml) yourself (preliminary question 1). Ensure thorough mixing by inverting the flask at least ten times. The standard solutions you will prepare will contain both copper and arsenic. Concentrations of both metals in the standards are to be 0, 5, 10, 15, 20 and 25 g/ml. Prepare the blank solution (0 g/ml) by adding 20 ml of 4M nitric acid to a 50 ml volumetric flask and making up to the mark with deionised water. Prepare the other standard solutions in a similar way by diluting the required amount of each stock solution (preliminary question 2 - the volumes you calculated for the copper stock solution will be the same required for the arsenic stock solution) plus 20 ml of 4M of nitric acid to 50 ml. Once again, ensure thorough mixing by inverting each flask at leask ten times. Don't forget that a "5 g/ml standard solution" will contain 5 g/ml of both copper and arsenic.



Analysis using Atomic Absorption Spectrophotometry The AA spectrophotometer you will be using is semi-automated. Consult the manual for settings and instructions on using the apparatus. Before proceeding with your analysis, consult a demonstrator. The line at 327.4 nm is best for detection of copper in the concentration range you will be using, while the line at 193.7 nm is best for detection of arsenic. The instrument is first calibrated with solutions of known concentration, giving a calibration curve of absorption versus metal concentration, which is printed out automatically. The AA uses a different lamp for each metal to be analysed, so complete the analysis for one metal (calibration curve and 2 sample measurements) before moving onto the second. Remember, the same set of solutions will be used to analyse for both copper and arsenic. Waste disposal Stock solutions and standard solutions containing arsenic must be disposed of in the waste Copper/Arsenic container. Do not pour any solutions containing arsenic (including the wood extract solutions) down the sink!. If you are unsure of how to dispose of a particular solution, feel free to ask a demonstrator. Questions 1. Calculate the amounts of copper and arsenic present per gram of treated wood

(NB - results for the two samples should not be averaged until an amount of metal per gram of wood for each sample has been calculated).

2. Consider sources of error which may have caused your final calculated amounts

of copper and arsenic to be incorrect. 3. Did the concentrations of copper and arsenic in the two samples match closely

(within 0.5 ppm)? If not, give a possible reason for this. (Hint: consider probable sampling techniques for the treated wood sample and the manner in which the wood may be treated.)




Report Your report should contain the following: quantities (weights and volumes) used to make up the stock solution and the

standard solutions containing both copper and arsenic - use table form wherever possible

weight of the two wood samples used calibration curves for both copper and arsenic calculations showing how the amounts of copper and arsenic per gram of wood

were calculated answers to questions Reference 1. This experiment is adapted from "Environmental Chemistry: Experiments and

Demonstrations" (2nd ed.); M.G. Ondrus; Wuerz Publishing, Winnepeg (1996).

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