ANALYST, JULY 1986, VOL. 111 849 Catalytic Reactions: Determination of Trace Amounts of Copper in Natural Samples Wilson F. Jardim and Jarbas J. R. Rohwedder lnstituto de Quimica, Universidade Estadual de Campinas, CP 6154, 73100 Campinas, SP, Brazil Under alkaline conditions, copper(l1) ions catalyse the oxidation of hydroxylamine to nitrite. This catalytic reaction allows the determination of trace amounts of copper in natural samples. The mechanism of oxidation relies on superoxide ions generated from dissolved oxygen. For tap water and lake water samples, the results obtained indicated satisfactory levels of precision and accuracy. Keywords : Catalytic reactions; copper determination; natural samples The determination of trace concentrations of some metals in aquatic systems has become an important factor because of the toxic effects they may have on living organisms. As the metal concentration in natural aquatic samples (i. e., rivers, lakes and tap water) is usually low, sensitive analytical techniques are required. Voltammetry, atomic absorption and catalytic reactions are examples of suitable techniques that can be used in the determination of trace metals. However, whereas voltammetry and atomic absorption may require technical support to operate the equipment and have a high cost per sample, catalytic reactions involve simple, rapid and low cost spectrophotometric measurements. Over the past decade, the use of catalytic reactions in trace analysis has developed substantially. 1 Although the technique often has poor selectivity and a high level of interfering species, sensitivity is very high. The determination of copper in blood serum has been carried out by following its catalytic effects on the reduction of iron(II1) salts with thiosulphate.2 Heller and Guyon3 used a method based on the ascorbic acid reduction of the isopoly- molybdate ion in the presence of minute amounts of copper. More recently, Nakano et aL4 determined nanogram levels of copper by the oxidative coupling reaction of N-phenyl-p- phenylenediamine with N, N-dimethylaniline. In this work, a very simple and sensitive method for determining copper is presented. Under alkaline conditions, trace amounts of copper catalyse the oxidation of hydroxyl- amine to nitrate ions. After a suitable period of time (up to 120 min), the resulting nitrite concentration is then determined spectrophotometrically as a strongly coloured azo compound. Experimental Reagents To avoid highly coloured blank values, de-ionised, doubly distilled water was used throughout. Glassware was cleaned by soaking in 10% V/V nitric acid overnight. BufSer solution. To prepare a 0.01 M sodium tetraborate buffer solution, 3.81 g of sodium tetraborate (Naz- B407. 10H20)were dissolved in 1 1 of water. The final pH was 9.18. Hydroxylamine solution. To prepare a 0.1 M solution, 1.74 g of hydroxylamine hydrochloride were dissolved in 250 ml of water. Mixed nitrite reagent. In a 1 1 calibrated flask, 5.00 g of sulphanilic acid { [4-(H2N)C6H4S03H. H20]} were dissolved in a mixture of 750 ml of water and 35 ml of 1 M hydrochloric acid. The solution was heated and, after cooling to room temperature, 25 ml of a 20% m/V solution of N-(1- naphthy1)ethylenediamine dihydrochloride (CloH7NHCH2- CH2.2HCl) were added. The final volume was adjusted to 1 1. Effects of pH The generation of nitrite as a function of [OH-] is shown in Fig. 1. In the presence of sodium tetraborate buffer, the production of nitrite was greatly enhanced. As the generation of nitrite follows a first-order reaction with respect to the concentration of hydroxyl ions, it was decided to work at a buffered pH of 9.18 (sodium tetraborate). At pH vaIues above 10,'any minor variation in this parameter will be reflected as larger variations in the absorbance readings. NH20H Concentration Fig. 2 shows the production of nitrite as a function of different concentrations of hydroxylamine. For both 1 X 10-7 and 5 x 10-7 M solutions of copper(II), nitrite generation increased as the concentration of NH20H increased, reaching a constant value above 10-3 M of hydroxylamine. Procedure Determination of copper in tap water To a 10-ml sample in a test tube, 5 ml of the sodium tetraborate buffer solution and 0.3 ml of the hydroxylamine solution were added. After a suitable period of time, 1.5 1.2 8 0.9 C m e v) Ll a 0.6 0.3 2 4 6 8 1 - Log [ OH- J I Fig. 1. Generation of nitrite as a function of [OH-]. The effect caused by the resence of sodium tetraborate buffer (1 X 10-2 M) is also shown (Or. Total copper concentration, 1 X M