Teaching Analytical Chemistry through
Mineral Analysis
W
estern Australia has extensive rich mineral ore deposits, including bauxite, gold, iron, lithium, tantalum, and nickel, as well as mineral sands (Figure 1). Many of these ores are processed downstream. As a result, there are numerous employment opportunities in the chemical industry, government organizations, and private analytical services for chemistry graduates. At Curtin University of Technology, to better prepare students for employment in those industries, our second-year undergraduate analytical chemistry course, or unit, focuses on mineral analysis through detailed lectures, laboratory exercises, and problem-based and realworld mini-projects. Although the emphasis of the unit is on mineral analysis, teaching our students principles such as the chemical basis of removing interfering species and techniques such as perchloric acid digestion and lithium metaborate fusions gives them a decided advantage in gaining employment. Table 1 shows the range of positions advertised in the local press and the large percentage of our graduates who fill them.
Students at Curtin University use mineral analysis to get a jump on real-world analytical problems.
David N. Phillips Curtin University of Technology (Australia) A U G U S T 1 , 2 0 0 2 / A N A LY T I C A L C H E M I S T R Y
427 A
somewhat vocation-oriented and contain a significant amount of analytical and instrumental chemistry. In keeping with the NSFsupported report Curricular Developments in the Analytical Sciences on the generic components of an analytical problem-solving teaching approach, we emphasize sampling, separating the analyte from interfering substances, performing the measurement, interpreting the data, and assessing the results (8).
Mineral analysis unit NaCl, Fe
Port Hedland Dampier
Telfer
NaCl, Fe
Au
Fe Fe
Newman
Paraburdoo
Fe NaCl
Yeleerie
U, V Ni Ni
Mt. Weld
Ta, V, P, rare earths
Ni, Cu, Zn, Pb
Leonora Ti
Eneabba
V
Perth Bunbury
Wundowie
Kalgoorlie Au, Ni Au, Cu, Co
Al Al Ti, Zr, rare earths, Ta, Nb, Sn, Li
FIGURE 1. Western Australian cities and their respective mineral ore deposits. The map represents an 1800 x 1500 km area.
In the United States, the trend over the past five years has been to use problem-based learning techniques to solve real-world problems in analytical chemistry classes. Wenzel of Bates College has extensively discussed the role of problem-based learning in undergraduate analytical chemistry teaching (1–3). Wilson, Anderson, and Lunte at the University of Kansas take a similar approach in their instrumental analysis course in which their students are asked to environmentally assess a proposed industrial park site, validate an analytical method for a pharmaceutical compound, and investigate a fish kill in a local river (4). Similarly, Werner, Tobiessen, and Lou of Union College use a real-world experience in their quantitative analytical chemistry laboratory with their water project (5). At Curtin University, our three-year undergraduate degree curriculum uses problem-based learning techniques with our miniprojects course and our experimental program design course (6, 7 ). However, before taking these courses, students must complete an intensive mineral chemistry course. Unlike at many traditional Australian universities, courses at Curtin University are 428 A
A N A LY T I C A L C H E M I S T R Y / A U G U S T 1 , 2 0 0 2
This unit, one of 25, is composed of 14 lectures and 60 h of laboratory work. It is complemented by an introductory analytical chemistry course in which topics such as semimicroanalysis of cations and anions, titrimetric and gravimetric analysis, and UV– vis spectrophotometry are taught. In the second and third year, more instrumental analysis topics are covered, such as atomic absorption spectrometry (AAS) and flame AAS (FAAS), using organic solvents, inductively coupled plasma (ICP) optical emission spectroscopy, ICPMS, X-ray fluorescence, and introductory Raman spectroscopy. The lecture program covers accuracy and precision; decomposition methods; removal of interfering species by precipitation; solvent extraction; ion exchange; masking; and methods for sampling solids, gases, and liquids. Removing interfering species constitutes a significant percentage of the lecture syllabus. Solids sampling is covered in more detail during the third-year inorganic chemistry course (9), and liquids sampling is taught in conjunction with an environmental chemistry exercise on pollutants in the Swan River in Perth. Students also take a statistics course that runs concurrently with the analytical chemistry unit. A simple treatment of determinant and indeterminant errors is considered necessary for this unit, because students only carry out duplicate analyses for each exercise. The group method of cation separation using solubility products to calculate the purity of precipitates is illustrated by the formation of hydroxides and sulfides. Fundamentals of Analytical Chemistry by Skoog, West, and Holler is the prescribed textbook for the lecture program (10).
Laboratory program and exercises The laboratory program covers many of the same analytical methods used in the local mineral processing industry, especially titrimetry and gravimetry. Volumetric and gravimetric techniques are required to train students in accuracy and allow for highly critical assessments of results, consistent with the findings of Kratochvil (11). This approach matches the findings of a recent report on the important practical skills required of new chemistry graduates (12), which concluded that, along with other requisites, titration skills and knowledge of concepts such as accuracy and precision were important to employers. Instrumental techniques such as FAAS and ICPMS are often used in industry and private analytical laboratories. In this unit, less emphasis is placed on accuracy and precision when assessing exercises that end with a FAAS analysis. The lab program also incorporates forensic and environmental chemistry exercises to broaden the students’ experience. There are 13 laboratory exercises used to emphasize the skills employers deemed important in the survey; 6 are carried out in any one year and chosen on a rotating basis. The following is a
brief summary of the chemistry and objectives of many of these exercises. In Exercise 1, reduced ilmenite is analyzed for its Fe(0) content by selective oxidation using CuSO4 (as described in Analysis of Fe(0) and TiO2 in Ilmenite from RGC Mineral Sands, copies of which are available from the author). The TiO2 content is determined in Exercise 2 by fusing ilmenite with potassium pyrosulfate. After reducing Ti(IV) and Fe(III) with Al, the Ti(III) is determined by selective oxidation using Ce(IV) as the titrant. In these two exercises, students gain an appreciation of selective oxidation and reduction and the need to construct a standard electrode potential diagram to gain a full understanding of the chemistry in the analytical procedure. The standard electrode potential diagram for Exercise 1 is shown in the scheme below. +1.33 +0.83 +0.77 +0.44 +0.34 –0.41 –1.66
Cr2O72–+ 14H+ + 6e r 2Cr3+ + H2O Indic(ox) r Indic(red) Fe3+ + e r Fe2+ in 1M HCl Fe3+ + e r Fe2+ in 1M H3PO4 Cu2+ + 2e r Cu Fe2+ + 2e r Fe Al3+ + 3e r Al
with FAAS analysis, in which simple acid standards may be used for the HClO4 determination, whereas matrix-matched standards are needed for the LiBO2 determination. The geochemical ore is analyzed for its nickel and manganese content, and the students are required to use their analytical data to compare and contrast the relative merits of HClO4 digests and LiBO2 fusions.
This course provides students with a solid basis for employment in an analytical chemistry laboratory environment.
The determination of the available Al2O3 content of bauxite in Exercise 3 illustrates preferential complexation and how to overcome difficulties with reaction kinetics using a back titration. The procedure is slightly modified from that used in the alumina industry and originally devised by Watts and Utley (13). The bauxite is digested in NaOH at atmospheric pressure. Although this adaptation produces lower results than those expected from a pressurized digestion, no loss of precision is experienced. Exercise 4 determines gold in gold-bearing ores. The gold is taken into solution as AuCl 4– using an oxidative leach mix of HCl and KMnO4, as described by Tewari and Gupta (14) and modified by Hoang et al. (15). The gold is coprecipitated as a mercury amalgam and dissolved in aqua regia. The gold is then determined by FAAS. In Exercise 5, iron is determined after using thioacetamide to remove the acidic sulfide group elements by precipitation. After precipitation of Fe2O3•xH2O and subsequent dissolution in HCl, the iron is reduced using Na2SO3 prior to determination with KMnO4. Replacement of the conventional Sn(II)/Hg(II) reductant by Na2SO3 represents a somewhat improved and environmentally friendly reduction method. Students are introduced to the use and handling of HClO4 as a powerful oxidizing agent in Exercises 8 and 9 with the digestion of a geochemical ore and a waterway sediment sample, respectively. The acidic and basic properties of LiBO2 are introduced in Exercise 10 by fusing a geochemical ore and a cement sample. The contrast in the silica content of these materials illustrates the need for differing sample-to-flux ratios. Both these exercises end
Mini-project
The waterway sediment exercise caters to students who are majoring in environmental chemistry. Students are provided with a sampling device designed for sediments, and they individually collect samples from prescribed locations on the local Swan River. At any particular location, students take two
