Analytical Chemistry Chapter 1. 1A THE ROLE OF ANALYTICAL CHEMISTRY. 1B CLASSIFYING QUANTITATIVE ANALYTICAL METHODS. 1C STEPPING THROUGH A TYPICAL QUANTITATIVE ANALYSIS. C-1 Picking a Method.
Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.
1A THE ROLE OF ANALYTICAL CHEMISTRY
1B CLASSIFYING QUANTITATIVE ANALYTICAL METHODS
1C STEPPING THROUGH A TYPICAL QUANTITATIVE ANALYSIS
C-1 Picking a Method
Analytical chemistry is a school of science consisting of a set of powerful ideas and methods that are useful in all fields of science and medicine.
“The world was captivated by the Pathfinder mission. As a result, the numerous World Wide Web sites tracking the mission were nearly overwhelmed by millions of Internet surfers who closely monitored the progress of tiny Sojourner in its quest for information on the nature of the Red Planet” (Skoogs and West).
The key experiment on Sojournerwas the APXS, or alpha proton X-ray spectrometer, which combines the three advanced instrumental techniques of Rutherford backscattering spectroscopy, proton emission spec-troscopy, and X-ray fluorescence.
C-2 Acquiring the Sample
C-3 Processing the Sample
C-4 Eliminating Interferences
C-5 Calibration and Measurement
C-6 Calculating Results
C-7 Evaluating Results by Estimating Their Reliability
ID. AN INTEGRAL ROLE FOR CHEMICAL ANALYSIS: FEEDBACK CONTROL SYSTEMS
D-1 Eliminating Interferences
D-2 Calculating the Concentration
D-3 Measuring the Amount Of The Analyte
“Analytical chemistry plays a vital role in the development of science. In 1894, Friedrich Wilhelm Ostwald wrote: Analytical chemistry, or the art of recognizing different substances and determining their constituents, takes a prominent position among the applications of science, since the questions which it enables us to answer arise wherever chemical processes are employed for scientific or technical purposes. Its supreme importance has caused it to be assiduously cultivated from a very early period in the history of chemistry, and its records comprise a large part of the quantitative work which is spread over the whole domain of science” (Skoog andwest)
Analytical chemistry has evolved from an art of court magicians to alchemist’s into a science with applications throughout industry, medicine, and all the sciences.
Examples Of uses
Analytical Chemists compute the results of a typical quantitative analysis from two measurments. One is the mass or the volume of sample to be analyzed. The second is the measurement of some quantity that is proportional to the amount of analyte in the sample, such as mass, volume, intensity of light or electrical charge.
This second measurement usually completes the analysis, and we classify analytical methods. according to the nature of this final measurement.
Figure Flow diagram showing the steps in a quantitative analysis. There are a number of possible paths through the steps in a quantitative analysis. In the simplest example represented by the central vertical pathway, we select a method, acquire and process the sample, dissolve the sample in a suitable solvent, measure a property of the analyte, calculate the results, and estimate the reliability of the results. Depending on the complexity of the sample and the chosen method, various other pathways may be necessary. (Skoogs, West)
A typical quantitative analysis involves the sequence of steps shown in the flow diagram. In some instances, one or more of these steps can be omitted. For example, if the sample is already a liquid, we can avoid the dissolution step. The first 23 chapters of this book focus on the last three steps in Figure 1-2. In the measurement step, we measure one of the physical properties mentioned in Section IB. In the calculation step, we find the relative amount of the analyte present in the samples. In the final step we evaluate the quality of the results and estimate their reliability.
We then present a case study to illustrate these steps in solving an important and practical analytical problem. The details of the case study foreshadow many of the methods and ideas you will explore as you study analytical chemistry.
The essential first step in any quantitative analysis is the selection of a method as depicted in. The choice is sometimes difficult and requires experience as well as intuition. One of the first questions to be considered in the selection process is the level of accuracy required. Unfortunately, high reliability nearly always requires a large investment of time. The selected method usually represents a compromise between the accuracy needed and the time and money that are available for the analysis.
A second consideration related to economic factors is the number of samples to be analyzed. If there are many samples, we can afford to spend a good deal of time in preliminary operations such as assembling and calibrating instruments and equipment and preparing standard solutions. If we have only a single sample or just a few samples, it may be more appropriate to select a procedure that avoids or minimizes such preliminary steps. Finally, the complexity of the sample and the number of components in the sample always influence the choice of method to some degree.
Acquiring the Sample
Processing the Sample
The third step in an analysis is to process the sample as shown in Figure. Under certain circumstances, no sample processing is required prior to the measurement step. For example, once a water sample is withdrawn from a stream, a lake, or an ocean, the pH of the sample can be measured directly. Under most circumstances, we must process the sample in any of a variety of different ways. The first step in processing the sample is often the preparation of a laboratory sample.
Arsenic can be separated from other substances that might interfere in the analysis by converting it to arsine, AsH3, a toxic, colorless gas that is evolved when a solution of H3AsO3 is treated with zinc. The solutions resulting from the deer and grass samples were combined with Sn-+, and a small amount of iodide ion was added to catalyze the reduction of H3AsO4 to H3AsO3 according to the following reaction:
H3AsO4 + SnCl2 + 2HC1 H2AsO3 + SnCl4 + H2O
The H3AsO3 was then converted to AsH3 by the addition of zinc metal as follows:
H3AsO3+ 3Zn + 6HC1 AsH3(g) + 3ZnCl2 + 3H2O
The entire reaction was carried out in flasks equipped with a stopper and delivery tube so that the arsine could be collected in the absorber. The arrangement ensured that interferences were left in the reaction flask and that only arsine was collected in the absorber in special transparent containers called cuvettes. Arsine bubbled into the solution in the cuvette, reacted with silver diethyldithiocar-bamate to form a colored complex compound according to the following equation: