analytical chemistrych1

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Analytical ChemistryChapter 1

Analytical chemistry is a school of science consisting of a set of powerful ideas and methods that areuseful in all fields of science and medicine.

The world was captivated by the Pathfinder mission. As a result, the numerous World Wide Websites 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 Sojourner was the APXS, or alpha proton X-ray spectrometer, whichcombines the three advanced instrumental techniques of Rutherford backscattering spectroscopy,proton emission spec-troscopy, and X-ray fluorescence.

1A THE ROLE OF ANALYTICAL CHEMISTRY

1B CLASSIFYING QUANTITATIVE ANALYTICAL METHODS1C STEPPING THROUGH A TYPICAL QUANTITATIVE ANALYSIS

C-1 Picking a Method

C-4 Eliminating Interferences

C-5 Calibration and Measurement

C-6 Calculating Results

C-2 Acquiring the Sample

C-3 Processing the Sample

C-7 Evaluating Results by Estimating Their Reliability

ID. AN INTEGRAL ROLE FOR CHEMICAL ANALYSIS: FEEDBACK CONTROL SYSTEMS

D-2 Calculating the ConcentrationD-3 Measuring the Amount Of The Analyte

D-1 Eliminating Interferences


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The Role of Analytical Chemistry

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 alchemists into a sci ence with applications throughoutindustry, medicine, and all the sciences.

Examples Of uses

The concentrations of oxygen and of carbon dioxide are determinedin millions of blood samples every day and used to diagnose andtreat illnesses.Smog-control is done by the measurement of quantities of hydrocarbons, nitrogen oxides, and carbon monoxide in automobileexhaust.Analytical chemistry helps diagnose parathyroid diseases inhumans measurements of ionized calcium in blood serum.Determination of nitrogen in foods establishes their protein contentand thus their nutritional value. Analysis of steel during its production permits adjustment in theconcentrations of such elements as carbon, nickel, and chromium toachieve a desired strength, hardness, corrosion resistance, andductility.


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Classifying Quantitative Analytical Methods

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.

Gravimetric methods determine the mass of the analyte or somecompound chemically related to it.

Volumetric method , the volume of a solution containingsufficient reagent to react completely with the analyte ismeasured.

Electroanalytical methods involve the measurement of suchelectrical properties as voltage, current, resistance, and quantity of electrical charge.

Spectroscopic methods are based on measurement of theinteraction between electromagnetic radiation and analyte atoms

or molecules or on the production of such radiation by analytes.Finally, there is a group of miscellaneous methods that includesthe measurement of such quantities as mass-to-charge ratio of molecules by mass spectrometry, rate of radioactive decay, heat of reaction, rate of reaction, sample thermal conductivity, opticalactivity, and refractive index.


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Stepping through a typical Quantitative anslysis

Figure Flow diagram showing the steps in a quantitativeanalysis. There are a number of possible paths through thesteps in a quantitative analysis. In the simplest examplerepresented by the central vertical pathway, we select amethod, 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 chosenmethod, various other pathways may be necessary. (Skoogs,West)

A typical quantitative analysis involves the sequence of steps shown in the flowdiagram. 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 chaptersof this book focus on the last three steps in Figure 1-2. In the measurement step, wemeasure one of the physical properties mentioned in Section IB. In the calculationstep, we find the relative amount of the analyte present in the samples. In the final stepwe evaluate the quality of the results and estimate their reliability.We then present a case study to illustrate these steps in solving an important andpractical analytical problem. The details of the case study foreshadow many of themethods and ideas you will explore as you study analytical chemistry.


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Acquiring the Sample

This step is a quantitative analysis it is done to acquire the sample To produce meaningful information, an analysis must be performed on a sample whose composition represents the bulk of material from whichit was taken.

Where the bulk is large and heterogeneous, great effort is required to get a representative sample. Sample problem below.

A railroad car containing 25 tons of silver ore. Buyer and seller must agree on a price, which will hebased primarily on the silver content of the shipment .The ore itself is inherently heterogeneous, consisting of many lumps that vary in size as well as in silvercontent.

Explanation of procedureThe assay of this shipment will be performed on a sample that weighs about one gram. For the analysis tohave significance, this small sample must have a composition that is representative of the 25 tons (orapproximately 22.700,000 g) of ore in the shipment.Isolation of one gram of material that accurately represents the average composition of the nearly23,000,000 g of bulk sample is a difficult undertaking that requires a careful, systematic manipulation of the entire shipment. Sampling involves obtaining a small mass of a material whose compositionaccurately represents the bulk of the material being sampled. (Skoogs, West)

Section C


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The third step in an analysis is to process the sample as shown in Figure. Under certain circumstances, no sampleprocessing 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 thesample in any of a variety of different ways. The first step in processing the sample is often the preparation of alaboratory sample.

Processing the Sample

Preparing a Laboratory Sample -Solid samplesA solid laboratory sample is ground to decrease particle size, mixed to ensure homogeneity, and stored for various lengths of timebefore analysis begins.(dry samples just before starting an analysis, to avoid absorption or loss of water)

Liquid sampleLiquid samples present a slightly different but related set of problems during the preparation step.(Liquid sample container must be kept inside a second sealed container, loss of liquid through evaporations change the concentration of the analyte .)Defining Replicate Samples-We perform most chemical analyses on replicate samples whose masses or volumes have been determined by careful measurementswith an analytical balance or with a precise volumetric device.Replication improves the quality of the results and provides a measure of their reliability. Quantitative measurements on replicates areusually averaged, and various statistical tests are performed on the results to establish their reliability.

Section C


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Preparing Solutions: Physical and Chemical Changes Most analyses are performed on solutions of the sample made with a suitable solvent. Ideally, the solvent should dissolve the entire sample, including the analyte, rapidly and completely. The conditions of dissolution should be sufficiently mild that loss of the analyte cannot occur or is minimized. Unfortunately, many materials that must be analyzed are insoluble in common solvents. Examples include silicate minerals, high-molecular-weight polymers, and specimens of animal tissue. Under this circumstance, we must follow the flow diagram to the box on the right and carry out some rather harsh chemistry. Conversion of the analyte in such materials into a soluble form is often the most difficult and time-consuming task in theanalytical process. The sample may require heating with aqueous solutions of strong acids, strong bases, oxidizing agents, reducing agents, orsome combination of such reagents. It may be necessary to ignite the sample in air or oxygen or perform a high-temperature fusion of the sample in the presenceof various fluxes. Once the analyte is made soluble, we then ask whether the solution has a property that is proportional to analyteconcentration and that we can measure. For example, in the determination of manganese in steel, manganese must be oxidized to MnO before the absorbance of thecolored solution is measured. At this point in the analysis, it may be possible to proceed directly to the measurement step, but more often than not, we musteliminate interferences in the sample before making measurements as illustrated in the flow diagram.



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Eliminating Interferences Once we have gotten the sample into solution and converted the analyte to an appropriate form for the measurement step, the nextstep is to eliminate substances from the sample that may interfere with the measurement step.Few chemical or physical properties of importance in chemical analysis are unique to a single chemical species.Instead, the reactions used and the properties measured are characteristic of a group of elements or compounds.Species other than the analyte that affect the final measurement are called interferences, or interferents.A scheme must be devised to isolate the analytes from interferences before the final measurement is made.There are no stead fast rules can be given for eliminating interferences; indeed, resolution of this problem can be the most demandingaspect of an analysis.

Calibration and MeasurementAll analytical results depend on a final measurement A' of a physical or chemical property of the analyte.This property must vary in a known and reproducible way with the concentration ca of the analyte.Ideally, the measurement of the property is directly proportional to the concentration.That is, where k is a proportionality constant. With two exceptions, analytical methods require the empirical determination of k withchemical standards for which ca is known. The process of determining k is thus an important step in most analyses; this step is called acalibration.

Calculating Results Computing analyte concentrations from experimental data is usually relatively easy, particularly with modern calculators or computers.These computations are based on the raw experimental data collected in the measurement step, the characteristics of the measurementinstruments, and the stoichiometry of the analytical reaction. Samples of these calculations appear throughout this book.


Evaluating Results by Estimating Their ReliabilityAnalytical results are incomplete without an estimate of their reliability.The experimenter must provide some measure of the uncertainties associated with computed results if the data are to have any value.


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An Integral Role For Chemical Analysis: Feedback Control System

Analytical chemistry is usually not an end in itself, but is part of a bigger picture in which we may use analytical resultsto help control a patient's health, to control the amount of mercury in fish, to control the quality of a product.. Chemical analysis is the

measurement elements in all of these examples and in many other cases. Consider the role of quantitative analysis in the determinationand control of the concentration of glucose in blood. Patients suffering from insulin-dependent diabetes mellitus develophyperglycemia, which manifests itself in a blood glucose concentration above the normal concentration of 60 to 95 mg/dL. We beginour example by determining that the desired state is a blood glucose level below 95 mg/dL. Many patients must monitor their bloodglucose levels by periodically submitting samples to a clinical laboratory for analysis or by measuring the levels themselves using ahandheld electronic glucose monitor.

Process of Analysis The first step in the monitoring process is to determine the actual state by collecting a blood sample from the patient and measuring theblood glucose level.If the measured blood glucose level is above 95 mg/dL, the patient's insulin level, which is a controllable quantity, is increased byinjection or oral administration.After a delay to allow the insulin time to take effect, the glucose level is measured again to determine if the desired state has beenachieved.If the level is below the threshold, the insulin level has been maintained, so no insulin is required. After a suitable delay time, the bloodglucose level is measured again, and the cycle is repeated.In this way, the insulin level in the patient's blood, and thus the blood glucose level, is maintained at or below the critical threshold,which keeps the metabolism of the patient in control.

I The process of continuous measurement and control is often referred to as a feedback system, and the cycle of measurement, comparison, and control is called a feedback loop. These ideas find wide application in biological and bio-medicalsystems, mechanical systems, and electronics. From the measurement and control of the concentration of manganese in steel tomaintaining the proper level of chlorine in a swimming pool, chemical analysis plays a central role in a broad range of systems.


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ELIMINATING INTERFERENCESArsenic can be separated from other substances that might interfere in the analysis by converting it to arsine, AsH 3, a toxic, colorless

gas that is evolved when a solution of H 3AsO 3 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 H 3AsO 4 to H 3AsO 3 according to thefollowing reaction:

H3AsO 4 + SnCl 2 + 2HC1 H 2AsO 3 + SnCl 4 + H 2O

The H 3AsO 3 was then converted to AsH 3 by the addition of zinc metal as follows:

H3AsO 3 + 3Zn + 6HC1 AsH 3 (g) + 3ZnCl 2 + 3H 2O

The entire reaction was carried out in flasks equipped with a stopper and delivery tube so that the arsine could be collected in theabsorber. The arrangement ensured that interferences were left in the reaction flask and that only arsine was collected in the absorberin 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:


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MEASURING THE AMOUNT OF THE ANALYTEThe amount of arsenic in each sample was determined by measuring the intensity of the red color formed in the cuvettes with an

instrument called a spectrophotometer.Spectrophotometer provides a number called absorbance that is directly proportional to the color intensity, which is also proportionalto the concentration of the species responsible for the color.To use absorbance for analytical purposes, a calibration curve must be generated by measuring the absorbance of several solutionsthat contain known concentrations of analyte. The color becomes more intense as the arsenic content of the standards increases from0 to 25 parts per million (ppm).

CALCULATING THE CONCENTRATION

The absorbance for the standard solutions containing known concentrations of arsenic are plotted to produce a calibration curve.The color intensity of each solution is represented by its absorbance, which is plotted on the vertical axis of the calibration curve.Note that the absorbance increases from 0 to about 0.72 as the concentration of arsenic increases from 0 to 25 parts per million.The concentration of arsenic in each standard solution corresponds to the vertical grid lines of the calibration curve as shown.This curve is then used to determine the concentration of the two unknown solutions shown on the right. We first find the absorbanceof the unknowns on the absorbance axis of the plot and then read the corresponding concentrations on the concentration axis.The lines leading from the cuvettes to the calibration curve show that the concentrations of arsenic in the two deer were 16 ppm and22 ppm, respectively.Arsenic in kidney tissue of an animal is toxic at levels above about 10 ppm, so it was probable that the deer were killed by ingesting

an arsenic compound.The tests also showed that the samples of grass contained about 600 ppm arsenic. This very high level of arsenic suggested that thegrass had been sprayed with an arsenical herbicide. The investigators concluded that the deer had probably died as a result of eatingthe poisoned grass.

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