Chapter 1

What is Analytical Chemistry?

What is Analytical Chemistry

-Analytical chemistry is a measurement science consisting of a set of powerful ideas and methods that are useful in all fields of science and medicine

-Used for qualitative and quantitative analysis of information

-Qualitative Analysis-Establishes the chemical identity of the species in the sample

-Quantitative Analysis-Determines the relative amount of species or analytes, in numerical terms

1A—The Role of Analytical Chemistry

• -Friedrich Wilhelm Ostwald– “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 chemical purposes.”

1A—The Role of Analytical Chemistry

• -All branches of chemistry draw on the ideas and techniques of analytical chemistry

– -Concentrations of oxygen and carbon dioxide are determined in millions of blood samples everyday and used to diagnose and treat illnesses

– -Quantities of hydrocarbons, nitrogen oxides, and carbon monoxide present in automobile exhaust gases are measured to determine the effectiveness of smog-control devices

– -Quantitative measurements of ionized calcium in blood serum help diagnose parathyroid disease in humans

– -Many other uses for analytical chemistry…Think about it!

– -Research in chemistry, Biochemistry, Biology, Geology, Physics, etc.

1B—Classifying Quantitative Analytical Methods

• -Gravimetric Methods determine the mass of the analyte or some compound chemically related to it

• -Volumetric Methods measure the volume of a solution containing sufficient reagent to react completely with the analyte

• -Electroanalytical Methods involve the measurement of such electrical properties as voltage, current, resistance, and quantity of electrical charge

• -Spectroscopic Methods are based on the measurement of the interaction between electromagnetic radiation and analyte atoms or molecules on the production of such radiation by analytes

• -Miscellaneous Methods include the measurement of such quantities as mass-to-charge ratio, rate of radioactive decay, heat of reaction, rate of reaction, sample thermal conductivity, optical activity, and refractive index

1C—Stepping Through A Typical Quantitative Analysis

• 1-Picking a Method

• -The Essential First Step

• -Choice is sometimes difficult and requires experience and intuition

• -Consider the level of accuracy required

• -Selected method usually represents a compromise between the accuracy needed and the time and money that are available for the analysis (time consuming task)

• -Consider the number of samples to be analyzed

• -Consider the complexity of the sample and the number of components in the sample

1C—Stepping Through A Typical Quantitative Analysis

• 2-Acquiring the Sample• -To produce a meaningful information, an analysis must be performed on a

sample whose composition faithfully represents that of the bulk of material from which it was taken

• -Bulk is large and heterogeneous• -Constituent parts can be distinguished visually or with the aid of a microscope

• -Assay is the process of determining how much of a given sample is the material indicated by its name

• -Analyze samples and determine substances• -Sampling involves obtaining a small mass of a material whose composition

accurately represents the bulk of the material being sampled• -Frequently the most difficult step in an analysis and the greatest source of error

1C—Stepping Through A Typical Quantitative Analysis

• 3-Processing the Sample• -This step may or may not be necessary, depending upon the type of sample you’re using,

whether it is already good to be used or not• -The first step in processing the sample is often the preparation of a laboratory sample

• -Preparing a Laboratory Sample– -A solid laboratory sample is ground to decrease particle size, mixed to ensure

homogeneity, and stored for various lengths of time before analysis begins– -Liquid samples should be prepared as not to allow evaporation because that

can change the concentration of the sample• -Defining Replicate Samples

– -Most chemical analyses are performed on replicate samples whose masses or volumes have been determined by careful measurements with an analytical balance or with a precise volumetric device

– -Replicate samples are portions of a material of approximately the same size that are carried through an analytical procedure at the same time and in the same way

• -Replication improves the quality of the results and provides a measure of their reliability through statistical analysis

1C—Stepping Through A Typical Quantitative Analysis

• 3-Processing the Sample

• -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.

• -Conversion of the analyte in insoluble materials into a soluble form is often the most difficult and time-consuming task in the analytical process

• -At this point in the analysis, it may be possible to proceed directly to the measurement step, but more often than not, we must eliminate interferences in the sample before making measurements

1C—Stepping Through A Typical Quantitative Analysis

• 4-Eliminating Interferences

• -Eliminate substances from the sample that may interfere with the measurement step

• -Species other than the analyte that affect the final measurement are called interferences

• -A scheme must be devised to isolate the analytes from interferences before the final measurement is made.

• -Techniques or reactions that work for only one analyte are said to be specific

• -Techniques or reactions that apply for only a few analytes are selective

1C—Stepping Through A Typical Quantitative Analysis

• 5-Calibration and Measurement

• -All analytical results depend on a final measurement X of a physical or chemical property of the analyte, which must vary in a known and reproducible way with the concentration of the analyte

• -Ideally, the measurement of the property is directly proportional to the concentration, where k is the proportionality constant

• -The process of determining k is called calibration

1C—Stepping Through A Typical Quantitative Analysis

• 6-Calculating Results

• -Computing analyte concentrations from experimental data is usually relatively easy

• -Computations are based on raw experimental data collected in the measurement step, the characteristics of the measurement instruments, and the stoichiometry of the analytical reaction

1C—Stepping Through A Typical Quantitative Analysis

• 7-Evaluating Results by Estimating Their Reliability

• -Analytical 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

1D—An Integral Role for Chemical Analysis: Feedback Control Systems

• -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.

• -The feedback system provides continuous monitoring and feedback to maintain the controllable quantity, and thus the actual state, at the proper level

1D—An Integral Role for Chemical Analysis: Feedback Control Systems

• -The feedback system goes as follows:

• 1-The desired state is determined

• 2-The actual state of the system is measured, and the two states are compared

• 3-The difference of the two states is used to change a controllable quantity that results in a change in the state of the system

• 4-Quantitative measurements are again performed on the system and comparison is repeated

• 5-The new difference between the desired and actual states is again used to change the state of the system if necessary.