Analytical Chemistry

Lecture #1: Easy and old-school stuffDr. Khoi Nguyen

Spring 2011

What is Analytical Chemistry?

Chemistry

Organic Chemistry

Inorganic Chemistry Physical

Chemistry

Analytical Chemistry

What is Analytical Chemistry?

• Analytical Chemistry provides the methods and tools needed for insight into our material world… for answering four basic questions about matters:– What?– Where?– How much?– What arrangement, structure or form?

Analytical Chemistry

Analytical Chemistrymedicine

industry

Environmental quality

food

forensicsArchaeology

Space science

How analytical chemistry originate?

• Archimedes weighing gold wreaths.• By the alchemists.• Robert Boyle the term “analyst.”• Antoine Lavoisier is considered the

father of analytical chemistry.

The evolution of Analytical Chemistry

• Gravimetry• Titrimetry• First analytical textbook: “Anleitung zur Quantitaven

Chemischen Analyse” – by Karl Fresenius 1845.• Second analytical textbook: “Die wissenschaflichen

Grundagen der analytischen Chemie” by Wilhem Ostwald in 1894.

• Analytical Chemistry has been growing fast in the 20th century. In 1927, methods as electroanalysis, conductimetric titrations, colorimetric methods had been added to textbooks of this field.

Chemical AnalysisDefine the Problem

Select a sound method

Obtain a Representative Sample

Sample Preparation

Chemical Separation

Measurement

Data analysis

Defining a problem

• What exactly do you want answers for?

• Would the findings be scientifically useful?

• Is the problem expensive?

• What’s the degree of accuracy?

Method selection

• Sample type• Sample size• Sample preparation• Sensitivity• Accuracy/Precision• Instrumentation • Experience/Expertise • Cost• Speed

Obtain a Representative Sample

• Sample type/size/homogenerity

• Sampling statistics/errors

• Sample statistical distribution

Sample Preparation

• Solid/liquid/gas?• Soluble? • Ash/digest?• Chemical separation needed?• Chemical concentration needed?• Need to alter the analyte for easier detection?• Need to compromise the conditions?

Measurement

• Calibration

• Validation/controls/blanks

• Number of replicates

Data Analysis

• Statistical analysis

• Report results with the appropriate level of confidence

Rules of thumb• 1. never handle objects to be weighed with bare

hands (use clean paper, tongs, tweezers…)• 2. do not weigh objects when it’s hotter or colder

than room temperature.• 3. always use weighing boats and keep the pan clean

at all times.• 4. Close the balance case door while tarring and

carrying out the reading.• 5. when moving the balance or loading/unloading

the samples and weights, engage the beam/pan arrests (with mechanical balances)

How to choose weighing boats/dishes/bottles?

• Are you weighing a liquid or solid sample?

• How heavy is your object being weighed?

• Is your sample hygroscopic or non-hygroscopic?

• Is your sample reactive to certain materials?

Rough weighing - Accurate weighing

• Accurate weighings require meticulous procedures and of course and GOOD balance.

• Rough weighings are usually followed by some sort of titrations or adjustments.

Operating principle of an analytical balance

• Fig 2.2 and explanation

Sources of errors

• Zero-point drift (due to temperature, humidity, static)

• Variation in air density ( affecting the buoyancy)

• On going chemical/physical processes during the course of weighing

• Samples being hygroscopic• Different elevation of the locations

http://www.surveymonkey.com/s/9QC7Q3Z

Measuring volumes

Volumetric toys

Or just simply this

Glass Pipets: are used for medium volumes

• Transfer/volumetric pipets:– Are used to measure AND transfer a volume from

one place to another.– the interior of these pipets may not be uniform.

• Measuring/graduate pipets:– The interiors of these pipets are uniform.– Are often used for measuring volumes (duh!)These pipets can be blowout pipets or not.

Volumetric flasks

• Are used to measure large volumes.

Mirco-pipets, micro-syringe: for tiny amounts

Burets: used for titrations

• Looks like a graduate measuring pipet • Used with a clamp stand and a stop-cock

Rules of thumb

• Always measure volumes at room temperature (why?)

• Think of possible chemical processes may occur between your liquids and the volumetric toys. (why?)

• Think of the appropriate means according to the desired amounts.

• Perform the calibration, if needed.

Calibration?

• Wvac = weight in vacuum (g)

• Wair = observed weight in air (g)

• Do = density of object

• Dw = density of weights• 0.0012: density of air

Examples

Qualitative - Quantitative

• Qualitative: answers the questions of:– Is there A, B, C?– Is it good or bad?– How do things look/smell?

• Quantitative: answers the questions of:• How much of A, B, C are there?• How much is good, how much is too much, how

much is too little.

Accuracy and precision

Errors/Uncertainty in chemical analyses

• Determinate (systematic) errors: errors that follow a predictable pattern; therefore, can be corrected.– Instrumental errors– Operative errors– Errors come from the methodology implemented

• Indeterminate (random) errors: these are accidental. These can also be eliminated with mathematical tools.

Systematic errors

• Systematic errors tend to produce inaccurate results by introducing a common shift into measured values. This shift can be an offset or a percentage change. – For example, if your wooden meter stick had the first mm

cut off, there would be an offset in all of your measurements. If, on the other hand, the humidity in the room had caused the meter stick to expand by 1%, there would be a percentage error in all of your measurements.

• Incorrect calibration of the equipment can cause systematic error; reduce by equipment recalibration.

• Systematic errors might also be caused by not correctly accounting for some phenomena in your model and might be corrected by adopting a more sophisticated model.

• The effects of systematic errors on an experiment should be estimated.

• If important, systematic errors should be reported separately from the random errors in the experimental results.

• Systematic errors may have no effect on the slope of your data.

• However, it leads to an incorrect value for the intercept.

• Such systematic errors may or may not be important in an experiment, depending on

whether the slope or the intercept (or both) provide critical information.

Question: In what kinds of experiments, systematic errors could lead to an incorrect value for the slope?

Random errors

• Many sources of random errors: such as equipment limitations, reading uncertainties, and statistical fluctuations.

• Examples: the uncertainties in reading scale divisions of an analog voltmeter or a ruler.

• Repeated measurements may help. However, random errors can never be completely eliminated.

Errors

• The error analysis is generally more tedious than the calculation of the numbers being measured.

• However, measurements can be quite meaningless without knowledge of their associated errors.

• Why?

Errors

If you are told that Sue is 162 cm tall and Beth is 165 cm tall you might conclude that Beth is taller than Sue. But if you then learn that the measurements had errors of ±5 cm, you should realize that you can’t determine who is taller.

For every measurement, you MUST record the uncertainty in the measured quantity.

Internal errors

• For repeated measurements of the same quantity, statistical analysis can be used to study the uncertainties in our measurements. This type of analysis yields internal errors, – i.e., the uncertainties determined from the data

themselves without requiring further estimates.– mean, the standard deviation and the standard

error (error of the mean)

Mean (Average)

Standard deviation (σ) is defined as:

• Sample error:

• Sample error:

or

Two variables x and y?

• If we measure x and y N times and we want to compare the measured x and y with the relationship y=f(x).

In many cases, the quantity that we wish to determine is derived from several measured quantities.

Addition or subtraction

Constant*parameter

Multiplication/division

If

If

then

then

Uncertainty in a power

If

then

General formula for error propagation

Significant figures

• Are the number of digits needed to express the results of a measurement consistent with the measured precision.

• How many significant figures are there in these numbers:– 0.216 90.7 800.0 0.0607– 35.63 0.5481 0.05300 1.1689

Handling divisions and multiplications

• The answer of a multiplication or division can be no more accurate than the LEAST accurately known operator.

• Example:

Addition and subtraction

• The answer of an addition or subtraction is known to have the same number of units as the number containing the LEAST significant unit.

• Example:

Logarithms

• The mantissa determines the number of significant figures.

• Example:

Ways to indicate errors

• Absolute errors: errors expressed in the same units as the measurement.

• Relative errors: expressed in percentage of the measurements.

• Example:– The results of an analysis are 36.97 g, compared

with the accepted value of 37.06 g. What is the relative error in parts per thousand?

• Abs error: 36.97 g-37.06 g = -0.09 g• Relative error:

Hypothesis testing

• Statements: something you claim to be true.– E.g.: 80% of people love eating Phở. – Guy #1 argues: That is not true. (Ho) negating guy

– Guy #2 talks back: Yes, it is true. (H1)– Terminologies to memorize:

• (Ho): null hypothesis

• (H1): research hypothesis

– Testing hypotheses= testing the research statements, based on the data collected from experiments/observations.