CHEMISTRY 59-320CHEMISTRY 59-320ANALYTICAL CHEMISTRYANALYTICAL CHEMISTRY

Fall - 2012Fall - 2012

Lecture 2

Scales of Measurements

• (left) Carbon-fiber electrode with a 100-nanometer-diameter (100 × 10−9 meter) tip extending from glass capillary. The marker bar is 200 micrometers (200 × 10−6 meter). [From W.-H. Huang, D.-W. Pang, H. Tong, Z.-L. Wang, and J.-K. Cheng, Anal. Chem. 2001, 73, 1048.] (middle) Electrode positioned adjacent to a cell detects release of the neurotransmitter, dopamine, from the cell. A nearby, larger counterelectrode is not shown. (right) Bursts of electric current detected when dopamine is released. Insets are enlargements. [From W.-Z. Wu, W.-H. Huang, W. Wang, Z.-L. Wang, J.-K. Cheng, T. Xu, R.-Y. Zhang, Y. Chen, and J. Liu, J. Am. Chem. Soc. 2005, 127, 8914.]

The possible scale of measurements in analytical chemistry is astounding, ranging from the atomic level to the size of galaxies!

Chapter 1: Measurements1-1 SI Units

Using prefixes as multipliers

We customarily use prefixes for every third power of ten, e.g. 10-9. 10-6, 10-3 …

Converting between unitsExample: Express the energy 20 Calories in terms of ? kilojoules (kJ)

Oops! In 1999, the $125 million Mars Climate Orbiter spacecraft was lost when it entered the Martian atmosphere 100 km lower than planned. The navigation error would have been avoided if people had written their units of measurement. Engineers who built the spacecraft calculated thrust in the English unit, pounds of force. Jet Propulsion Laboratory engineers thought they were receiving the information in the metric unit, newtons. Nobody caught the error.

Why should we be concerned of units?

1-2 Chemical concentrations

A few concepts

• Solution: homogeneous mixture of two or more substances.

• Solute: a minor species in a solution.• Solvent: the major species in a solution.• Concentration: how much solute is contained in

a given volume or mass of solution or solvent.• Strong & weak electrolytes.

1-2 Chemical concentrations (Continued)

Molarity and Molality:Molarity (M) is the number of moles of a

substance per liter of solution.Molality (m) is concentration expressed as

moles of a substance per kilogram of solvent.

Problem 1-14. What is the formal concentration (expressed as mol/L = M) of NaCl when 32.0 g are dissolved in water and diluted to 0.500 L?

Answer: The molecular mass of NaCl is 58.44 g/mol The moles of sale in 32.0 g are 32.0 g /58.44(g/mol) = ……

1.3 Preparing solutions

con con dil dilM V M V

• Preparing a solution with a desired molarity

• Dilution

Problem 1-32: A bottle of concentrated aqueous sulfuric acid labeled 98.0wt % H2SO4, has a concentration of 18.0 M. HowMany millilitres of reagent should be diluted to 1.000 L to give 1.00 M H2SO4?

Solution:

1.4 Stoichiometry Calculations for gravimetric Analysis

One must make sure that the reaction equation is balanced!!

Gravimetric calculation (2)

• Problem 1-34: How many millilitres of 3.00 M H2SO4 are required to react with 4.35 g of solid containing 23.2% Ba(NO3)2 if the reaction is Ba2+ + SO4

2- →BaSO4(s)?

• Answer: (in class demon.)

1-5 Introduction to Titrations

• A titration is a form of volumetric analysis in which increments of reagent solution—the titrant—are added to analyte until their reaction is complete.

• The equivalence point occurs when the quantity of added titrant is the exact amount necessary for stoichiometric reaction with the analyte.

• The equivalence point is the ideal (theoretical) result we seek in a titration. What we actually measure is the end point.

• The difference between the end point and the equivalence point is an inescapable titration error.

More terminologies associated to titration

• Primary standard

• Standardization

• Standard solution

• Direct titration

• Back titration

• Gravimetric titration:

• Volumetric titration:

1-6 Titration calculationsExercise 1-E. A solution of NaOH was standardized by titration of a known quantity of the primary standard, potassium hydrogen phthalate (page 223):

The NaOH was then used to find the concentration of an unknown solution of H2SO4:

(a) Titration of 0.824 g of potassium hydrogen phthalate required 38.314 g of NaOH solution to reach the end point detected by phenolphthalein indicator. Find the concentration of NaOH (mol NaOH/kg solution).

(b) A 10.00-mL aliquot of H2SO4 solution required 57.911 g of NaOH solution to reach the phenolphthalein end point. Find the molarity of H2SO4.

Chapter 2: Tools of the Trade

• 2.1 Safe, ethical handling of chemicals and waste

-- Chemical experimentation creates hazards!

-- The primary safety rule is to familiarize yourself with the hazards and then to do NOTHING that you consider to be dangerous.

-- Minimize waste production and RESPONSIBLY dispose of waste.

-- Recycling chemicals.

-- Clean spills immediately.

-- Label all vessels to indicate what they contain.

What safety information can be obtained from the following label?

2.2 The lab notebook

• State what you did and what you observed in a way that it can be understood by a stranger.

• Record the names of computer files where programs and data are stored.

• Copy important data into your notebook. (Computers may crash and become garbage at any time).

2.3 Analytical balance

• Tare: the mass of an empty vessel that is used to receive the substance to be weighted .

• Sensitivity: the smallest increment of mass that can be measured.

• Chemicals should NEVER be placed directly on the weighting pan

• Handle the vessel you are weighting with a paper towel so that the tare value does not change.

Principle of operation (1)

Principle of operation (2)