Analytical Chemistry (UVU Chemistry 3000) Fall 2018

the absorption spectrum (visible)

320 Violet absorbed - 680-780 red absorbed -

How to do stoichiometry calculations.

Balance the equation. Convert units of a given substance to moles. Using the mole ratio, calculate the moles of substance yielded by the reaction. Convert moles of wanted substance to desired units.

Beers law

Beer's law states that absorbance is proportional to the concentration of the absorbing species

•Heterolytic cleavage:

Both electrons stay with one fragment

Gaussian error distribution

If an experiment is repeated a great many times and if the errors are purely random, then the results tend to cluster symmetrically about the average value (Figure 4-1). The more times the experiment is repeated, the more closely the results approach an ideal bellshaped curve called the Gaussian distribution.

What is a complex? When and why do they form? What effect do they have on solubility of ionic solids?

If anion X2 precipitates metal M1, it is often observed that a high concentration of X2 causes solid MX to redissolve. The increased solubility arises from formation of complex ions, such as MX2 2, which consist of two or more simple ions bonded to one another.

What information can we find using the Method of Continuous Variation? How does spectrophotometry help us do it

If one complex predominates, the method of continuous variation (also called Job's method) allows us to identify the stoichiometry of the predominant complex.

When Beer's Law Fails

If the absorbing molecule participates in a concentration-dependent chemical equilibrium, the absorptivity changes with concentration. For example, in concentrated solution, a weak acid, HA, may be mostly undissociated. As the solution is diluted, dissociation increases. If the absorptivity of A2 is not the same as that of HA, the solution will appear not to obey Beer's law as it is diluted.

How can mixtures be analyzed using absorbance?

If we know the spectra of the pure components, we can mathematically disassemble the spectrum of a mixture into those of its components a solution containing any number of species, the absorbance at any wavelength is the sum of absorbances of all species in the solution.

what is the equivalence point

It is when the moles of a standard solution (titrant) equal the moles of a solution of unknown concentration (analyte).

What are the different types of chemical concentrations?

Molality-the number of moles of solute per kilogram of solvent . Molarity- moles of solute per litre of solution Mole Fraction- It is equal to the moles of one component divided by the total moles in the solution or mixture Mole Ratio - the ratio of moles of one substance to the moles of another substance in a balanced equation. Mass Fraction - moles of one component divided by the total moles in the solution or mixture

What is the mass balance?

The mass balance, also called the material balance, is a statement of the conservation of matter. The mass balance states that the quantity of all species in a solution containing a particular atom (or group of atoms) must equal the amount of that atom (or group) delivered to the solution

To obtain a mass spectrum

To obtain a mass spectrum, gaseous species desorbed from condensed phases are ionized, the ions are accelerated by an electric fi eld, and then ions are separated according to their mass-to-charge ratio

what is the Charge balance?

The charge balance is an algebraic statement of electroneutrality: The sum of the positive charges in solution equals the sum of the negative charges in solution.

Linear Quadrupole Ion-Trap Mass Spectrometer

has higher trapping efficiency and higher storage capacity than the three-dimensional quadrupole ion trap. used a direct current voltage and a radio-frequency voltage to select ions of a particular m/z to be transmitted through the filter. The linear ion trap in Figure 22-19 adds sections at each end of the quadrupole to create a potential well. If the ends are sufficiently positive with respect to the center section, cations become trapped in the center section. Ions are confined in the radial direction (the xy-plane) by a radio-frequency field applied to the central section. By manipulating the voltages, ions of a specific m/z value can be expelled through slits in the x-direction or out the ends in the z-direction to one or more detectors.

What is titration error?

by definition, the difference between the volume of titrant added to reach the end point and the volume of titrant necessary to reach a stoichiometrically defined equivalent point.

describe electrolysis

electrolysis is a technique that uses a direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction.

How do Immunoassays work and when are they used?

employ antibodies to detect analyte An antibody is a protein produced by the immune system of an animal in response to a foreign molecule called an antigen. An antibody recognizes a specifi c antigen. The formation constant for the antibody-antigen complex is large, whereas the binding of the antibody to other molecules is weak.

What is voltammetry?

information about an analyte is obtained by measuring the current as the potential is varied

double-focusing mass spectrometer

ions ejected from the source pass through an electric sector before the magnetic sector. The electric sector only permits a narrow range of ion kinetic energies to pass to the magnetic sector.

Three-Dimensional Quadrupole Ion-Trap Mass Spectrometer

is a compact device that is well suited as a chromatography detector. substances emerging from the chromatography column enter the cavity of the three-dimensional ion-trap mass analyzer from the lower left through a heated transfer line. The gate electrode periodically admits electrons from the fi lament at the top into the cavity through holes in the end cap. Molecules undergo electron ionization in the cavity formed by the two end caps and a ring electrode, all of which are electrically isolated from one another. Alternatively, chemical ionization is achieved by adding a reagent gas such as methane to the cavity, ionizing it with the electron beam to form reagent ions, and then allowing reagent ions to react with analyte molecules. Some gas chromatography-mass spectrometry systems and all liquid chromatography-mass spectrometry systems produce ions outside the ion trap and inject them into the trap.

What is ionic strength? How does it affect the solubility of salts?

is a measure of the total concentration of ions in solution. The more highly charged an ion, the more it is counted. Addition of an "inert" salt increases the solubility of an ionic compound.

Weight percent=

mass of solute/mass of total solution or mixture x100

what is the end point and how is it detected?

reaction completion An endpoint is indicated by some form of indicator at the end of a titration.

Volume percent=

vol solute/vol total solution

What is a titration?

technique where a solution of known concentration is used to determine the concentration of an unknown solution. Typically, the titrant (the know solution) is added from a buret to a known quantity of the analyte (the unknown solution) until the reaction is complete.

The chelate effect is

the ability of multidentate ligands to form more stable metal complexes than those formed by similar monodentate ligands

Under what circumstances is the systematic treatment of equilibrium used? How is it applied?

a way to deal with all types of chemical equilibria, regardless of their complexity. After setting up general equations, we often introduce specific conditions or judicious approximations that allow simplification. Even simplified calculations are usually tedious, so we make liberal use of spreadsheets for numerical solutions.

Ways to detect systematic error:

1. Analyze a known sample, 2. Analyze blank samples containing no analyte being sought. 3. Use different analytical methods to measure the same quantity 4. Round robin experiment: Different people in several laboratories analyze identical samples by the same or different methods.

What is a Scatchard plot and how is it used?

A Scatchard plot is a plot of the ratio of concentrations of bound ligand to unbound ligand versus the bound ligand concentration. It is a method for analyzing data for freely reversible ligand/receptor binding interactions.

What's a Blank solution?

A blank solution is a solution containing little to no analyte of interest, usually used to calibrate instruments such as a colorimeter

A blank titration?

A blank titration is carried out by titrating a fixed and known concentration of titrant into a solvent with zero analyte. The only difference from the regular titration is the absence of analyte.

What is a galvanic cell?

A galvanic cell (also called a voltaic cell) uses a spontaneous chemical reaction to generate electricity.

How are K and ΔG related?

A non-spontaneous reaction has a positive delta G and a small K value. When delta G is equal to zero and K is around one, the reaction is at equilibrium. You have learned the relationship linking these two properties. This relationship allows us to relate the standard free energy change to the equilibrium constant.

What is a representative sample?

A random sampling in different random areas consisting of random heterogeneous material and segregated heterogeneous material

What are reference electrodes used for?

A reference electrode is an electrode which has a stable and well-known electrode potential.

absolute uncertainty

Absolute uncertainty expresses the margin of uncertainty associated with a measurement. If the estimated uncertainty in reading a calibrated buret is 60.02 mL, we say that 60.02 mL is the absolute uncertainty associated with the reading.

Accuracy

Accuracy: nearness to the "truth"

describe the methods of cyclic voltammetry

After the application of a linear voltage ramp between times t0 and t1 (typically a few seconds), the ramp is reversed to bring the potential back to its initial value at time t2. The cycle might be repeated many times

What is the selectivity coefficient?

An electrode intended to measure ion A also responds to interfering ion X. The selectivity coefficient gives the relative response to different species with the same charge

What is "Nernstian behavior"?

An electrode is said to behave "nernstially" if the equilibrium electrode potential obeys the Nernst equation when the concentration (strictly speaking, activity) of a species involved in the electrode reaction changes. Opposite: non-nernstian behavior

Internal Standards

An internal standard is a known amount of a compound—different from analyte—that is added to the unknown. Internal standards are especially useful for analyses in which the quantity of sample analyzed or the instrument response varies slightly from run to run. For example, gas or liquid fl ow rates that vary by a few percent

What is the ionic atmosphere?

An ionic atmosphere, shown as a spherical cloud of charge s+ or s-, surrounds ions in solution. The charge of the atmosphere is less than the charge of the central ion. The greater the ionic strength of the solution, the greater the charge in each ionic atmosphere.

How is the activity coefficient affected by ionic strength, ion size and charge?

As ionic strength increases, the activity coefficient decreases .The activity coefficient (g) approaches unity as the ionic strength (m) approaches 0. 2. As the magnitude of the charge of the ion increases, the departure of its activity coefficient from unity increases. Activity corrections are more important for ions with a charge of +/-3 than for ions with a charge of +/-1 3. The smaller the ion size (a), the more important activity effects become.

method assessment

Assessment is the process of (1) collecting data to show that analytical procedures are operating within specifi ed limits and (2) verifying that fi nal results meet use o bjectives

Buffer capacity

Buffer capacity, b, is a measure of how well a solution resists changes in pH when strong acid or base is added. Buffer capacity is defi ned as

Calculate concentrations

C1V1 = C2V2 M1V1 = M2V2 G/l= mass of solute(g)/Vol soln Molarity(M)=moles of solute/Vol of soln ppm=units of a solute/1mill units

What is a cathode or an anode?

Cathode: where reduction occurs Anode: where oxidation occurs

what is common ion effect?

Common ion effect: A salt is less soluble if one of its ions is already present in the solution.

Density =

Density= mass/volume=g/mL

Limits of detection

Detection limit. ... In analytical chemistry, the detection limit, lower limit of detection, or LOD (limit of detection), is the lowest quantity of a substance that can be distinguished from the absence of that substance (a blank value) with a stated confidence level (generally 99%).

Describe the EDTA tiration techniques: Direct, Back, Displacement, Indirect, and Masking.

Direct - analyte is titrated with standard EDTA Back - a known excess of EDTA is added to the analyte. Excess EDTA is then titrated with a standard solution of a second metal ion Displacement - Hg is treated with excess Mg(EDTA) to displace Mg which is titrated with standard EDTA) Indirect- Anions that precipitate with certain metal ions can be analyzed with EDTA by indirect titration. Masking -A masking agent is a reagent that protects some component of the analyte from reaction with EDTA

What is EDTA and how is it used?

EDTA is a versatile chelating agent. It can form four or six bonds with a metal ion, and it forms chelates with both transition-metal ions and main-group ions. EDTA is frequently used in soaps and detergents, because it forms a complexes with calcium and magnesium ions. One mole of EDTA reacts with one mole of metal ion.

When are auxiliary complexing agents used?

EDTA titration conditions in this chapter were selected to prevent metal hydroxide precipitation at the chosen pH. To permit many metals to be titrated in alkaline solutions with EDTA, we use an auxiliary complexing agent. This reagent is a ligand, such as ammonia, tartrate, citrate, or triethanolamine, that binds the metal strongly enough to prevent metal hydroxide from precipitating, but weakly enough to give up the metal when EDTA is added. Zn21 is usually titrated in ammonia buffer, which fi xes the pH and complexes the metal ion to keep it in solution.

What is a line notation?

Electrochemical cells are described by a notation employing just two symbols: | phase boundary || salt bridge

What is a Lewis acid? what is a Lewis base?

Electron pair acceptor Electron pair donor

Excitation vs Emission

Emission: variable wavelength. measure emitted radiation Excitation: constant wavelength. An excitation spectrum looks very much like an absorption spectrum because, the greater the absorbance at the excitation wavelength, the more molecules are promoted to the excited state and the more emission will be observed.

fraction of dissociation

Fd=[A-]/[A-]+[HA]

what is luminescence?

Fluorescence and phosphorescence are examples of luminescence, which is emission of light from an excited state of a molecule. Luminescence is inherently more sensitive than absorption.

What is the analytical process? Describe the steps in a chemical analysis.

Fq SAP A RI DC Formulate question Selecting analytical procedure Analysis analyte etc Reporting and interpretation Drawing conclusions

What are the 4 classes of ISEs?

Glass - pH Crystalline Gas sensing polymer membrane

Mixing a Weak Acid and Its Conjugate Base

If you mix A moles of a weak acid with B moles of its conjugate base, the moles of acid remain close to A and the moles of base remain close to B. Little reaction occurs to change either concentration.

What's a standard solution?

In analytical chemistry, a standard solution is a solution containing a precisely known concentration of an element or a substance, A known weight of solute is dissolved to make a specific volume. It is prepared using a standard substance, such as a primary standard.

What is the matrix of a sample?

In chemical analysis, matrix refers to the components of a sample other than the analyte of interest. The matrix can have a considerable effect on the way the analysis is conducted and the quality of the results obtained; such effects are called matrix effects.

standard addition

In standard addition, known quantities of analyte are added to the unknown. From the increase in signal, we deduce how much analyte was in the original unknown

Time-of-Flight Mass Spectrometer

Ions can be produced inside the source region by electron ionization in the gas phase or by laser irradiation of a solid sample on the surface of the backplate.

How does Le Châtelier's Principle relate to chemical equilibrium

Le Chatelier's principle is an observation about chemical equilibria of reactions. It states that changes in the temperature, pressure, volume, or concentration of a system will result in predictable and opposing changes in the system in order to achieve a new equilibrium state.

What is mass spectrometry?

Mass spectrometry is a technique for studying the masses of atoms or molecules or fragments of molecules.1,4 A mass spectrum displays the number of ions detected at each value of mass to-charge ratio,

What are matrix effects?

Matrix effect is the effect on an analytical method caused by all other components of the sample except the specific compound to be quantified. Matrix effects and selectivity issues have long been associated with bioanalytical techniques.

What are the two general classes of indicator electrode?

Metal electrodes described in this section develop an electric potential in response to a redox reaction at the metal surface. Ionselective electrodes described later are not based on redox processes. Instead, selective binding of one type of ion to a membrane generates an electric potential.

How do metal ion indicators work?

Metal ion indicators are compounds that change color when they bind to a metal ion. Useful indicators must bind metal less strongly than EDTA does.

What is a metal-chelate complex? What are they created from?

Metal ions are Lewis acids, accepting electron pairs from electron-donating ligands that are Lewis bases. Cyanide (CN2) is called a monodentate ligand because it binds to a metal ion through only one atom (the carbon atom). Most transition metal ions bind six ligand atoms. A ligand that attaches to a metal ion through more than one ligand atom is said to be multidentate ("many toothed"), or a chelating ligand

Understand titration calculations.

MolaracidVolacid = MolarbaseVolbase A 25 ml solution of 0.5 M NaOH is titrated until neutralized into a 50 ml sample of HCl. What was the concentration of the HCl? Step 1/2Determine the number of moles of OH # of moles OH- = (0.5 M)(.025 L) Determine the number of moles of H+ same as OH when neutralized Determine the concentration of HCl Molarity of HCl = (0.0125 mol)/(0.050 L) Molarity of HCl = 0.25 M The concentration of the HCl is 0.25 M.

Precision

Precision: reproducibility

How are the ions created for introduction into the spectrometer?

Molecules entering the ion source are converted into ions by electron ionization. Electrons emitted from a hot filament (like the one in an incandescent light bulb) are accelerated through 70 V before interacting with incoming molecules. Some (~0.01%) molecules (M) absorb as much as 12-15 electron volts (1 eV 5 96.5 kJ/mol), which is enough for ionization:

Ohm's Law

Ohm's law states that current, I, is directly proportional to the potential difference (voltage) across a circuit and inversely proportional to the resistance, R, of the circuit.

Homolytic cleavage:

One electron remains with each fragment

relative uncertainty

Relative uncertainty compares the size of the absolute uncertainty with the size of its associated measurement. The relative uncertainty of a buret reading of 12.35 6 0.02 mL is a dimensionless quotient

What is Ksp?

Solubility product constant is simplified equilibrium constant (Ksp) defined for equilibrium between a solids and its respective ions in a solution. Its value indicates the degree to which a compound dissociates in water. The higher the solubility product constant, the more soluble the compound.

Ex.Preparing a Solution with a Desired Molarity

Solution An 8.00 mM solution contains 8.00 x10 -3mol/L. We need 8.00 x 10-3 mol/L x 0.500L = 4.00 x 10-3 mol CuSO4 x 5H2O The mass of reagent is (4.00 x 10-3 mol) x(249.68 g/mol)= 0.999 g.

What is a standard potential?

Standard Electrode Potentials. In an electrochemical cell, an electric potential is created between two dissimilar metals. This potential is a measure of the energy per unit charge which is available from the oxidation/reduction reactions to drive the reaction.

Errors in pH Measurement

Standards Junction Potential Junction potential Drift Sodium error acid error equilibration time Hydration of glass Temp Cleaning

students t is?

Student's t is a statistical tool used most frequently to express confidence intervals and to compare results from different experiments. It is the tool you could use to evaluate the probability that your red blood cell count will be found in a certain range on " normal" days.

Systematic error(determinate) is

Systematic error is a consistent error that can be detected and corrected.

What is the activity of a substance? How does the activity coefficient relate? When is it used?

The activity of species C is its concentration multiplied by its activity coefficient. The activity coefficient measures the deviation of behavior from ideality. If the activity coefficient were 1, then the behavior would be ideal

Buffer

The buffer is a mixture of an acid and its conjugate base

Weak Is Conjugate to Weak

The conjugate base of a weak acid is a weak base. The conjugate acid of a weak base is a weak acid.

describe the methods of polarography

The dispenser suspends one drop of mercury from the bottom of the capillary. After current and voltage are measured, the drop is mechanically dislodged. Then a fresh drop is suspended and the next measurement is made. Freshly exposed Hg yields reproducible current-potential behavior.

Transmission Quadrupole Mass Spectrometer

The electric field deflects ions in complex trajectories as they migrate from the ionization chamber toward the detector, allowing only ions with one particular mass-to charge ratio to reach the detector. Other ions (non resonant ions) collide with the rods and are lost before they reach the detector. Rapidly varying voltages select ions of different masses to reach the detector. Transmission quadrupoles can record 2-8 spectra per second

What is the difference between the formation constant, Kf, and the conditional formation constant, K'f?

The equilibrium constant for the reaction of a metal with a ligand is called the formation constant, Kf , or the stability constant: With the conditional formation constant, we can treat EDTA complex formation as if all free EDTA were in one form.

Define Resolving power

The higher the resolving power of a mass spectrometer, the better it is able to separate two peaks with similar mass

Sig Figs

The number 1.4270 x 102 has five significant figures. 9.25 x 104 -3 significant figures 9.250 x 104- 4 significant figures 9.2500 x 104 -5 significant figures

Ohmic potential

The ohmic drop (IR drop) describes the over- potential due to the electron flow through a material. In electrochemistry, it often refers to the potential induces by the resistance of the electrolyte or any other interface

Orbitrap Mass Spectrometer

The orbitrap is a high-resolution mass analyzer that does not require a magnetic field or a radio-frequency field

chromophore

The part of a molecule responsible for light absorption is called a chromophore

What happens at the interface of a glass electrode to allow for a pH electrode to work?

The two surfaces swell as they absorb water. Metal ions in these hydrated gel regions of the membrane diffuse out of the glass and into solution. H+ can diffuse into the membrane to replace metal ions. The reaction in which H+ replaces cations in the glass is an ion-exchange equilibrium. A pH electrode responds selectively to H+ because H+ is the main ion that binds significantly to the hydrated gel layer.

What is an ion-selective electrode (ISE)?

They respond selectively to one ion. These electrodes are fundamentally different from metal electrodes in that ion-selective electrodes do not involve redox processes. The key feature of an ideal ion-selective electrode is a thin membrane capable of binding only the intended ion.

How can we use spectrophotometry to find equilibrium constants?

To measure an equilibrium constant, we measure concentrations (actually activities) of species at equilibrium. Spectrophotometry can be used for this purpose. We examine a graphical procedure called the Scatchard plot

transmittance

Transmittance, T, is defined as the fraction of the original light that passes through the sample.

Use objective:

Use objective: states purpose for which results will be used

What is the method of Least Squares

We use the method of least squares to draw the "best" straight line through experimental data points that have some scatter and do not lie perfectly on a straight line.5 The best line will be such that some of the points lie above and some lie below the line

Absorption of light

When a molecule absorbs a photon, the energy of the molecule increases. We say that the molecule is promoted to an excited state. If a molecule emits a photon, the energy of the molecule is lowered. The lowest energy state of a molecule is called the ground state.

What is the junction potential and why is it so important in ISEs?

Whenever dissimilar electrolyte solutions are in contact, a voltage difference called the junction potential develops at their interface. This usually small voltage (usually a few millivolts) is found at each end of a salt bridge connecting two half-cells. The junction potential puts a fundamental limitation on the accuracy of direct potentiometric measurements, because we usually do not know the contribution of the junction to the measured voltage.

Describe what random heterogeneous material

a material in which there are differences in composition w/ no pattern or predictability and on a fine scale. when you collect a portion of the material for analysis, you obtain some of each of the different compositions

What is meant by the confidence interval?

a range of values so defined that there is a specified probability that the value of a parameter lies within it.

describe the methods of stripping voltammetry

analyte from a dilute solution is concentrated into a thin fi lm of Hg or other electrode material, usually by electroreduction. The electroactive species is then stripped from the electrode by reversing the direction of the voltage sweep. The potential becomes more positive, oxidizing the species back into solution. Peak current measured during oxidation is proportional to the quantity of analyte that was deposited

What does an analytical chemist do?

analyze chemicals in food etc qualitatively and quantitatively

random (indeterminate) error is

arises from uncontrolled (and maybe uncontrollable) variables in the measurement. Random error has an equal chance of being positive or negative. It is always present and cannot be corrected. There is random error associated with reading a scale.

what is concentration polarization?

concentration polarization denotes the part of the polarization of an electrolytic cell resulting from changes in the electrolyte concentration due to the passage of current through the electrode/solution interface.

What are overpotentials

overpotential is the potential difference (voltage) between a half-reaction's thermodynamically determined reduction potential and the potential at which the redox event is experimentally observed. The term is directly related to a cell's voltage efficiency.

How do the different types of indicators work?

pH indicators used for acid-base titration end point detection are weak acids or bases

Primary standard?

primary standard is typically a reagent which can be weighed easily, and which is so pure that its weight is truly representative of the number of moles of substance contained.

what is alphay4-

relates to EDTA and its 6 ligands

Indicator (working) electrode:

responds to analyte activity

In general, the most intense peaks correspond to

the most stable fragments.

Standardization?

the process of determining the exact concentration (molarity) of a solution. Titration is one type of analytical procedure often used in standardization. In a titration, an exact volume of one substance is reacted with a known amount of another substance.

what is Fluorescence?

the visible or invisible radiation emitted by certain substances as a result of incident radiation of a shorter wavelength such as X-rays or ultraviolet light. the property of absorbing light of short wavelength and emitting light of longer wavelength.

What are u and o?

u-the mean o-standard deviation

Wavelength, l, is the crest-to-crest distance between waves. Frequency, n, is the number of complete oscillations that the wave makes each second

wv x Freq = c (speed of light)

what is segregated heterogeneous material

• a material in which differences in composition are on a large scale • different regions have obviously different composition

What are the advantages of an ISE?

• less expensive than competing techniques such as atomic spectroscopy and ion chromatography • linear response to log A over a wide range • nondestructive • noncontaminating • short response time • unaffected by color or turbidity

Specifications might include:

• sampling requirements • accuracy and precision • rate of false results • selectivity • sensitivity • acceptable blank values • recovery of fortification • calibration checks •quality control samples