AC Exam 1

C0 | Analytical chemistry

The branch of chemistry concerned with identifying what substances are present and determining how much of each substance is present in a sample.

C0 | Qualitative analysis

The process of determining the identity of the constituents of a substance.

C0 | Quantitative analysis

The process of measuring how much of a constituent is present in a substance.

C0 | Analyte

The substance being analyzed or the chemical substance of interest being measured.

C0 | Species

An element, compound, or ion of interest; the word is both singular and plural.

C0 | Sampling

The process of selecting and collecting a portion of material that represents the whole sample being studied.

C0 | Homogeneous

Having the same chemical composition throughout.

C0 | Heterogeneous

Not uniform throughout; composition differs from region to region.

C0 | Random heterogeneous material

A material whose composition differences occur randomly and on a fine scale.

C0 | Random sample

A bulk sample constructed by taking portions of the entire lot at random.

C0 | Segregated heterogeneous material

A material in which composition differences occur on a large scale, producing obviously different regions.

C0 | Composite sample

A representative sample prepared from heterogeneous material by taking portions from the different regions in amounts proportional to their sizes.

C0 | Standard solution

A solution whose composition is known because it was made from a reagent of known purity or through reaction with a known quantity of a standard reagent.

C0 | Calibration curve

A graph of a measured property or detector response versus analyte concentration, used to determine the concentration of an unknown.

C0 | Aqueous

In water; an aqueous solution uses water as the solvent.

C0 | Aliquot

A portion of a sample or solution used for an individual analysis.

C0 | Slurry

A suspension or heterogeneous mixture of solid particles dispersed in a liquid.

C0 | Supernatant liquid

The liquid remaining above a solid after precipitation or centrifugation; also called supernate.

C0 | Decanting

Carefully pouring liquid off a solid or another immiscible liquid while leaving the undesired material behind.

C0 | Quantitative transfer

Moving a sample from one vessel to another without losing material, usually by rinsing the original vessel and transferring the rinses.

C0 | Sample preparation

Transforming a representative sample into a form suitable for chemical analysis, often by dissolving, concentrating analyte, and removing or masking interferents.

C0 | Interference/interferent

An effect or species other than the analyte that changes the analytical response and makes the measured amount appear too high or too low.

C0 | Masking

Transforming an interfering species into a form that is not detected by the analytical method.

C0 | General analysis step 1

Formulate the question by translating a general question into specific questions that can be answered through chemical measurement.

C0 | General analysis step 2

Select analytical procedures by consulting the chemical literature or developing an appropriate procedure.

C0 | General analysis step 3

Sample: select representative material to analyze.

C0 | General analysis step 4

Prepare the sample by converting the representative sample into a form suitable for analysis.

C0 | General analysis step 5

Perform the chemical analysis by measuring analyte concentration in replicate aliquots and evaluating variability/uncertainty.

C0 | General analysis step 6

Interpret and report the results clearly, completely, and with relevant limitations for the intended audience.

C0 | General analysis step 7

Draw conclusions from the reported results.

C0 | Why is representative sampling essential?

A poorly chosen sample or a sample that changes between collection and analysis can make the analytical result meaningless: 'garbage in, garbage out.'

C0 | Why use replicate measurements?

Replicates assess variability/uncertainty and help guard against a gross error in a single aliquot.

C0 | Why might different analytical methods be used?

To check that the result is consistent across methods and that the selected method is not biasing the result.

C0 | Why can several bulk samples be analyzed?

To evaluate variation introduced by the sampling procedure.

C0 | Chocolate-bar sample preparation example

Fat was removed because it could interfere with chromatography, and the desired analytes were dissolved.

C0 | Core idea of analytical chemistry workflow

The process begins with a question, includes sampling, preparation, measurement, and reporting, and ends with a conclusion; chemical analysis is only the middle portion.

C0 | Replicates and standard deviation

Multiple replicate results can be used to assess reproducibility; standard deviation describes the variability of the replicate results.

C28 | Lot

The total material from which samples are taken.

C28 | Bulk/gross sample

A sample taken from a lot for analysis; it must represent the lot.

C28 | Laboratory sample

A smaller, homogeneous sample formed from the bulk sample that should have the same composition as the bulk sample.

C28 | Aliquots in sample preparation

Smaller portions of the laboratory sample used for individual analyses.

C28 | Why sample storage matters

Sample composition can change with time because of chemical reactions, exposure to air, or interactions with the storage container.

C28 | Why can glass alter trace-ion concentrations?

Glass can act as an ion exchanger, allowing mobile ions near its surface to exchange with ions in solution.

C28 | Hydration layer

A swollen, gel-like layer formed when water penetrates the outer few nanometers of glass.

C28 | Ion swapping in glass

Mobile ions near the glass surface diffuse out into solution.

C28 | Charge balancing in glass

Ions from solution diffuse into the glass matrix to occupy vacant negative sites created by ion exchange.

C28 | Recommended storage-container practice

Plastic, especially Teflon, is frequently used; plastic containers should be washed with distilled/deionized water or another suitable solvent before storage.

C28 | Laboratory notebook and sample history

The notebook should document how a sample was collected, stored, handled, and analyzed.

C28 | Overall sampling variance equation

For random errors, overall variance is the sum of analytical-procedure variance and sampling variance: s_o² = s_a² + s_s².

C28 | Variance

The square of the standard deviation; it reflects variability in sampling or analysis.

C28 | Sampling standard deviation

The standard deviation in the number or composition of analyte particles entering a sample due purely to random sampling.

C28 | Why does larger sample mass reduce sampling uncertainty?

A larger sample contains more particles, so random differences in the number of analyte particles become smaller relative to the expected amount.

C28 | Sampling constant K_s

A measure of how much sample mass is required to achieve a specified sampling precision; it corresponds to the mass that would give an expected sampling RSD of 1%.

C28 | Small K_s

Indicates relatively homogeneous material, easier representative sampling, and good precision with smaller sample masses.

C28 | Large K_s

Indicates heterogeneous material, more difficult representative sampling, and a need for larger sample masses.

C28 | Relationship between sample mass and RSD

Sampling variance decreases as sample mass increases; the slides express this using the sampling constant relationship involving K_s, sample mass m, and RSD R.

C28 | Choosing sample replicates

The number of replicate samples can be selected using the sampling standard deviation, desired uncertainty, confidence level, and Student's t.

C28 | Requirement for s_s and e in replicate calculations

The sampling uncertainty s_s and desired uncertainty e must be expressed in the same type of units: both absolute or both relative.

C28 | Student's t for estimating sample replicates

For 95% confidence, t is taken from the appropriate degrees of freedom; because n is initially unknown, t for n approaching infinity can be used to estimate n, then the calculation is repeated until n stabilizes.

C28 | Important assumption in sample-replicate calculations

The analytical uncertainty is assumed to be much smaller than the sampling uncertainty unless analytical uncertainty is explicitly included.

C28 | Why grind solid samples?

To make the laboratory sample more uniform and chemically accessible and to help ensure it has the same composition as the bulk sample.

C28 | Why dissolve/digest the entire sample?

If the entire sample is not dissolved or digested, not all analyte is available for quantitative analysis.

C28 | Steel mortar and pestle

Relatively inexpensive but can contaminate samples with iron, nickel, or chromium and can corrode.

C28 | Agate mortar and pestle

Chemically inert and a standard choice in analytical chemistry, although expensive and brittle.

C28 | Boron carbide mortar and pestle

Much harder than agate and less prone to contaminate samples.

C28 | Acid dissolution of metals

Nonoxidizing acids such as HCl, HBr, HF, H3PO4, dilute H2SO4, and dilute HClO4 can dissolve metals capable of reducing H+ to H2.

C28 | Oxidizing acids for difficult metals

Oxidizing acids can dissolve metals that do not react with nonoxidizing acids by providing a stronger oxidizing environment.

C28 | Aqua regia

A mixture of concentrated HCl and HNO3 used as a powerful oxidizing/dissolving medium for metals that resist individual acids.

C28 | Hydrofluoric acid (HF) caution

HF is useful for dissolving silicate-containing materials but is highly hazardous; its use requires appropriate safety controls.

C28 | Fusion

A sample-preparation method in which a solid is heated with a reagent so that the material forms a melt or a soluble product that can subsequently be dissolved.

C28 | Acid digestion

Dissolving or decomposing a sample using acids, often with heat, to place analytes into solution for analysis.

C28 | Dry ashing

Heating a sample strongly in air/oxygen to destroy organic matter and leave a residue containing the desired inorganic components.

C28 | When is dry ashing useful?

The slides identify stable elements such as Fe, Cu, Mg, Ca, and Zn as suitable examples; it is relatively simple and has low reagent-related contamination risk but can be time-consuming.

C28 | Wet ashing

Chemical decomposition of organic material using liquid reagents, often oxidizing acids or oxidizing systems.

C28 | Why is wet ashing common?

It can reduce loss of volatile metals and produces a liquid sample that can be directly used with analytical instruments.

C28 | Fenton's reagent

The Fe2+/H2O2 system used to oxidize organic material in dilute aqueous solutions.

C28 | Extraction

Separating an analyte from a matrix based on differences in solubility between phases.

C28 | Liquid-liquid extraction (LLE)

Separating a compound according to its relative solubility in two immiscible liquids, commonly an aqueous phase and an organic solvent.

C28 | Solid-liquid extraction (SLE)

Removing a soluble analyte from a solid matrix by dissolving it into an appropriate liquid solvent.

C28 | Derivatization

Chemically converting an analyte into a derivative with properties that make it easier to separate, detect, or measure by the chosen analytical method.

C28 | Slurry after grinding

A heterogeneous mixture of solid particles dispersed in liquid; centrifugation can produce a supernatant above the packed solid.

C28 | Quantitative transfer in sample preparation

Transferring the entire amount of material from one vessel to another so the complete analyte amount is accounted for.

C1 | SI system

The internationally accepted system of measurement based on fundamental SI units from which derived units are built.

C1 | Seven SI base quantities

Time, length, mass, electric current, thermodynamic temperature, amount of substance, and luminous intensity.

C1 | SI base unit for time

Second (s).

C1 | SI base unit for length

Meter (m).

C1 | SI base unit for mass

Kilogram (kg).

C1 | SI base unit for electric current

Ampere (A).

C1 | SI base unit for temperature

Kelvin (K).

C1 | SI base unit for amount of substance

Mole (mol).

C1 | SI base unit for luminous intensity

Candela (cd).

C1 | Avogadro constant

6.02214076 × 10^23 mol^-1; exactly defined as the number of elementary entities in one mole.

C1 | Force

A derived quantity measured in newtons: N = kg·m/s².

C1 | Pressure

A derived quantity measured in pascals: Pa = N/m² = kg/(m·s²).

C1 | Energy/work/heat

A derived quantity measured in joules: J = N·m = kg·m²/s².

C1 | Power

A derived quantity measured in watts: W = J/s = kg·m²/s³.

C1 | Frequency

Measured in hertz: Hz = 1/s.

C1 | Electric charge

Measured in coulombs: C = s·A.

C1 | Electric potential

Measured in volts: V = W/A.

C1 | Electrical resistance

Measured in ohms: Ω = V/A.