Exam 1 Lecture 1 Flashcards
what does analytical chemistry deal with?
methods for determining the chemical composition of samples of matter; classified as what do we have? and how much do we have?
what are qualitative methods?
deals with the identity of one or more constituents of a substance (ex: does this mineral contain copper?)
what are quantitative methods?
provide numerical info of these components (ex: does this mineral contain enough copper to be an economical source of copper?)
hcG
hormone related to pregnancy
how does a home pregnancy test work?
urine containing hcG is applied to the sample pad and moves down the conjugate pad which contains antibodies for hcG. hcG and antibodies bind and continue to move down until they bind to further antibodies. if pregnant, two lines will glow. the second line always glows because it binds a conjugate reagent to it’s antibody
what does it mean if the control line of a pregnancy test is not glowing?
it’s a faulty test; no conjugate reagant at all… no antibodies for the conjugate pad to attach to
what is the chemical analysis process?
sampling, sample preparation, chemical analysis
sampling (chemical analysis process)
procuring a representative sample to measure
(have to average many samples)
(ex: choosing the representative parts of the chocolate bar)
sample preparation (chemical analysis process)
preparing the sample for analysis through: removing interferents, weighing the sample, quantitatively transfer the sample for analysis
(ex: find out how much caffeine is in the sample)
interferent
a species other than the analyte increases or decreases the response of the analytical method (interferents give inaccurate results and skew data)
chemical analysis (chemical analysis process)
chromatography, calibration curve
chromatography
separates analytes and measures the quantity of each compound
calibration curve
measure detector response for known concentrations of each analyte, graph peak height vs. concentration, prediction step (the response signal obtained for the analyte and used to predict its concentration from the calibration curve or best-fit equation)
what are the general steps in chemical analysis?
formulate the question, select the analytical procedure, sample, prepare the sample, analyze, report and interpret, draw conclusions
electronic balance
uses electromagnetic force to compensate for a load placed on the pan
readability: the smallest increment of mass that can be indicated
microbalance: measures milligram quantities with a readability of 1 microgram
hygroscopic reagents
reagents that capture moisture rapidly from the air (must weight by difference… weigh capped bottle of it, quickly pour your reagent into receiver, cap bottle again and weigh)
minimizing weighing errors
- minimize vibrations by locating analytical balances on a heavy table
- keep object to be weighted close to the center of the pan
- keep sample at ambient temperature (temp. of the surroundings) to prevent errors due to convective air currents
- minimize fingerprints (they can affect the apparent mass of an object)
balance troubleshooting errors
- erratic readings: balance door open, object hot or cold, static charge on object
- reading drifts down: liquid evaporating
- reading shifts up: hygroscopic material absorbing moisture from air
burets
precisely manufactured glass tubes; include graduations for measuring the volume of liquid delivered through the stopcock at the bottom
operating a buret
- wash the buret with new solution
- eliminate air bubbles in the stopcock before use
- read the bottom of the concave
- avoid the parallax
micropipet
- delivers volumes from 1 to 1000 microliters
- liquid held in disposable polypropylene tip, stable to most aq solutions and many organic solvents except chloroform (CHCl3)
- tip not resistant to concentrated nitric or sulfuric acids
- to prevent aerosols from entering pipet shaft, some tips have polyethylene filters
experimental error
- difference between “true” value and measured value
- error is associated with every measurement
- there is no way to measure the true value of anything, best we can do is carefully apply a technique that experience tells us is reliable
what are the two types of experimental errors?
random and systematic
systematic (determinate) error
- flaw in equipment or equipment design
- if you conduct experiment again in exact manner, error is reproducible
- discovered by analyzing a known sample, using different methods to measure same quantity, analyzing blank sample and observing nonzero result
- corrected by calibration of glassware and instruments
serial dilution
- process of making successive dilutions to obtain a desired concentration of reagent
- purpose is to accurately transfer small amounts of material that are too little to weigh accurately
- practice these!
random error (indeterminate error)
- arises from uncontrolled variables in measurement
- equal chance of being positive or negative
- always present; cannot be eliminated
- might be reduced with better technique
(ex: subjective reading of a scale which varies with individual, electrical noise in an instrument)
precision
describes the reproducibility of a result
(several times and values agree = precise) (dots close to each other on bullseye target)
accuracy
how close a measured value is to the “true” value
(dots all close to the bullseye on target but not really close together)
uncertainty
the variability within a set of measurements
absolute uncertainty
expresses the margin of uncertainty associated with any measurement (with units)
relative uncertainty
compares the size of the absolute uncertainty with the size of its associated measurement
relative uncertainty= absolute uncertainty/magnitude of the measurement
