Techniques Flashcards
What is gravimetric analysis used to do?
to determine the mass of an element or compound in a substance
In gravimetric analysis, how is the mass of the analyte present in a substance determined?
The substance is converted into another solid substance (with an appropriate reagent) of known chemical composition which can be readily isolated, purified and weighed.
The conversion can either occur through precipitation or volatilisation.
In gravimetric analysis, what is heating to a constant mads/steps of gravimetric analysis
Used to dry final product completely
heating the substance->allowing it to cool in a dessicator (dry atmosphere so no water is absorbed)->reweighing it all in a crucible
This is repeated until a constant mass is obtained (shows no water)
In gravimetric analysis, when heating to a constant mass, what are the conditions to heating with a bunsen burner?
the lid should be left partially off to allow water to escape
A blue flame should be used to avoid a built up of soot on the outside of the crucible (could affect mass)
Heating should be started off gently and then more strongly
What does volumetric analysis involve?
using a solution of known concentration (standard solution) in a quantitative reaction to determine the concentration of the other solution
In volumetric analysis, what procedure is used?
titration in the form: standard, complexometric or back titrations
What do titrations involve?
measuring one solution quantitatively into a conical flask using a pipette . The other solution is added to a burette until a permanent colour change of an indicator is seen in the conical flask.
A rough titration is carried out first followed by more accurate ones until concordant titre values are achieved (+-0.1ml). The average is used in calculations.
What can a standard solution be directly prepared from?
a primary standard
What characteristics must a primary standard have?
a high state of purity (>99.9%)
stability in air and water
solubility (usually in water)
reasonably high formula mass (reduces errors)
standard solution preparation steps
-calculate mass of primary standard
-weigh out primary standard as accurately as possible
-dissolve primary standard in small volume of deionised water in beaker
-transfer solution and all rinsings into standard flask
-make the solution up to mark with deionised water
-i vert the stoppered flask several times to mix
substances used as a primary standard
oxalic acid
sodium carbonate
potassium hydrogen phthalate
potassium iodate
potassium dichromate
substances like sodium hydroxide cannot be used ad they readily absorb h2o and co2 from atmosphere
What does the use of controls in chemical reactions do?
validates a technique and may consist of carrying out a determination on a solution of known concentration to give a referencing point on which to base all other results from impure samples e.g. in the determination og vitamin c content in fruit juice, a sample of absorbic acid (pure vitamin c)woulf be analysed
what is a back titration used for?
to find the number of moles of a substance by reacting it with an excess volume of reactant of known concentration. This resulting mixture is then titrated to work out the number of moles of reactant in excess.
Useful when trying to work out quantity of substance in an i solublesolif
What relationship does colorimetry use?
between colour intensity of a solution and the concentration of the coloured species present to determine concentration
What is distillation used for?
to purify and identify a compound by separating it from a non-volatile or less-volatile material by boiling points
-stirrer/heat plate
-anti-bumping granules
-round-bottom flask
-condenser with water out (top) and water in (bottom)
-cooling bath
-thermometer
What is refluxing?
a technique used to apply heat energy to a chemical reaction mixture over an extended period of time
-round-bottom flask
-anti-bumping granules
-condenser
-dropping funnel
-thermometer
Why is vacuum filtration used instead of normal filtration?
faster
What can vacuum filtration be carried out using?
buchner, hirsch or sintered glass funnel
What is recrystallisation used for?
To purify solids based upon solubility
What is thin-layer chromatiography used for?
To assess product purity
weighing accurately approximately
weigh out as close as possible to a certain mass but ensure that you record the actual mass given on the balance
weighing by difference
a way to weigh materials accurately
the weight of the material is the difference between the two weights of the ‘vial’ before and after you transfer the material from the vial
why is deionised water used to make a standard solution
tap water contains dissolved salts that may react with our sample and affect the concentration of the solution
What is standardisation?
the process of exact determination of concentration of a solution
You use primary standard A to determine the concentration of a solution B (not a primary standard). This is done by titrating B (in a conical flask) with A (in burette)
Once you have found the concentration of B, you can use B to titrate other substances
cv stock=cv diluted
benefits of refluxing
prevents vapours escaping
allows extended heading
more vigorous heating
to dissolve/extract maximum
more efficient
Colorimetry-how is a calibration graph produced?
1-standard solutions prepared
2- a suitable filter used
3 - mention of a blank / solvent
only Measurement
4- absorbance of each is plotted
colorimetry-how to find concentration of unknown
the absorbance of unknown is measured
and
a mention of using the calibration graph
What is a primary standard?
Primary standards are materials containing a known concentration of a substance. They provide a reference to determining unknown concentrations or to calibrate analytical instruments
recrystallisation steps
-dissolve an impure solid gently i. a minimum volume of hot solvent
-hot filtration of the resulting mixture to remove any insoluble impurities
-cooling the filtrate slowly to allow crystals of the pure compound to form, leaving soluble impurities dissolved in the solvent
-filtering, washing and drying the pure crystals
How is the solvent for crystallisation chosen?
so that the compound being purified is completely soluble at high temperatures and only sparingly soluble at lower temperatures
How do you make a solution from a solution?
c1v1=c2v2
then e.g. ‘Measure 50cm3 of stock solution with a pipette and transfer to a 250cm3 standard flask. Add deionised water up to the mark, stopper and invert
Simple explanation of
-TLC
-Heating under reflux
-Recrystallisation
-Distillation
-TLC separates chemicals with different polarities/size
-Heating under reflux allows volatile chemicals without reactant escape
-Recrystallisation allows chemical to be extracted from impurities due to solubility in a second solvent
-Distillation separates chemicals with different boiling points
Explain whether gravimetric or volumetric analysis is more suitable for measuring substances at low concentrations.
Volumetric analysis is more suitable for low concentrations because it can
accurately measure small volumes of solution using precise apparatus like burettes and pipettes.
In contrast, gravimetric analysis relies on forming and weighing a solid precipitate, but at low concentrations, too little solid may form to be weighed reliably.
This makes gravimetric analysis less accurate and less practical for low concentrations.
What properties do appropriate gravimetric analysis reagents have?
the precipitate must be of LOW SOLUBILITY
high purity and known composition
name a step in gravimetric analysis that you forget
test filtrate to ensure no more precipitate forms
What happens in gravimetric analysis
a precipitate is formed
SO
(aq) + (aq) -> (s)
What is best to measure
1. Cl- concentration in sea water and
2. Ca2+ concentration in milk
- volumetric analysis
- complexometric analysis titration (volumetric)
recrystallisation steps
1-minimum/small volume
2-hot solvent
3-solvent cool
what properties must a solvent have for
crystallisation?
-doesn’t react with solute
-solute more soluble in hot than cold solvent
-impurities to be soluble in hot solvents
-boiling point
-polarity
What must the filter be for colorimetry?
opposite colour to transmitted wavelength (so the absorbed wavelength)
how would you remove separated samples from a furnace
gas-›collected at top and cooled in condenser
liquid->collected at bottom
does distance solvent travels affect the Rf
No as solvent and solute are proportional
what affects rf?
solvent used
How would make filtration faster?
Use vacuum filtration
Fluted shape filter paper
What does drops of silver nitrate on the filtrate do?
check reaction is complete (e.g. all Cl- ions have reacted)
How would you reduce the uncertainty in a titre value?
titrate larger samples
dilute sample less
use class A glassware
What do you need to mention when describing how a standard solution is made in the
transferring to standard flask part and the
made up to mark part
SOLUTION is transferred to the standard flask
Made up to the mark WITH DEIONISED WATER
Gravimetric-What happens in precipitation conversion?
-The substance undergoes a precipitation reaction
-The precipitate is separated from the filtrate
-The filtrate is tested to ensure the reaction had gone to completion
-The precipitate is washed, dried to a constant mass and then weighed
Gravimetric-What happens in volatilisation conversion?
The substance is heated and any volatile products (often water) are evaporated.
The substance is heated to a constant mass and the final mass is recorded.
Why may a sample for colorimetry be diluted
because the absorbance was outwith the calibration range
Why may experimentally determined concentration be higher than the actual value?
other ingredients/impurities may be reacting with the reactant
control for an experiment to find concentration
pure substance
