C10- Chemical analysis Flashcards
Rf(retention factor)
A measurement from chromatography: the distance a spot of substance has been carried above the baseline(distance travelled by dissolved substance - solute) divided by the distance of the solvent front
Pipette
A glass tube used to measure accurate volumes of liquids
How to test the purity of a sample
By measuring its melting/boiling point and comparing it with the melting or boiling point of the pure substance(which you can find from a data book)
Consequences of impurities in a sample
Impurities in a sample will:
Lower the melting point and increase the melting range of a substance
Increase the boiling point and result in the sample boiling at a range of temperatures
Formulations
Useful mixtures with a precise purpose - with each component in a formulation present on a measured quantity, and contributing to the properties of the formulation so it meets its required function
Examples of formulations
Paints - composed of:
1) Pigment - gives the paint colour e.g. titanium oxide is used as a pigment in white paints
2) Solvent - used to dissolve the other components and alter the viscosity.
3) Binder(resin)- forms a film holding the pigment in place after it’s been painted on
4) Additives - added to further change the physical and chemical properties of the paint.
Uses of formulations
Pharmaceutical industry e.g. by altering the formation of a pill, chemists can ensure it delivers the drug to the correct part of the body at the right concentration, that it’s consumable and has a long enough shelf life(how long it stays fresh).
Cleaning products
Fuels
Cosmetics
How can you know a product’s a formulation?
Due to information on the product about its composition on the packaging(ratio/percentage of each component), lets you choose a formulation with the right composition for a particular use
Phases in chromatography
A mobile phase - where the molecules can move(liquid or gas).
A stationary phase - where the molecules can’t move(solid/really thick liquid)
Chromatography stages
1) During a chromatography experiment, the substances in the sample constantly move between the mobile and stationary phases - an equilibrium is formed between the two phases.
2) The mobile phase moves through the stationary phase, and anything dissolved in the mobile phase moves with it,How quickly a chemical moves depends on how it’s distributed(where it spends more time in) the mobile phase or stationary phase.
3) The chemicals that spend more time in the mobile phase than the stationary phase will move further.
4) The components in a mixture will normally separate through the stationary phase, as long as all the components spend different amounts of time in the mobile phase. The number of spots will change in different solvents as the distribution of the chemical will change depending on the solvent. A pure substance will only form one spot in any solvent as there’s only one substance in the sample.
Paper chromatography(phases)
Stationary phase - chromatography paper(often filter paper)
Mobile phase- solvent(e.g. ethanol or water)
Paper chromatography steps
Molecules with a higher solubility in the solvent and which are less attracted to the paper will spend more time in the mobile phase, so are carried further up the paper.
Calculating Rf values(required practical safety)
Be careful of any food colouring allergies
Capillary tubes can be fragile and care should be taken so they don’t break
Calculating Rf values(required practical) method
1) Draw a horizontal/pencil line across the chromatography/filter paper near the bottom. This is the origin.
2)Place a dot of each colour
on the line using a capillary tube.
3) Place the paper in a beaker so the bottom of the paper is just in the water. The solvent level should be below the origin line.
4) Put a lid on the beaker.
5) Use the ruler to measure the distance of each dot(colour) from the starting line. Let the solvent rise more than 75% of the paper and mark on the solvent front using a pencil.
6) Let the paper dry.
7) Measure from the origin to the centre of the colour. Calculate the Rf value of each colour.
Why is chromatography carried out?
To see if a certain substance is present in a mixture.
Run a pure sample of a substance(reference) alongside the unknown mixture and if the Rf values of the reference and one of the spots in the mixture match the substance may be present(although you haven’t yet proved they’re the same).
How solvent affects Rf value
If you change the solvent,the Rf value of a substance will change.
Can test both the mixture and reference in a number of different solvents:
1) If the Rf value of the reference compound matches the Rf value of one of the spots on the mixture in all the solvents, then the reference compound is present in the mixture.
2) If the spots in the mixture and reference only have the same Rf value in some of the solvents, then the reference compound isn’t present in the mixture.
Chlorine test
Chlorine bleaches damp litmus paper, turning it white(it may turn red for a moment first though, because a solution of chlorine is acidic).
Oxygen test
If you put a glowing splint inside a test tube containing oxygen, the oxygen will relight the glowing splint.
Carbon dioxide test
Bubbling carbon dioxide through an aqueous solution of calcium hydroxide/limewater causes the solution to turn cloudy.
Hydrogen test
If you hold a lit splint at the open end of a test tube containing hydrogen, you’ll get a “squeaky pop.”
Pure substance in everyday life
A substance that has had nothing added to it
What would happen to the rate of a reaction if you increase the temperature of a reaction by 10C(at room temperature)?
The rate of reaction will roughly double
Precaution before heating is stopped when collecting oxygen
End of delivery tube being removed from water to stop cold water being sucked back, into hot test tube causing it to crack
How the pattern of spots produced in a chromatography experiment can be used to distinguish a pure substance from an impure substance?
Pure substance - single spot on chromatography paper
Impure substance - multiple spots
Formulations
Alloys e.g. stainless steel cutlery
Medicines
Food
Paint
Fuels
Paper(mobile and stationary phases)
Mobile phase - solvent
Stationary phase - paper
TLC(mobile and stationary phase)
Mobile phase - Solvent
Stationary phase - powder
Column(mobile and stationary phase)
Mobile phase - solvent
Stationary phase - powder
Gas(mobile and stationary phase)
Mobile phase - gas
Stationary phase - powder
Paper chromatography: problem with start line in ink
Ink would run on paper
How paper chromatography separates substances like dyes in food colouring
Solvent moves through paper, carrying different substances(different distances), depending on their attraction to the paper and solvent
