Topic 5: Chemicals in analysis

Stages in analysis 5A

Qualitative: Type of chemicals present in sample

Quantitative: How much of each chemical is present in the sample

Standard procedure = agreed method of working

Safe – Effective – Accurate

Sample taken (represents bulk material)

Replicate samples ensure reliability -> find average
Easier to analyse when chemicals are in liquid

Accurately measure mass of samples and chemicals in it
Find a property to measureg. how much acid is needed to neutralise the alkali
Equation + conc. of acid is needed to find out -> volume of alkali needed to neutralise acid
Compare values from replicate samples

Closer values = more repeatable = more confidence

Sampling 5B

Representative samples give an accurate reflection of the material as a whole

Homogenous – composition is uniform Heterogeneous – composition varies

Considerations

No of samples + Mass of sample 2) No of sample repeats (ensures reliability
Storage (prevents tampering with) 4) Transport (prevents contamination)

Analysing water

Bottled water = homogenous b/c clear

Single sample needed -> straightforward procedure

Stream water = heterogeneous b/c may be: cloudy; contain organisms; samples vary in different parts/ times of year.

Lots of samples need to be stored -> complex procedure

Soil samples

Take the sample:

Where there is a change in the soil
Where you can’t see a change in the soil
Anywhere there is a risk of damaged/contaminated soil

Sports sample

Athletes pee in x2 bottles with witness watching to ensure no cheating
Bottles are sealed and labelled with code (don’t want any influences if the name is revealed)
Packaged in polystyrene
One bottle analysed. The other is put in a freezer as backup.

Paper Chromatography 5C

Chromatography is used to separate and then identify chemicals in a mixture.

Mobile phase is where the molecules can move, so in this case it is the solvent.

Stationary phase is where the molecules can’t move, so in this case it is the chromatography paper.

The type of solvent is important because some chemicals are more/less soluble in different solvents. The solubility enables you to separate the chemicals and identify them.

Types of solvents

Aqueous: Water (the chemicals only dissolve in water)

Non-aqueous: Not water (the chemicals dissolve in something other than water e.g. ethanol)

Process (qualitative)

Mixture/ substance is placed in a concentrated spot on the baseline of the paper

The paper is placed in a beaker containing a solvent. This is the mobile phase.
The spot must be above the solvent, only the end of the paper can touch the solvent

Solvent moves up the chromatography paper (stationary phase) and hits the sample spot

The chemicals in the sample spot dissolve in the solvent.

Some chemicals dissolve more = more soluble = less attracted to paper (SP) and more attracted to solvent (MP). They spend more time in the MP so they travel faster up the paper (SP).

Some chemicals dissolve less = less soluble = more attracted to paper and less attracted to solvent. So they travel slower up the paper.

When the molecules are moving at an equal rate between the SP and MP, they are at a dynamic equilibrium. This doesn’t mean that they are moving at equal speeds. It means that an equal number of molecules are transferring from either phase.

Each chemical is moving at a different speed so some may be travelling faster/ slower. This means that they have reached separate spots on the paper.

The colours have separated. You can identify the chemicals present but not the quantity of each.

Thin layer chromatography (TLC)

In TLC, The stationary phase is an absorbent layer of solid.

The mobile phase is a solvent.

Pros of TLC

Quick – Small volume of solution required
Lots of samples run at once – Cheap

Preparation of chromatogram

Reference materials: the known sample is run alongside the unknown sample.

If the two unknown sample has the same colour spots as the known sample, this means that they have the same chemicals present.

Sample is dissolved in a solvent that is not used as the MP

A small, concentrated drop of the sample is placed on the baseline and left to dry
If dilute, squeeze further drops on the spot
If sample spreads -> won’t separate properly b/c chemicals will mix

The MP solvent is placed in a tank which is covered with a lid. This lid makes the atmosphere inside saturated with a solvent vapour.

Place the chromatography paper in the tank. The spots must be above the level of the solvent.

The lid is closed again to prevent evaporation of the MP on the SP.

The molecules move between the MP and SP in dynamic equilibrium. The chemicals separate.

The paper is taken out before the solvent reaches the top of it.

The position that the last chemical travelled up is marked as the Solvent Front.

Mark the centre of the spots of colours with a pencil b/c the colours may fade.

Locating agents

Some spots may be colourless or faded. To locate them you need to use a:

Locating agent: reacts with substances and forms coloured compounds.

UV lamp used on fluorescent TLC plates, makes the spots appear violet.

Interpreting the chromatogram

The Retardation Factor (R.F) can be used to identify a chemical.

R.F = Distance moved by chemical (on the paper) / Distance moved by solvent (on the paper)

OR: C/S

Gas chromatography 5D

Separates complex mixtures
Measures how much of the chemical is present (quantitative)
Detects chemicals that are present in the mixture, but in small quantities

the harmful chemical is a contaminant.

In this case the:

MP = un-reactive gas Carrier gas
SP = viscous liquidg. oil

Process

Carrier gas supplies the MP which travels through a column.

Down the column there is an oven. Part of the column is placed in the oven. The hole where you insert the mixture (via a syringe) is located in the oven

The oven makes the chemicals in the mixture/sample turn into gas b/c of the high heat.
If you alter the oven temperature, you can separate your compounds because they have different boiling points.

The carrier gas is now travelling to the oven section where it mixes with the sample.

The gases travel to the column which is packed with SP.
The column is long, thin, coiled and sealed

The chemicals in the gas are distributed between the phases

The chemicals travel up the column and go through the detector

The detector sends signals to a recorder/computer every time a compound is identified

Interpreting chromatograms

Retention time: How long it takes the compound to pass the column

Peak height: Shows how much of the compound was present

Titrations 5E

Titration is quantitative b/c measured volume, concentration and quantity of solutions that react with each other.

Usually used for neutralisation of acid with alkali.
Add known volume of alkali to a conical flask.
Add indicator to the flask. This will change the solution’s colour at neutralisation, which will indicate the end point.
Fill a fixed volume of acid into the burette (more precise)
Use the burette to add the acid to the alkali drop by drop.
Swirl regularly to mix acid and alkali.
When you’re nearing the end point, add smaller drops.
The end point is reached when the colour changes and solution has neutralised
Record the volume of acid needed. (Starting volume of acid – remaining acid = acid needed)
Repeat titration x3 -> repeatable -> more confidence

Calculations

Concentration (g/dm³) = Mass (g) x Volume (dm³)

E.g. what mass of NaCl is in 8000cm³ of solution with concentration 9dm³?

Rearrange: Mass = Volume x Concentration 2) 28000 / 1000 = 0.8 dm³3) 9 X 0.8 = 2g

Interpreting results

Find the mass of the acid and alkali using M = V x C
Remember to covert scales
Find the RFM of the known solute
Find the RFM of the unknown solute using the formula:

Mass of known solute/ RFM of known solute = Mass of unknown solute = RFM of unknown solute

OR: Known Mass/ Known RFM = Unknown Mass/ Unknown RFM

Once you have the answer to the equation, find the RAM that matches it on the periodic table.

Worked example

E.g. 25cm³ of unknown MOH is at a concentration of 192 g/dm³ has been titrated with 40cm³ of HCL acid at a concentration of 182.5g/dm³. Determine what the metal hydroxide is.

192g of MOH per dm³. So in 0.025 dm³ (25cm³ / 1000) there is: 0.025 x 192 = 4.8g MOH

182.4G of HCL per dm³. So in 0.04 dm³ (40/1000) there is: 0.04 x 182.g = 7.3g of HCL

RFM of HCL (known): 1 + 35.5 = 36.5
RFM of MOH (Unknown) -> use the equation (above): 7.3 / 36.5 = 4.5 / RFM of MOH Rearrange: MOH = 24
You know the RFM of the OH part: 16 + 1 = 17.

So, 24 – 17 = 7. In the periodic table lithium has an RAM of 7.

So the unknown metal hydroxide in lithium hydroxide

Standard Solutions

Standard solution = accurate concentration of solution

Accurately weigh the mass of solute (remember M = V x C)
Dissolve solute in a small amount of solvent e.g. distilled water
Using a funnel, transfer the dissolved solution to a graduated flask and stir it.
Rinse the beaker with the solution with more solvent. Make sure that no solvent is clinging to it.
Add solvent drop by drop with pipette to reach the top of the mark on the flask.
Place a stopper and shake the flask to ensure all the particles are mixed up.

Evaluating results 5F

Results are given in a range b/c measurements have uncertainties.

E.g. 90 +/- 0.1 -> 90 is the average value but the true value lies between 89.9 and 90.1

Outliers are caused by human error or faulty equipments.

Random error – Repeated measurement gives different values

Systematic error – Repeated measurements give too high/low results than the true value

Accuracy: How close to the true value

Precision: Spread between measured values

Valid: Techniques and procedures fit for the experiment

Justifiable: Conclusion is backed by reliable evidence.