Practical Session 1 Flashcards
What is Laboratory Practical 1 about?
Accuracy and Measurement.
What are the aims of the practical 1?
- To show how to use correctly the weights and measure equipment in science to get accurate and precise results.
- To calculate the accuracy and precision of results.
- To show the correct use of sample t-tests.
What are the types of analysts?
Forensic scientists.
Chemists.
Biologists.
Environmental scientists.
What are the analysts required for?
To weigh and measure volumes of different types of materials, solids or liquids.
What is the purpose of this lab?
To revise the analytical skills.
For what are the analytical skills essential?
Sampling.
Accurate measure substances.
How is sampling and accurately measure substances known as?
Quantitative analysis.
On what does the type of balance used to weigh materials depend?
On the accuracy and the amount to be weighed.
What is the rule about weighing and accuracy?
The smaller the amount to be weighed, the greater the accuracy required.
What is used when only a rough weight is required?
A top-loading/top-pan balance.
What is the weigh of the balances used for a rough weight?
0.1g - 300g.
What is the accuracy of the balances used in a rough weight?
0.1g with 2-3 significant figures.
For what are the analytical balances used?
For accurate work.
How much do the materials weigh in analytical balances of accuracy?
1mg-50g.
How precise are the weighing materials in analytical balances of accuracy?
1 in 100,000.
At their maximum capacity.
How many significant figures does the analytical balance give for weights of 100mg/more?
At least 4.
What is the modern analytical balance?
A single pan.
What is the function of the single pan as a modern analytical balance?
Direct reading electronic balance.
Weigh objects rapidly.
On what does the quality of any sampling or analysis technique depend?
On all the glassware that the sample has been in contact with directly.
What should all the glassware be?
Clean.
On what does the glassware used depend?
On the volume measured.
On the accuracy required.
Which are the precision glassware?
Volumetric flasks.
Volumetric pipettes.
Volumetric burettes.
Which are the inexact glassware?
Beakers.
Conical flasks.
Measuring cylinders.
How are all glassware available?
In different capacities.
By what is the precision expressed?
By the uncertainty of a reading.
What is required to get the degree of precision required in analytical methods involving volumetric measurements?
A quantity is ‘accurately measured’.
What must the devise chosen be?
Suitable to the volume required.
When is the devise chosen suitable to the volume required?
When bearing in mind volumetric flasks and pipettes.
When should careful choices be made?
When using graduated pipettes and burettes.
What would we use if we are required to pipette 0.10mL?
A 1mL graduated pipette.
How is accuracy/tolerance expressed?
As the maximum allowable error.
How many ‘grades’ do volumetric flasks and pipettes come?
2.
What are the 2 grades volumetric flasks and pipettes come?
Grade A.
Grade B.
By what are the 2 grades of volumetric flasks and pipettes accompanied?
By an expected tolerance/’limit of error’.
At what degrees are tolerance/’limit of error’ of grade A and B quoted?
At 20 oC.
What will the glass do?
Expand/contract.
What will happen to the volume of a glassware when it will expand?
It will be slightly different at different temperatures.
Not enough to matter if temperature stays close to 20 degrees.
What happens to the limit of error (mL) when volume increases?
It increases.
What happens to the limit of error as a percentage when volume increases?
It decreases.
How much is the limit of error of grade B glassware compare to A?
Grade B has twice the limit of error as grade A.
How can the volume of grade A 100mL volumetric flask be expressed?
As 100+/- 0.08mL.
As what must the volume of grade A 100mL volumetric flask be written in terms of significant figures?
As a volume of 100mL.
Why must the volume of grade A 100mL volumetric flask be written as 100mL in terms of significant figures?
Because the first decimal place is uncertain.
What do volumetric flasks do?
They make up standard solutions of accurately known concentrations.
What is the method description of making up standard solutions of accurately known concentrations with using volumetric flasks?
- Weigh desired amount using appropriate balance and a weighing boat.
- Transfer material to a beaker.
- Rinse weighing boat twice with small amounts of solvent.
- Dissolve material in a beaker.
- Transfer this solution to the flask with a funnel.
- Rinse the beaker 3 times with small portions of solvent.
- Transfer into funnel/flask slowly.
- Transfer all the solution into the flask.
What should happen to the funnel after all the solution is transferred into the flask?
It should be rinsed 3 times with small portions of solvent.
Transferred into the flask.
What do we have to make sure during the method of solutions using volumetric flasks?
The liquid level does not rise above etched mark on the flask.
What should we never do to a volumetric flask?
Heat it to dissolve the sample.
Why we should never heat a volumetric flask to dissolve the sample?
Because it may break.
What is really important for the solution?
It should be thoroughly mixed.
How can diluting to the etched mark be done?
By adding solvent from a wash bottle.
By using a Pasteur pipette and a small clean beaker with fresh solvent.
What is the use o pipettes?
They transfer aliquots/measured volumes accurately of liquids from one container to another.
Why are the pipettes transfer liquid from one container to another?
For analysis or dilution.
What we must do before using a pipette?
Rinse it with the solution we will use.
How many different types do pipettes have?
2.
Which are the types of pipettes?
- Volumetric/transfer pipettes.
2. Measuring/graduated pipettes.
What is the use of volumetric/transfer pipettes?
They deliver a single volume: 5, 10, and 25mL of a liquid.
A small amount of the liquid remains in the tip when it is emptied –> must be blown out.
What is the use of measuring/graduated pipettes?
They deliver variable quantities of a liquid.
They can have a scale or a blow-out volume.
What do the pipettes with a blow-out volume scale have printed close to the manufacturer’s name?
BLOW-OUT.
What happens when a blow-out pipette is used?
The small amount of the liquid remaining in the tip when it’s emptied must be used to obtain the correct volume.
What does the pipette have printed on it when it does not have a blow-out scale?
DELIVERY.
Or it does not have anything printed.
What happens to the pipette when it does not have BLOW-OUT printed and it has nothing or DELIVERY on it?
The small amount of the liquid that remains in the tip when it is emptied must not be blown out.
What are the main rules of correct use of a pipette?
- Never suck the liquid into the pipette by mouth.
- Use a pipette bulk to suck the liquid inside, 2cm above stem mark/line.
- Wipe outside of the pipette with paper towel.
- Allow pipette to drain slowly down to the line by releasing pressure.
- Place tip of the pipette on the inside wall of the container –> known volume of the liquid –> transferred –> allow liquid to drain.
How can the concentration units be expressed?
In Molarity, M.
Weight/volume vasis (wt / vol) and volume / volume basis (vol/ / vol).
How is the molarity of a solution expressed?
As the number of moles of the solute per litre/dm3 of the solution.
Or as a number of millimoles/millilitre.
How can the results for liquid samples be reported?
On a weight/volume basis.
Or on a volume / volume basis.
What is the: weight/volume basis?
The mass of the solute / volume of the solution.
What is the: volume / volume basis?
The volume of the solute / the volume of the solution.
How are the weight/volume basis and volume/volume basis expressed?
As percentages: % (w/v) or % (v/v).
What units are used for trace concentrations?
Smaller units.
What are the most common units used for concentrations?
Parts / million.
Parts / billion.
How are parts / million expressed?
mg /L.
ug / mL.
How are parts/ billion expressed?
ug /.
ng / mL.
What is the equation of parts / million?
Parts per million (ppm), wt/vol = [ wt. of solute (g) / vol. of sample (mL) ] x 10000000.
What is the equation of parts / billion?
Parts per billion (ppb), wt/vol = [ wt. of solute (g) / vol. of sample (mL) ] x 10000000000.
What is often required based on the dilutions?
To prepare dilute solutions from more concentrated stock solutions.
With what are the moles of the solute / analyte taken for dilution from the stock solution same?
With the moles in the final diluted solution.
What is the eqaution of dilutions?
C1 x V1 = C2 x V2.
What does the C1 x V1 = C2 x V2 mean?
C1 = concentration of stock solution in the given units.
V1 = volume of the stock solution taken for dilution.
C2 = concentration of the diluted solution in the same units.
V2 = volume the diluted solution.
How should we record our measurements?
In triplicate.
What must the results indicate?
Reproducibility.
Uncertainties.
Precision in the measurements.
How can we record our measurements?
With statistical measure.
What statistical measure should we use to record our measurements?
Relative standard deviation.
Significant figure.
Why should we use statistical measure to record our measurements?
To indicate reliability of a final reported result.
What do we have to define for the statistical measure to record our measurements?
Arithmetic mean/average.
Accuracy.
Precision.
Population and standard deviation.
What is the arithmetic mean/average?
The sum of the individual measurements divided by the number of measurements.
How is the arithmetic mean/average expressed mathematically?
X = (X1+ X2+ X3 + … + Xn ) / N = ΣXi / N.
What does the mathematic expression of the arithmetic mean/average mean?
X = mean. Xi = individual results. N = total number of measurements. Σ = summation.
What does accuracy define?
How close the result / best value is to the true value, x, which is known.
What does the difference between an individual result and the true value give?
The absolute error.
How is the absolute error divided by the true value expressed?
In percent ( x 100). Or parts / thousand ( x 1000).
What does the absolute error divided by the true value define?
The relative error.
What does precision define?
The closeness of the results in replicate measurements.
How can the precision be calculated?
- Largest minus, smallest value measured –> range/spread of the results.
- Range / mean value in percent –> relative range.
- Absolute value of difference between given result and the mean –> deviation.
- Average of individual deviations for all replicate measurements –> average deviation.
What happens when the number of results is infinite/ >20?
The population mean: x, is taken as equal to the true value of the measured quantity.
What happens when the number of results, N is small?
The replicate measurements/observations are called a sample.
Mean value, X differs from x.
When does the difference between X and x decrease?
As N increases to a value > 20.
What does the standard deviation measure?
The precision.
What is the standard deviation?
An indicator of the scatter of data.
How is the standard deviation, S defined for a sample/small data set?
S = [Σ (Xi - X)2 / (N - 1) ]1/2.
What does the definition of the standard deviation, S, mean?
( Σ(X1-X) = (a-x)2 + (b-x)2 + (c-x)2 + (d-x)2 ) .
Difference di = (x1 - x)2.
std. dev. S = route of Σdi2 / n-1.
What do standard deviations and measured parameter have?
Same units.
How many results should at least obtained?
3.
Why should at least 3 results be obtained?
To justify the use of the equation of standard deviation and estimate it.
What is the relative standard deviation?
The standard deviation divided by the mean.
How is the relative standard deviation expressed?
In %.
What does the relative standard deviation measure?
Precision.
What is the equation of the relative standard deviation?
RSD (%) = (S/X) X 100.
How is the equation of the relative standard deviation known?
As the coefficient of variation, CV.
What do the relative standard deviation and the coefficient of variation allow?
To compare one set of data with another, with a classmate.
Weighing 1: Solids
You will be given a vial containing a white powder and some plastic weighing boats.
- Accurately weigh approximately 1 g of the white powder.
- Record your data (to 3 decimal places).
- Repeat steps 1 and 2 three times (you should have weighted out the powered 4 times.
- Calculate the mean, standard deviation and % coefficient of variation (CV) of your data.
- Reflect on the accuracy and precision of your weighing technique and record suitable comments in your lab notebook
Weighing 2: Liquids
You are provided with a conical flask containing ultra-pure water (UPW), a 10 mL volumetric pipette + filler and 5 small beakers.
- In turn weight each beaker and then accurately pipette 10 mL of UPW into it. Reweigh the beaker + UPW. Record all your data in tabular form in your lab notebook.
- Calculate the mean and standard deviation and %CV of the 5 masses of UPW.
- Reflect on the accuracy and precision of your pipetting technique and record suitable comments in your lab notebook.
Dilution 1: Method
You will be given a solution of potassium permanganate (0.1 M).
- Rinse all equipment before use and after use with UPW. Rinse 5 mL pipette with 0.1 M potassium permanganate solution to remove any water from the water wash before filling the pipette (volumetric flasks do not need this step as we are adding water to our sample anyway).
- Accurately pipette 5.0 mL of the 0.1M potassium permanganate solution into a 100 mL volumetric flask and make up to the mark with UPW. Calculate the concentration of this solution.
- Accurately pipette 1.0, 2.0, 3.0, 4.0, and 5.0 mL of this new solution into 5 x 100mL flasks and make each up to the mark with distilled water. Calculate the concentrations of these solutions.
- Fill 5 cuvettes with each of the above solutions and fill another cuvette with UPW for use as a blank. Using a spectrometer record the absorbance of the solutions at 530nm. Tabulate your results in your lab notebook.
- Plot a graph of Absorbance at 530 nm against solution concentration. Draw a line of best fit and calculate the gradient of the line. Examine you graph carefully and identify any solutions that deviate from the line.
- Reflect on and comment on you graph and try to explain any anomalous results.
- Outside of the lab plot your result in excel and compare the gradient calculated from the graph and from excel.
Dilutions 2: Method
You will be given a solution of potassium permanganate of unknown concentration.
- Note which solution you were given A, B, C or D.
- Accurately pipette 10.0 mL of this solution into a 100 mL volumetric flask and make up to the mark with UPW.
- Accurately pipette 5.0 mL of this solution into another 100 mL volumetric flask and make up to the mark with UPW.
- Accurately pipette 1.0 mL of this solution into another 10 mL volumetric flask and make up to the mark with UPW.
- Measure the absorbance of this final solution at 530 nm.
- Using the graph prepared above, determine the concentration of the final solution.
- Also, using the line equation from the graph above, calculate the concentration of the final solution.
- With both values and the dilution factor you used to prepare the final solution, calculate the concentration of the original unknown potassium permanganate solution.
- Record all your data and calculations in your lab notebook.
- You will need to enter your data on the spreadsheet at the front of the class.
- Use the pooled data to carry out a 1 sampled T-test analysis of the results (this will be covered in the statistics sessions)
