Praktijkvoorschriften pw6 1
Praktijkvoorschriften pw6
Determination of zinc in multivitamin tablet using atomic
absorption spectrophotometry
Determination of ethanol in beer using GC method: internal standard
Determination of phosphate in diet Coca Cola using visible spectrophotometry
Determination of caffeine in diet Coca Cola using HPLC
Determination of fat content of coffee cream using extraction
Determination of citric acid in Hubba-Bubba chewing gum using acid-base titration
Determination of total acid content of fruit juice using acidbase titration
Determination of phosphoric acid in diet Coca Cola
using potentiometry.
Determination of sugar using polarimetry
Het onderdeel calculation is bij de voorschriften weggelaten.
Dit wordt behandeld in de theorielessen.
Determination of zinc in multivitamin tablet using atomic
absorption spectrophotometry
1. Scope
The method is suitable to determine the zinc content of effervescent multivitamin tablets (max. 8 mg/tablet) that usually contain several metal ions and other substances (sugar, sweeteners, etc.) as well.
2. Principle
The tablets under investigation are dissolved in water. The zinc content is
measured by atomic absorption spectrometer (AAS) after adding hydrochloric
acid to the solution. A calibration curve is made by using zinc solutions with
known concentration and then the zinc concentration of the sample solution can be determined by using the calibration curve. The zinc content of the sample (effervescent tablets) is calculated from the concentration of the sample solution.
3. Apparatus
3.1. Equipment
3.1.1. Instruments
• Calibrated analytical balance
• Atomic absorption spectrometer (Type e.g.: SHIMADZU AA-680)
3.1.2. Glassware
• 100 mL volumetric flasks (calibrated as Class ‘A’)
• 200 mL volumetric flasks (calibrated as Class ‘A’)
• 1000 mL cylinder
• 5 mL pipette (calibrated as ‘Class A’)
• 10 mL pipette (calibrated as ‘Class A’)
• 10 mL burette
• 1 mL syringe
• 200 mL beaker (tall)
• Weighing funnels
• Funnels
• Beaker
• Watch-glass
3.1.3. Other equipment
• Filtering stand
• Filtering ring
3.2. Materials and their CAS numbers: see Table 1.
Table 1: Materials and their CAS numbers
Name CAS
Zinc oxide 1314-13-2
(e.g.:Fluka zinc oxide, pss. p. a.)
Concentrated hydrochloric acid
(e.g.:Fluka hydrochloric acid fuming, pss. p. a.)
7647-01-0
3.3. Reagent solutions
3.3.1. ZnCl2 standard solution (1 mg/ mL Zn2+ content)
Weigh 0.1245 g of ZnO on an analytical balance as precisely as possible and dissolve it in 5 mL concentrated hydrochloric acid. Wash this solution into a 100 mL volumetric flask using distilled (or ion exchanged) water, fill it to the mark and mix well.
Note: Working with concentrated hydrochloric acid is very dangerous.
The operator must know and obey the safety rules!
3.3.2. ZnCl2 solution (0.05 mg mL Zn2+ content)
This is made with a 20 times dilution of the ZnCl2 standard solution: pipette 5 mL of the ZnCl2 standard solution, wash it into a 100 mL volumetric flask using distilled water, fill it to the mark and mix well.
3.3.3. Hydrochloric acid solution (0.02 mol/ L)
About 1 mL concentrated hydrochloric acid (using a 1 mL syringe) is diluted to 600 mL, in a cylinder, with distilled water and mixed well.
4. Preparation
4.1. Preparation of apparatus
4.1.1. Calibration of the analytical balance
Calibrate the analytical balance according to the instruction manual.
4.1.2. Calibration of the volumetric flasks and pipettes
See on a separate part of the web site.
4.2. Preparation of sample
Weigh each of the tablets in the box on an analytical balance and calculate the average mass (m). Put one of the tablets (make a note about its mass) into a tall 200 mL beaker, fill the beaker up to 1/3 of its height with distilled water, cover it with a watch-glass and wait until it is completely dissolved. Wash all the material spattered on the watch-glass into the solution with distilled water, mix the solution and wash it into a 200 mL volumetric flask. Fill the flask, to the mark, with distilled water and mix well.
If the solution is completely clear and sediment-free, pipette 10 mL of it into a 100 mL volumetric flask and then fill it up to the mark with 0.02 mol/L hydrochloric acid. (This way we model the effect of the hydrochloric acid found in the stomach). In this case the preparation of the sample is finished. If the solution is not completely clear and sediment-free shake the contents of the 200 mL volumetric flask thoroughly and pour about half of the solution into a 3 200 mL beaker put it onto a magnetic stirrer. While stirring, pipette 10 mL of the solution into a 100 mL volumetric flask, fill it up to the mark with 0.02 mol/L hydrochloric acid and mix well. (Thus we model the case when the consumer drinks the stirred solution.)
Let the remaining solution settle in the 200 mL beaker. Carefully pipette 10 mL of the solution into a 100 mL volumetric flask (so that the sediment is not stirred up), fill it up to the sign with 0.02 mol/L hydrochloric acid and mix well the solution. (This way we model the case when the consumer drinks the settled solution without its sediment.)
4.3. Preparation of the series of the calibrating solutions
This is done by diluting the 0.05 mg/mL ZnCl2 solution, so that we have a series of calibrating solutions with the mass concentration of 1, 2, 3, and 4 mg/L Zn2+ respectively. This means that using a 10 mL burette we have to measure 2, 4, 6 and 8 mL parts of the 0.05 mg cm-3 ZnCl2 solution into 100 mL volumetric flasks fill them up to the mark with 0.02 mol/L hydrochloric acid, mixing well.
5. Procedure
The atomic absorption spectrometer is switched on and the zinc lamp is adjusted if necessary. After the warming up time and the self-test of the instrument, the necessary parameters (e.g. lamp number, lamp current, wavelength, slit, measuring method, integration time, concentrations of the calibration solutions, etc.) are set and then the measurement is started.
Atomise (vaporize) the 0.02 mol/L hydrochloric acid solution as BLANK, the
calibrating solutions as STANDARDs and then (once the calibration is finished)
the UNKNOWN solution(s) (the one with sediment and the other without
sediment in the latter case of the point 4.2). As well the absorbance, the
instrument gives the measured mass concentration (ρ [mg/L]).
7. Expression of results
The results of the measurements are given in [mg Zn2+/tablet] units, e.g. 3.6 mg Zn2+/tablet.
Questions
Choose the correct answer!
1. The temperature of the acetylene – air flame is
a) 1500 - 2000 K
b) 2000 - 2500 K
c) 2500 - 3000 K
d) 3000 - 3500 K
2. How does the solution under investigation get into the flame?
a) A pump presses it.
b) Its flow is helped by gravity.
c) The gases feeding the flame suck it.
d) An auxiliary gas sucks it.
3. The lamp used in atomic absorption spectrometers is a
a) tungsten bulb
b) deuterium lamp
c) mercury vapour lamp
d) hollow cathode lamp
4. The wavelength of the absorbed light
a) depends on the type of flame.
b) is characteristic of the quality of the substance under investigation.
c) is characteristic of the quantity of the substance under investigation.
d) does not depend on the quality of the substance under investigation.
5. The absorbance
a) depends only on the concentration
b) is proportional to the concentration
c) is inversely proportional to the concentration
d) does not depend on the concentration
Choose the answer that is incorrect!
6. Among the following events which is the one that is not caused by the heat of the flame
a) The evaporation to dryness of the solution.
b) The evaporation of the substance.
c) The atomisation.
d) The excitations of the atoms.
7. Choose the statement that is not true! Using metal halogenide compounds in atomic absorption spectrometry is advantageous, because
a) they are not corrosive substances.
b) they usually dissolve easily.
c) they evaporate easily.
d) they can be easily atomised.
8. Choose the incorrect statement! It is not good if large drops get into the
flame, because
a) the flame cools down.
b) the evaporation and the atomisation will be uneven.
c) too many atoms get into the flame.
d) the substance is atomised to an unsatisfactory extent.
Determination of ethanol in beer using GC
method: internal standard
1. Scope
The goal of this analytical procedure is to determine the percentage ethanol in beer and other beverages. Dutch normal beer must contain no more than 5 vol% ethanol.
2. Principle
We add the same amount of 1-propanol to each solution to be measured. The
analyte is injected into a GC with polar column. The height of two peaks for ethanol and propanol is measured. By calculating the ratio (height ethanol : height propanol), the result does not depend on the injection volume or any air bubbles present in the needle during injection. Comparing the ratio with a calibration of a known ethanol concentration will give the exact amount of ethanol in the sample.
3. Apparatus
3.1. Equipment
3.1.1. Instruments
· Gas chromatograph with polar column Carbowax or Tenax. Nitrogen
carrier gas and FID detector. (capillary column could also be used)
3.1.2. Glassware and other equipment
· 6x25 mL volumetric flasks (calibrated as Class ‘A’)
· pipettes
· GC injection syringe 0-5 micro litre
3.2. Materials and their safety codes
Name No. Remarks
ethanol 64-17-5 absolute or 100% ethanol is not recommended
95 vol% is satisfactory
1-propanol 71-23-8 99 vol% is satisfactory free of other alkanoles
acetone 67-64-1
4. Preparation
Prepare the gas chromatograph as described in the user manual. Make sure that temperatures of oven, injector and detector are stabilised.
Column 1.6 metre Carbowax 20 M by GC- use:
Carrier gas Nitrogen 20 mL per minute
Oven 100 oC
Detector FID 150 oC
Injector 150 oC
Range 10-9
Attenuation 1-1024 to give max peak heights
Note: Use the attenuator to give the largest peaks possible. This will give better measurements of the peak heights on paper. Since both peaks will be equally larger or smaller at different degrees of attenuation, this will have no effect on the ratio.
5.Procedure
Preparation of calibration curve:
Take 4 volumetric flasks of 25 mL and pipette into each flask 1 mL
1-propanol.
Put respectively 0.5 ; 1 ; 2 and 3 mL ethanol in the flasks. Fill up with distilled water and mix well.
Preparation of the beer sample:
De-gas 30 cL Heineken beer (in can or small flask) by adding a few drops of acetone.
Pipette 2x 20 mL into two volumetric flasks of 25 mL
Add 1 mL 1-propanol to each flask and fill up with distilled water.
Inject 1 μL of the calibration solutions from low to high ethanol vol%
Measure the height of both peaks.
Inject 1 μL of the duplo sample Heineken Beer.
Measure the height of both peaks.
6. Expression of results
The results will be given in the % by volume (v/v%) of beer sample.
A conclusion should be drawn: whether the average of the duplo does not exceed 5 vol%
7. Precision
The precision of this method is 10 % based on 20 student results.
8. Questions
1
The main goals of gas chromatography are to:
I Separation of mixtures into their components
II quantative determination of components
a Normally first I followed by II
b Normally II first, followed by I
c Its doesn’t matter in what order I,II or II,I
d I and II both are not main goals.
2
For good separation the injector must be heated:
a For better gas flow
b To evaporate the sample
c To remove any fluctuations in temperature
d For warmer components.
3
When the sample contains polar components then:
a A polar stationary phase will lead to better separation
b A polar stationary phase will decrease retention times.
c A non-polar stationary phase will increase retention times.
d Polar or non-polar it doesn’t make any difference.
4
The injector temperature and detector temperature must be set :
a Lower than the column temperature to avoid condensation
b Equal in temperature to the column.
c Higher than the column temperature to avoid condensation.
d to any temperature lower of higher.
5
I Using a FID any water in the sample will cause an extra peak.
II Using a catharometer (TCD) will give a air-peak from any air in the needle.
a I is right but II is wrong.
b II is right and II is wrong.
c Both I and II are right.
d Both I and II are wrong
6
A sample containing both ethanol and 1-butanol is analysed on a polar column.
a Ethanol will come out first
b 1-butanol will come out first
c They will come out at the same time.
d It cannot be predicted.
7
The use of the internal standard method is often used because
a Standards for chromatography are cheap.
b It will save the amount of used chemicals
c It doesn’t make any difference when injecting less or more sample
d It is rarely used because it is a bad method.
8
When drawing the calibration graph with horizontally concentration ethanol and vertically is set to the quotient of areas
a buthanol : ethanol
b area ethanol
c it doesn’t make any difference, all are fine.
d ethanol : butanol
9
A sample liquid is analysed by GC using internal standard. The measurement is performed using 10 mL liquid and put it into a volumetric flask of 25 mL adding the internal standard and filling up to the mark. From this solution a chromatogram is used to determine the ethanol concentration. In the calibration graph the student reads 5.00 vol % ethanol. The original liquid will contain:
a 2.00 vol %
b 5.00 vol %
c 10.0 vol %
d 12.5 vol %
10
When the ethanol used for the standard, is common denaturised ethanol this means that :
a The ethanol is produced chemically and not in a natural way.
b Men cannot drink it because butanol is added.
c Extra methanol is added to make it undrinkable
d It is a brand name like Merck or Sigma.
11
When denaturised ethanol is used an extra peak might appear.