English
中文A convenient way of illustrating the principles governing phase changes within three component systems is to use a partially miscible liquid system. In the past, the acid-chloroform-water system has been employed as a practical example to investigate ternary systems and the miscibility gap (Wright et al., 1891; Brancker et al., 1940). However, since the status of chloroform has been changed to a suspected carcinogen, these experiments have been rendered unsuitable for teaching laboratories. A fitting alternative is offered in the form of an alcohol-water-hydrocarbon system (Washburn et al., 1931). Two undergraduate laboratory experiments have been proposed in the past, based on the water-n-heptane-n-propanol system and are similar to the previously popular acid-chloroform-water experiments. However, whereas the acid-chloroform-water experiments solely involve titration, these experiments utilise other techniques. Udale and Wells (1995) outline a method whereby a phase diagram can be constructed based upon titration results but also describe how pairs of phases in equilibrium can be quantified chromatographically. Karukstis et al. (2000) describe a novel method whereby the phase diagram is determined spectroscopically. The following suggested experimental procedure describes both methods, aimed at teaching students the principles of i) Gibbs phase law ii) phase diagrams iii) the three component system and iv) the miscibility gap.
Gibbs Phase Law
The number of degrees of freedom, F, within a system is given by:
Where C is the number of components and P is the number of phases in coexistence at equilibrium. Based upon this, a three component, two phase system has three degrees of freedom. Under both isothermal and isobaric conditions, only one degree of freedom remains characterising the composition of the system. The ternary system can therefore be plotted on a triangular graph.
The Phase Diagram
The two-dimensional phase diagram is an equilateral triangular graph with each apex representing a pure component. All compositions are expressed as percentage by weight and all mixtures can be prepared by weight using a top loading balance with a sensitivity of 0.001 g. Once completed, the phase diagram contains two regions: a single phase region where all components form a miscible solution and a two phase region where n-heptane and water exist as two immiscible phases.
Identifying the phase boundary
First, several different mixtures of aqueous propanol must be prepared. (Udale and Wells (1995) suggest preparing solutions of 20%, 40%, 60% and 80% aqueous propanol). The mixtures should be tightly stoppered, shaken and then suspended in a 25 oC thermostated bath to equilibrate. Each mixture is then titrated with n-heptane until the saturation point is reached (the first perceptible cloudiness in the mixture). The mass of heptane used to create the second liquid phase must be noted, either by weighing the amount used, or by converting the volume used (using the density of n-heptane at 25 oC = 0.68 g mL-1 from the MSDS (see section on safety information)).
The same procedure is used for several solutions of n-propanol in n-heptane. Udale and Wells (1995) suggest preparing solutions containing 20%, 40%, 60% and 80% n-propanol in n-heptane). The mixtures should be shaken and left to equilibrate to 25 oC before they are titrated against water. Again, the final weight composition of each mixture should be noted.
The end point compositions, when plotted on the triangular graph, mark the phase boundary which can be extended to the n-heptane-water axis based on the solubilities of water in n-heptane (0.01%) and n-heptane in water (0.0002%) (Shaw, 1989).
At this stage, Karusktis et al. (2000), in their method, suggest measuring the absorption or emission spectra of the mixtures using either a conventional UV-visible spectrophotometer or a steady-state fluorescence spectrophotometer. This involves placing an aliquot of each sample into a quartz cuvette with a requisite amount of the absorbance probe, methyl orange or the fluorescence probe, Prodan. The wavelength of maximum absorption or emission is then obtained from each sample and noted next to each point of the phase boundary line on the triangular graph.
Tie Lines
At this point, the phase boundary has been identified and so the experiment could be regarded as complete, if deemed appropriate. Time permitting; the next step of the experiment involves adding tie lines to the phase diagram. This involves creating samples with an overall composition that lies within the two phase region (Udale and Wells (1995) suggest mixtures of n-propanol:n-heptane:water that are composed of 40:40:20, 35:35:30, 20:45:35, and 15:45:40). The samples should be gently mixed and then left at 25 oC until the phases have separated. If the spectroscopic method has been employed then the next step involves taking an aliquot of each layer and mixing each with the selected probe. The wavelengths of maximum absorption or emission are then obtained for each sample. The results can be plotted on the phase diagram based upon the absorbance/fluorescence results obtained for the phase boundary measurements. The tie lines connect the results obtained for each single phase layer measurement, and should pass through the original overall composition value.
Alternatively, the composition of the single phase layers can be obtained by measuring the concentration of n-propanol in each sample. This can be done using gas chromatography with n-butanol as an internal standard (Udale and Wells, 1995). If a calibration curve is first created then the n-propanol concentrations can be estimated from the ratios of peak heights for n-propanol and n-butanol (Udale and Wells (1995) created standards of 15%, 30%, 45%, and 60% n-propanol in acetone. All of their standards and samples were diluted tenfold with a mixture containing 5% butanol, 10% heptane, 10% water, and 75% acetone before they were separated on a Varian Aerograph model 940 gas chromatograph fitted with a Carbowax column (20 m, isothermal at 50 oC). They detected their samples with a flame ionisation detector (injector and detector at 150 oC). Assuming that the top layer of the samples is heptane rich then the composition of the layers can be determined as they must lie on the phase boundary and from this the tie lines can be drawn.
Safety Information
Based upon the information provided on http://www.coshh-essentials.org.uk/ (Date accessed 16/11/08), the chemicals used in this process require engineering control, guidance sheets provided by the website include: Local exhaust ventilation (G200), fume cupboards (G201) and ventilated workbenches (G203). The chemicals cause harm via skin contact and so guidance sheets are also provided for general advice (S100) and for the selection of personal protective equipment (S101). A COSHH assessment can be performed on this website.
R-phrases
Highly flammable (R11); Irritating to skin (R38);Very toxic to aquatic organisms, may cause long-term adverse effects in the aquatic environment (R50/53); Harmful: may cause lung damage if swallowed (R65); Vapours may cause drowsiness and dizziness (R67).
S-phrases
Keep container in a well-ventilated place (S9); Keep away from sources of ignition – No smoking (S16); Do not empty into drains (S29); Take precautionary measures against static discharges (S33); This material and its container must be disposed of as hazardous waste (S60); Avoid release to the environment. Refer to special instructions/Safety data sheets (S61); If swallowed, do not induce vomiting: seek medical advice immediately and show this container or label (S62).
R-phrases
Highly flammable (R11); Risk of serious damage to eyes (R41). Vapours may cause drowsiness and dizziness (R67).
S-phrases
Keep container tightly closed (SR7); Keep away from sources of ignition – no smoking (S16); Avoid contact with skin (S24); In case of contact with eyes, rinse immediately with plenty of water and seek medical advice (S26); Wear eye/face protection (S39).
Flammable (R10); Harmful if swallowed (R22); Irritating to respiratory system and skin (R37/38); Risk of serious damage to eyes (R41); Vapours may cause drowsiness and dizziness (R67).
S-phrases
Keep container tightly closed and in a well-ventilated place (S7/9); Keep away from food, drink and animal feedingstuffs (S13); In case of contact with eyes, rinse immediately with plenty of water and seek medical advice (S26); Wear suitable gloves and eye/face protection (S37/39); If swallowed, seek medical advice immediately and show this container or label (S46).
R-phrases
Highly flammable (R11). Irritating to eyes (R36). Repeated exposure may cause skin dryness or cracking (R66). Vapours may cause drowsiness and dizziness (R67).
S-phrases
Keep container in a well-ventilated place (S9). Keep away from sources of ignition – No smoking (S16). In case of contact with eyes, rinse immediately with plenty of water and seek medical Advice (S26).
methyl orange
R-phrases
Toxic if swallowed (R25).
S-phrases
In case of accident or if you feel unwell, seek medical advice immediately (show the label where possible) (S45).
MSDS can be found at (Date accessed 16/11/08):
http://www.sigmaaldrich.com/catalog/search/ProductDetail/FLUKA/32624
prodan
Caution: Substance not yet fully tested (EU).
MSDS can be found at (Date accessed 16/11/08):
http://www.sigmaaldrich.com/catalog/search/ProductDetail/SIGMA/41525
Thin layer Chromatography (Tlc) stage 2
