Experiment 5 Phase diagram for a three component system Dated April 12 2010 I INTRODUCTION It is sometimes necessary to know the mutual solubilities of liquids in a two phase system For example you may need to know how much water is dissolved in an organic liquid with which it is in contact and also the amount of the organic compound that is in the aqueous phase In this experiment you will consider a three component mixture 1 butanol water acetic acid at 25 o C and barometric pressure and construct the corresponding ternary phase diagram Examples of experiments using this methodology are given in Refs 1 3 II THE PHASE RULE Information regarding phase equilibria can be predicted by a simple rule Gibbs phase rule f c p 2 1 where c is the number of components and p is the number of phases present in the system The degrees of freedom f or variance gives the number of variables e g pressure temperature composition etc that must be given to completely describe the system or to locate the state of the system on the phase diagram Example For pure gas e g one component system c 1 pure gas only one component and p 1 only one phase the gas phase and by Eq 1 we have f 2 This means that two variables are required for complete description of pure gas i e any two of the three variables P V or T Example Consider water with the phase diagram shown in Fig 1 FIG 1 A schematic drawing of the phase diagram for water For pure phases solid liquid or vapor f 2 When two phases are present simultaneously e g the point resides on the lines separating the phases in Fig 1 only P or T can be varied independently and the phase rule gives f 1 When all three phases are present i e the triple point all variables must be fixed and the phase rule says that f 0 For ternary systems i e consisting of three components we have c 3 and f 5 p If the system consists of only one phase f 4 The required four variables for describing such system are two for describing the relative composition mass fractions and one of the pairs P V P T or T V Note that if only two mass fractions x1 and x2 are given the third can be obtained by x3 1 x2 x1 Also in practice P T pair is chosen If the system separates into two different phases only f 5 2 3 variables are needed one mass fraction and P T Typeset by REVTEX FIG 2 A triangular phase diagram showing the representation of the mass fractions for ternary systems The colors indicate how concentrations for different species should be read from the diagram The point marked in the diagram represents 30 1 butanol 10 water and 60 acetic acid The one and two phase regions have been separated by a black line The line drawn is only demonstration and does not correspond to experimental observation Pressure and temperature are assumed to be fixed Phase diagrams for ternary systems are usually represented using a triangle shown in Fig 2 This graph accounts for the fact that only two variables are required Along the phase boundary only one variable is required Regions where one or two phases appear have also been indicated in Fig 3 Note that the line drawn is hypothetical the real curve will be determined in this experiment When the solution is stirred the transition from one region to another can be observed by appearance or disappearance of cloudiness or turbidity in the solution The turbidity results from scattering of light by the large number of very small oily droplets of the second phase that are produced when the system is stirred Sometimes it is easier to see this when stopping the stirring briefly If the three components are mixed to give an overall system composition that falls in the 2 phase region the system will separate into two phases a phase rich in water and another rich in 1 butanol The compositions of the phases that form are given by the intersections of a tie line with the phase boundary The tie line must also contain the point describing the overall system composition A graphical demonstration and an interpretation of a tie line are given in Fig 3 Note that in the case of Fig 3 only the mass fraction of 1 butanol must be given when the system remains on the phase boundary line This determines the mass fractions for water and acetic acid Hence the phase rule holds with f 5 p 3 i e mass fraction for 1 butanol temperature and pressure If the system was initially in the two phase region the tie line uniquely connects the points along the phase separating line Given the point A in Fig 3 depending on the 1 butanol mass fraction in phase 1 the points B and C are uniquely determined Thus only the 1 butanol mass fraction in phase 1 temperature and pressure are required for complete description of the system which had separated into two phases This is again in accordance with the phase rule III EXPERIMENTAL Task overview In the first part of the experiment solubility of 1 butanol in water and solubility of water in 1 butanol will be determined The switch to the two phase region can be observed as appearance of the turbidity in the stirred solution This gives the first two points in the phase diagram that lie along the horizontal axis see Fig 2 the starting points for the arc In the second part the points defining the arc will be determined by starting from the two phase region and adding 17 5 M acetic acid molecular weight 60 05 g mol 1 until the system switches into one phase This transition can be observed by disappearance of the turbidity in the stirred solution In the third part one of the tie lines is determined experimentally This will be done by choosing a point from the two phase region 2 FIG 3 Drawing of a tie line in a triangular phase diagram The diagram may contain one or many tie lines The point A denotes the composition of phase 1 B denotes the initial composition of the system and C the composition of phase 2 and determining the compositions of the two phases formed Part 1 1 Place 20 mL of water in a 50 mL Erlenmeyer flask Cover the flask with Parafilm Poke the buret containing 1 butanol through the Parafilm Add 1 butanol to the water drop by drop with continuous stirring until the turbidity small oily droplets form this is difficult to see be careful appears and remains for at least 5 min It will take less than 2 mL of 1 butanol to reach this point Record the amount 1 butanol required 2 Place 20 mL of 1 butanol in a 50 mL Parafilm covered Erlenmeyer flask Add water to the 1 butanol drop by drop with continuous stirring until the …