LABORATORY 20 · NATURAL SCIENCES

Three-Component Liquid-Liquid Phase Equilibrium and Ternary Phase Diagrams

To determine the binodal liquid-liquid solubility curve of the ternary system water-acetic acid-chloroform at 25.0 °C via turbidimetric cloud-point titrations, establish coexisting conjugate phase compositions and tie lines on an equilateral triangular phase diagram, evaluate the Nernst-Christian distribution of acetic acid between water and chloroform, and extract both the monomer partition coefficient (K_D) and the carboxylic acid dimerization constant (K_dim) in the organic phase via linearized regression. Explore interactive analytical gravimetry, Class A volumetric glassware and density determination.

MEASURE · CALIBRATE · ANALYSE

Your virtual chemistry laboratory.

TITRANT VOLUME V_NAOH FOR ORGANIC ALIQUOT—

MEASUREMENT STATUS

Ready to dispense 0.6 M of liquid.

Equilibrium aqueous acid concentration C_aq (mol/L)0.6 MSet aqueous acid concentration (M)

Precision 250 mL borosilicate separatory funnels with PTFE stopcocks mounted on a temperature-controlled mechanical shaker (25.00 ± 0.05 °C).

Adjust aqueous acid concentration V, then weigh the dispensed aliquot.

Mass observations are determined by the high-precision thermodynamic equation of state for pure liquid water. Readings reflect thermal equilibrium and analytical balance force restoration.

Physical chemistry assumptions and metrological limits

The Nernst-Christian dimerization model assumes that higher-order oligomers (trimers, tetramers) are negligible and that activity coefficients are approximately unity in both phases. At very high acid concentrations (> 2 M in water) approaching the plait point, mutual solubility of water in chloroform and non-ideal activity coefficients require multi-parameter NRTL or UNIQUAC models. The physical simulation model computes ternary liquid-liquid equilibrium (LLE) at 25.00 °C using the Nernst-Christian distribution model coupled with cyclic carboxylic acid dimerization. Monomer partition coefficient: K_D = [HAc]_org,mon / [HAc]_aq = 0.1000 (dimensionless). Dimerization constant of acetic acid in the chloroform organic phase: 2 HAc ⇌ (HAc)₂ with K_dim = [(HAc)₂]_org / [HAc]_org,mon² = 100.0 L/mol. Total organic acid concentration satisfies C_org = K_D·C_aq + 2·K_dim·K_D²·C_aq² = 0.100·C_aq + 2.000·C_aq². The distribution ratio D = C_org / C_aq follows the exact linear relation D = K_D + m_slope·C_aq = 0.100 + 2.000·C_aq. For aqueous acid concentrations C_aq from 0.20 M to 1.20 M, total organic concentrations span C_org = 0.100 M to 3.000 M. Titrating a 5.00 mL aliquot of the organic phase with standardized 0.2500 M NaOH requires titrant volumes V_NaOH = C_org × 5.00 / 0.250 = 20.0 × C_org, spanning 2.00 mL to 60.00 mL. Ordinary least-squares linear regression of D against C_aq extracts slope m = 2.000 L/mol and intercept b = 0.1000 with R² = 1.00000; the dimerization constant is evaluated without bias as K_dim = m / (2·b²) = 2.000 / (2 × 0.0100) = 100.0 L/mol, perfectly matching the accepted thermodynamic benchmark.

Analytical chemistry apparatus & tools.

Borosilicate Separatory Funnels (250 mL)

Airtight liquid-liquid extraction vessels fitted with ground glass stoppers and precision PTFE stopcocks for vigorous mixing and sharp gravitational phase separation of conjugate liquids.

Class A Micro-Burette (50.00 mL)

High-accuracy Schellbach burette with blue vertical reading stripe and 0.05 mL graduations for precise volumetric titration of acid aliquots with 0.2500 M NaOH.

Thermostatic Shaking Water Bath

Reciprocating mechanical shaker bath regulated to 25.00 ± 0.05 °C to ensure isothermal phase equilibrium and prevent temperature-induced binodal boundary shifts.

Class A Volumetric Pipettes & Chilled Syringes

Calibrated 5.00 mL transfer pipettes and gas-tight syringes used to withdraw representative aliquots from lower organic and upper aqueous phases without cross-contamination.

Magnetic Stirrer & Turbidimetric Titration Cell

Variable-speed magnetic stirrer and optical dark-field illumination stage used to detect the exact onset of persistent milkiness (cloud point) during binodal mapping.

Analytical Reagents: Glacial HAc, CHCl₃, & 0.250 M NaOH

High-purity glacial acetic acid (M = 60.05 g/mol), stabilized chloroform (ρ = 1.489 g/cm³), standardized 0.2500 M NaOH titrant, and 1% phenolphthalein indicator.

FOLLOW THE METHOD

A reason behind every step.

To determine the binodal liquid-liquid solubility curve of the ternary system water-acetic acid-chloroform at 25.0 °C via turbidimetric cloud-point titrations, establish coexisting conjugate phase compositions and tie lines on an equilateral triangular phase diagram, evaluate the Nernst-Christian distribution of acetic acid between water and chloroform, and extract both the monomer partition coefficient (K_D) and the carboxylic acid dimerization constant (K_dim) in the organic phase via linearized regression. Suggested independent volumetric settings: 0.2, 0.4, 0.6, 0.8, 1.0, 1.2 M.

Mapping the Binodal Solubility Curve via Cloud-Point Titrations

Prepare six known binary mixtures of water and chloroform (from 5% to 95% water by weight) in conical flasks, titrate dropwise with glacial acetic acid under vigorous stirring until the persistent turbidity just clears to a brilliant transparent single phase, and record the mass of acid added to locate the binodal boundary.

Synthesis of Two-Phase Mixtures inside the Miscibility Dome

In six 250 mL separatory funnels, prepare mixtures of water, chloroform, and acetic acid designed to lie well inside the two-phase region corresponding to target aqueous acid concentrations C_aq = 0.20, 0.40, 0.60, 0.80, 1.00, and 1.20 M.

Isothermal Equilibration in Shaking Bath

Stopper the separatory funnels firmly, mount them in the 25.00 °C thermostatic shaking bath, agitate vigorously for 20 minutes to achieve intimate liquid-liquid contact, and allow funnels to stand undisturbed for 30 minutes until both phases become completely crystal clear.

Sampling and Separation of Conjugate Phases

Carefully drain the lower dense organic phase (chloroform layer, ρ ≈ 1.48 g/cm³) through the PTFE stopcock, discarding the first 0.5 mL bore volume, into dry Erlenmeyer flasks, and draw representative aliquots of the upper aqueous phase.

Titrimetric Determination of Organic Acid Content

Pipette a 5.00 mL aliquot of the organic phase into a flask containing 25 mL of neutral deionized water and 3 drops of phenolphthalein indicator, and titrate with standardized 0.2500 M NaOH to a faint, persistent pink endpoint lasting 30 seconds.

Data Processing, Tie Line Construction, and Dimerization Modeling

Compute total acid concentrations C_aq and C_org, evaluate distribution ratio D = C_org / C_aq, construct equilibrium tie lines on the triangular phase diagram, and perform linear regression of D against C_aq to extract K_D and K_dim.

YOUR NOTEBOOK · 0 OBSERVATIONS

Turn measurements into quantitative chemical insight.

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Points are your recorded gravimetric observations. The regression line validates the linear mass-volume proportionality and extracts density.

ORDINARY LEAST-SQUARES FIT · m = ρ·V + m₀

D = K_D + 2 \cdot K_{\text{dim}} \cdot K_D^2 \cdot C_{\text{aq}} ==> D = 0.100 + 2.000 \cdot C_{\text{aq}}

#	Equilibrium aqueous acid concentration C_aq (mol/L) (M)	Titrant volume V_NaOH for organic aliquot (mL)	Aqueous acid concentration C_aq (mol/L)	Distribution ratio D = C_org / C_aq	Remove

No readings yet. Dispense and weigh a volumetric aliquot to record it.

Calculate it yourself.

Your estimate of DimerizationConstant_AceticAcid (L/mol)

CHECK YOUR UNDERSTANDING

Explain what the gravimetric measurements mean.

For a three-component system exhibiting two coexisting liquid phases at constant temperature and pressure, how many degrees of freedom exist according to the Gibbs phase rule?