Dual-Beam UV-Vis Spectrophotometer
Split-beam optical spectrophotometer with holographic grating monochromator and matched photodiode detectors for measuring absorbance (A = -log₁₀ T) at 450.0 nm with photometric accuracy ±0.002 AU.
Chemistry / Job's Method of Continuous Variation
FREE TO EXPLORELABORATORY 42 · NATURAL SCIENCES
Job's Method of Continuous Variations and Complex Ion Stoichiometry
Determine the coordination stoichiometry (n = 1.000) and thermodynamic formation constant (Kf = 1050 M⁻¹) of the iron(III)-thiocyanate complex ([Fe(SCN)]²⁺) using Job's method of continuous variations at 450.0 nm and 25.0 °C. Explore interactive analytical gravimetry, Class A volumetric glassware and density determination.
MEASURE · CALIBRATE · ANALYSE
Your virtual chemistry laboratory.
COMPLEX ABSORBANCE AT 450 NM (A₄₅₀)—
MEASUREMENT STATUS
Ready to dispense 5.0 mL of liquid.
Adjust V_L 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
["Job's method is strictly valid only when a single predominant complex species is formed under the experimental conditions.", 'At higher thiocyanate concentrations (C_L > 0.01 M), higher-order complexes ([Fe(SCN)₂]⁺) begin to form, shifting the apparent apex.', "Equilibrium constants extracted from Job's plots are conditional constants valid at the specified ionic strength (I = 0.100 M) and acidity."] The continuous variation model evaluates equimolar 0.00200 M Fe³⁺ and 0.00200 M SCN⁻ in 0.100 M HNO₃ at 25.00 °C (total volume V_total = 10.00 mL, C_T = 2.000 mM). Across the six calibration mixtures V_L = [1.00, 2.00, 3.00, 4.00, 5.00, 6.00] mL (corresponding to X_L = [0.10, 0.20, 0.30, 0.40, 0.50, 0.60]), measured absorbances at 450.0 nm are [0.573, 1.060, 1.435, 1.674, 1.756, 1.674] AU. The parabolic apex occurs at X_L,max = 0.500, confirming 1:1 stoichiometry ([Fe(SCN)]²⁺). Linear regression of log([ML]/[M]) versus log[L] across the six points yields an empirical slope m = 1.00020 (stoichiometry n = 1.000, difference = 0.0203%) and Y-intercept b = 3.02188 with R² = 0.9999989, extracting a thermodynamic formation constant Kf = 1052 M⁻¹ (log Kf = 3.022), in flawless agreement with accepted literature benchmarks.
Analytical chemistry apparatus & tools.
Matched Quartz Spectrophotometric Cuvettes (1.000 cm)
Pair of precision optical quartz cuvettes with certified internal optical path length b = 10.00 ± 0.01 mm and optical transmission matching within ±0.001 AU.
Automatic Precision Variable-Volume Pipettes
Calibrated micropipettes (P1000 and P5000, ISO 8655 certified) used for dispensing continuous variation aliquots of metal and ligand solutions.
Class A Volumetric Flasks & Pipettes (10.00 mL)
Certified Class A volumetric glassware (tolerance ±0.02 mL) used for preparing continuous variation equimolar mixtures.
Thermostatted Water Bath & Jacketed Cell Holder
Circulating constant-temperature water bath with external jacket maintaining the cuvette chamber at 25.00 ± 0.02 °C.
Four-Decimal Electronic Analytical Balance
Electromagnetic analytical balance (readability ±0.0001 g) used to weigh primary standard ferric nitrate nonahydrate (Fe(NO₃)₃·9H₂O) and potassium thiocyanate (KSCN).
FOLLOW THE METHOD
A reason behind every step.
Determine the coordination stoichiometry (n = 1.000) and thermodynamic formation constant (Kf = 1050 M⁻¹) of the iron(III)-thiocyanate complex ([Fe(SCN)]²⁺) using Job's method of continuous variations at 450.0 nm and 25.0 °C. Suggested independent volumetric settings: 1.0, 2.0, 3.0, 4.0, 5.0, 6.0 mL.
01
Equimolar Stock Solution Synthesis
Accurately prepare equimolar 0.00200 M stock solutions of iron(III) nitrate and potassium thiocyanate in 0.100 M HNO₃. The nitric acid matrix suppresses the hydrolysis of Fe³⁺ to Fe(OH)²⁺ and maintains constant ionic strength.
02
Preparation of Continuous Variation Mixture Series
Into a series of ten clean, dry 10.00 mL volumetric test vessels, dispense complimentary volumes of 0.00200 M Fe³⁺ and 0.00200 M SCN⁻ such that the total volume remains constant at V_total = 10.00 mL: V_L = [1.00, 2.00, 3.00, 4.00, 5.00, 6.00, 7.00, 8.00, 9.00] mL, corresponding to ligand mole fractions X_L = [0.10, 0.20, 0.30, 0.40, 0.50, 0.60, 0.70, 0.80, 0.90].
03
Thermal Equilibration and Homogenization
Mix each solution thoroughly by inversion and equilibrate in the 25.00 °C water bath for 15 minutes. Complex formation between Fe³⁺ and SCN⁻ is rapid (diffusion-controlled rate constant k ≈ 10⁸ M⁻¹·s⁻¹), establishing instantaneous thermodynamic equilibrium.
04
Spectrophotometer Baseline Calibration
Power on the dual-beam spectrophotometer, allow 30 minutes for thermal lamp stabilization, and set the wavelength to 450.0 nm with a 1.0 nm slit width. Zero the instrument against a reagent blank containing 0.100 M HNO₃.
05
Sequential Absorbance Acquisition
Rinse the sample quartz cuvette twice with the first mixture (X_L = 0.10), fill to 80% capacity, wipe optical windows with lens tissue, and seat in the sample cell holder. Record the steady-state absorbance A₄₅₀. Repeat sequentially across all nine continuous variation mixtures.
06
Job's Plot Construction & Linear Regression Analysis
Plot measured absorbance A₄₅₀ versus ligand mole fraction X_L to generate the classic triangular Job's plot. Extrapolate the linear branches to determine the peak maximum X_L,max = n / (1 + n). Calculate free equilibrium concentrations and perform linear regression of log([ML]/[M]) versus log[L] to determine the complex stoichiometry n and stability constant Kf.
YOUR NOTEBOOK · 0 OBSERVATIONS
Turn measurements into quantitative chemical insight.
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
\log([\text{ML}]/[\text{M}]) = 1.0000 \cdot \log[\text{L}] + 3.0212 \implies n = 1.000 \implies K_f = 1050\text{ M}^{-1}
| # | Ligand Solution Volume V_L (0.00200 M SCN⁻) (mL) | Complex Absorbance at 450 nm (A₄₅₀) (AU) | log([L]_free) | log([ML] / [M]_free) | Remove |
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