Dropping Mercury Electrode (DME) Assembly
Marine-grade marine capillary tube (internal bore ~0.05 mm) connected to a leveling mercury reservoir establishing an exact 50.0 cm hydrostatic head (m = 2.000 mg/s, drop life t = 3.50 s).
Chemistry / Polarography & Ilkovič Equation
FREE TO EXPLORELABORATORY 38 · NATURAL SCIENCES
Classical Polarography, Diffusion Currents, and Heavy Metal Speciation (Ilkovič Equation)
Determine the aqueous diffusion coefficient of the cadmium(II) ion (Cd²⁺) and verify the Ilkovič equation in classical DC polarography using a dropping mercury electrode (DME) across concentrations from 0.200 mM to 2.500 mM in 0.100 M KCl supporting electrolyte. Explore interactive analytical gravimetry, Class A volumetric glassware and density determination.
MEASURE · CALIBRATE · ANALYSE
Your virtual chemistry laboratory.
MEAN LIMITING DIFFUSION CURRENT I_D—
MEASUREMENT STATUS
Ready to dispense 1.0 mM of liquid.
Adjust c(Cd²⁺) 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
['Classical DC polarography is limited to analyte concentrations above 10 μM due to non-faradaic capacitive charging currents (i_c ∝ m²/³ t⁻¹/³).', 'Capillary bore clogging by particulate debris or mercury oxidation can alter the capillary characteristic m²/³ t¹/⁶, requiring re-calibration.', 'Mercury cannot be utilized at potentials positive of +0.25 V vs SCE due to the anodic dissolution of metallic mercury (2 Hg + 2 Cl⁻ → Hg₂Cl₂ + 2 e⁻).'] The Ilkovič equation for the mean diffusion current at a dropping mercury electrode is i_d = 607 · n · D¹/² · m²/³ · t¹/⁶ · c, where n = 2 for Cd²⁺, m = 2.000 mg/s is the calibrated mercury mass flow rate, and t = 3.500 s is the drop lifetime. The capillary constant m²/³ t¹/⁶ equals 1.956 mg²/³·s⁻¹/². Across cadmium concentrations from 0.200 mM to 2.500 mM, the half-wave potential remains strictly invariant at E_1/2 = -0.600 ± 0.002 V vs SCE, confirming reversible two-electron amalgam formation. Linear least-squares regression of mean diffusion current i_d against cadmium concentration c yields a slope of 6.3719 μA·mM⁻¹ with R² = 0.9999996, extracting an aqueous Cd²⁺ diffusion coefficient D = 7.20 × 10⁻⁶ cm²·s⁻¹, in complete agreement with authoritative electrochemical literature benchmarks.
Analytical chemistry apparatus & tools.
Classical Heyrovský Polarographic Cell
H-type double-jacketed polarographic cell featuring a quiet mercury pool anode and constant-temperature circulation at 25.00 ± 0.02 °C.
Saturated Calomel Reference Electrode (SCE)
External SCE reference cell connected via an agar-saturated KCl salt bridge with a fine porous ceramic plug to provide stable potential reference.
Low-Noise Polarographic Potentiostat
Linear potential ramp generator (-0.200 V to -1.000 V vs SCE at 2.0 mV/s) with a low-pass analog RC damping filter (time constant τ = 1.0 s) to measure mean diffusion currents.
High-Purity Nitrogen Deoxygenation Subsystem
Gas train with vanadous chloride deoxygenating scrubbers and water presaturator to remove electroactive dissolved O₂.
Precision Drop-Time Chronometer
Optical photodiode drop detector linked to a quartz microsecond chronometer to measure individual mercury drop lifetimes with ±0.005 s resolution.
FOLLOW THE METHOD
A reason behind every step.
Determine the aqueous diffusion coefficient of the cadmium(II) ion (Cd²⁺) and verify the Ilkovič equation in classical DC polarography using a dropping mercury electrode (DME) across concentrations from 0.200 mM to 2.500 mM in 0.100 M KCl supporting electrolyte. Suggested independent volumetric settings: 0.2, 0.5, 1.0, 1.5, 2.0, 2.5 mM.
01
Mercury Reservoir Leveling & Flow Rate Calibration
Adjust the mercury leveling bulb to establish a precise 50.0 cm column height. Collect 50 falling drops under deionized water into a tared weighing bottle to calibrate mercury mass flow rate m = 2.000 ± 0.005 mg/s.
02
Standard Electrolyte Formulation & Maxima Suppressor
Prepare standardized Cd²⁺ solutions (0.200, 0.500, 1.000, 1.500, 2.000, 2.500 mM) in 0.1000 M KCl containing 0.002% w/v Triton X-100 to suppress polarographic streaming maxima.
03
Cell Loading & Nitrogen Deoxygenation
Transfer 25.0 mL of analyte into the jacketed H-cell at 25.00 °C. Bubble high-purity nitrogen for 15 minutes to purge dissolved oxygen, then divert gas flow over the solution surface.
04
Potential Ramp Configuration & Residual Current Baseline
Configure the polarograph from E_initial = -0.200 V to E_final = -1.000 V vs SCE at a slow scan rate of 2.0 mV/s. Record the residual non-faradaic charging current prior to the reduction wave onset.
05
Polarographic Wave Acquisition & Limiting Current Measurement
Initiate the linear potential ramp. Record the sigmoidal polarographic wave. Measure the limiting plateau current (i_lim) between -0.80 V and -0.90 V and subtract the extrapolated residual current to obtain the true diffusion current (i_d).
06
Ilkovič Calibration & Diffusion Coefficient Extraction
Verify half-wave potential invariance (E_1/2 = -0.600 V vs SCE). Plot mean diffusion current i_d against cadmium concentration c. Perform linear least-squares regression to extract slope d(i_d)/dc and calculate D using the Ilkovič equation.
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
\bar{i}_d = 607 \cdot n \cdot D^{1/2} \cdot m^{2/3} \cdot t^{1/6} \cdot c \implies \bar{i}_d = 6.372 \cdot c \implies D = 7.20 \times 10^{-6}\text{ cm}^2\cdot\text{s}^{-1}
| # | Cadmium(II) Concentration c (mM) | Mean Limiting Diffusion Current i_d (μA) | Cadmium(II) Concentration c (mM) | Mean Limiting Diffusion Current i_d (μA) | Remove |
|---|
No readings yet. Dispense and weigh a volumetric aliquot to record it.
Calculate it yourself.
CHECK YOUR UNDERSTANDING