Techniques {conductivity measurement} {conductance measurement} can measure total electrolytes in solution.
purposes
Conductivity can detect bath and electrolyte acidity, scrubbing-tank basicity, and water, soil, milk, biological tissue, and ion-exchange-chromatography ions. Conductivity is fast, accurate, and non-destructive.
AC current
AC current prevents decomposition. Low frequency is for high resistance, and high frequency is for low resistance, to keep capacitance low.
ions
Conductivity in solutions depends on solute-ion transport. If ion charge is low, velocity is high, and conductivity is high. Proton goes from water molecule to water molecule directly and so is fast. Big ions have small hydration and small effective size. Ion hydration and ionic interactions affect conductivity.
ions: electrolyte
Strong electrolytes slightly reduce conductivity as concentration increases, because there are more collisions. Weak electrolytes have low molar conductivity, because they do not dissociate. Weak electrolyte causes more variation.
standard
Potassium chloride is standard.
solvent
Solvent-ion collisions affect solute-ion velocity through solvent.
solvent: viscosity
Higher viscosity makes more-random flow and resists conduction.
temperature
High temperature increases conductance.
Techniques {coulometry} can measure charge that has flowed, by weighing metal or gas produced by electrolysis. 1 Faraday = 96487 Coulomb = 1 mole of electrons.
Instruments {coulometer} can have silver anode and platinum cathode in silver perchlorate solution, in series with cell to test. Deposited silver weight indicates total charge that flowed.
Two platinum electrodes can be in iodine and potassium iodide solution. Titration with reducing agent finds iodide-concentration change.
Two platinum electrodes in potassium sulfate solution can produce hydrogen and oxygen gas.
Opposite-charge ions {counterion} surround ions. Opposite charges attract and make ions closer together than in random arrangements, lowering potential energy and chemical potential. Counterions shield ions and reduce effective charge, lowering potential energy and chemical potential.
Potential-energy and chemical-potential lowering depend on ion solubility {Debye-Hückel theory}.
Electric current can reduce metal ion to make metal, separating metal from solution or mixture, for weighing {electrogravimetry}.
Voltage can separate charged molecules in solution {electrophoresis}|. Electrophoresis can use pH gradient.
purposes
Electrophoresis can separate nucleic acids and peptides by size and charge.
process
Solution at constant, buffered pH is in paper or gel. Voltage applied across paper or gel moves charged molecules. The most-highly-charged molecules move most. Molecular size and shape also affect movement.
gel
Gel can be polyacrylamide {polyacrylamide gel electrophoresis} (PAGE). Gel can be agarose sugar, which can have different concentrations to separate different size ranges. Thinner gels allow more resistance and so more voltage compared to current.
detergent
During electrophoresis {SDS-gel electrophoresis}, detergent can coat molecules. Molecules then have same shape and charge, so only molecule size determines separation.
pH
During electrophoresis {zone electrophoresis} {moving boundary electrophoresis}, solution pH can change as solution moves. During electrophoresis {disc electrophoresis}, solution pH in gels can change over time in one direction. During electrophoresis {slab electrophoresis}, solution pH in gels can change over time in two directions.
Iodine in potassium iodide solution is oxidizing agent {iodimetry}. I3- [3 is subscript and - is superscript] ion is at pH 6 to 8. Starch is indicator.
Iodide ion is reducing agent {iodometry}. Sodium thiosulfate titrates iodine. Starch is indicator.
Ion solutions have charge concentration {ionic strength} depending on ion charge and concentration: I = 0.5 * z1^2 * c1 + 0.5 * z2^2 * c2, where z = charge and c = concentration.
pH gradient across solution can move molecules to minimum-charge point {isoelectric point} {isoelectric focusing}.
Charge separation at electrode makes voltage {overvoltage} above standard potential.
Oxidation or reduction voltage change {oxidation-reduction titration} can indicate compound amount. Indicator oxidation increases voltage, by millivolts. Indicator reduction decreases voltage. Indicators and samples can change color.
types
Potassium permanganate is oxidizing agent, which standardizes with sodium oxalate or iron (II) sulfate. Potassium chromate is standard oxidizing agent. Cerium +4 ion in acid is standard oxidizing agent, with ferroin as indicator.
Thiosulfate is reducing agent, which oxygen in water does not oxidize and which standardizes with perchloric acid. Fe+2 ion titrates cerium, chromium, and vanadium ions, with ferroin as indicator. Tin (II) chloride reduces iron.
reactions
Carbon dioxide or acid removes sodium sulfate and sulfur dioxide reducing agents. Acid removes metals like zinc and lead. Phenol removes bromine and chlorine. Hydrazine boiling removes permanganate and peroxide.
Measuring galvanic-cell potential {potentiometry} can find ion concentration.
purposes
Potentiometry measures body-fluid, column-effluent, waste-water, pool, detergent, silver-thiocyanate solution, iodide, bromide, chloride, calcium, nitrate, copper, lead, sulfate, aluminum, phosphate, metal-plating cyanide wastes, bleach-chlorine, paper-bleach, water-pollution, and sewage ions. Titration by potentiometer is accurate.
Potentiometry is for redox reactions, precipitations, acid-base reactions, complexing, or indicators. pH meters and ion detectors use direct potentiometry or potential change followed by titration.
potentiometer
Potentiometers measure voltage at zero current, to eliminate internal resistance. Potentiometer has reference electrode and indicator electrode. Circuit has equal and opposite voltage.
Exact-voltage cells {Weston cell} can calibrate potentiometers.
At halfway to equilibrium, potential equals sample potential. At equivalence, potential is half sum of sample potential and titration ions, if valences are equal. Otherwise, it is weighted average. On graph, steepest slope is equivalence point.
potentiometer: reference electrode
Reference electrodes can change voltage by salt-bridge ion-flow-rate change, temperature change, pH change, electrical-resistance change, and mercury, potassium, or chloride sample contamination.
potentiometer: indicator electrode
Indicator electrodes are for redox reactions. Indicator electrodes must be rapid and exact. If both molecules are ions, electrodes are platinum. If ions are strong reducing agents, electrodes are gold. Strong reducing agents are chromium, titanium, or vanadium. If system uses metal and ion, and metal does not react in water, electrodes are gold. Silver, cadmium, mercury, and copper do not react in water. If system uses metal and low-solubility salt, electrodes are gold. Hydrogen electrode is for pH.
Voltage can determine solution metal concentration {voltametry}.
Techniques {polarography} can measure solution concentration, by diffusion-controlled oxidation or reduction at electrode surface. Voltage relates to metal-ion concentration.
purposes
Polarography measures transition metal ions, inorganic ions, water and blood oxygen levels, and ion resonance in organic compounds, aldehydes, acids, ketones, nitrogen compounds, and halides. Inorganic ions are sulfide, oxide, hydroxide, and chromate. Polarography is sensitive, with ion concentrations from 0.01 M to 0.000001 M.
process
A small electrode receives potential that depletes ions near it. With no stirring, electrode and solution have a concentration gradient. Polarized electrodes block ion flow, so nearby ion concentration is due only to ion diffusion from solution. Diffusion rate depends on concentration, if concentration gradient is constant. Potassium chloride minimizes electrostatic effects. Impurities and capacitances can cause other currents.
types
In electrodes {dropping mercury electrode} (DME), mercury can oxidize, removing oxygen gas from solution. Dropping mercury electrodes, rotating disk electrodes, or ring-disk electrodes remove surface layer.
Polarography methods {amperometry} can measure oxidation or reduction by titration, instead of directly.
Metals {electrode}| can contact solutions. Corrosion, electrolysis, electroplating, and batteries involve electrodes.
oxidation
At anode, ions enter solution, so anode is negative, and solution is positive. Oxidation is at anode surface, and reduction is at cathode surface. Opposite charges surround electrode charges, and solution ions solvate, with high attraction at surfaces. Charge gradient is higher for higher concentration and higher ion mobility. Higher temperature reduces attraction, by breaking up surface layer. Applied electric force reduces attraction.
current
Ion formation or discharge rate is current density, which is 0 at equilibrium. Ion far from electrode feels net force. At 10^-7 meters, ion sees widely distributed charges, as it enters ion layer around electrode and feels constant voltage. When ion reaches electrode surface, voltage changes rapidly to opposite sign. Finally, ion reaches electrode pure metal.
At high current, potential can be constant, such as at hydrogen electrode or calomel electrode. If high overvoltage causes high current density, diffusion can be too slow, and electrode can become polarized. Adding extra potential or moving electrodes reduces polarization. Solution friction causes slower cation flow than electron flow, causes ohmic resistance, and decreases current. Power generation maximizes if concentration polarization is just below limiting current.
Reference electrodes {calomel electrode} can use mercury and saturated mercury chloride. Potassium chloride reduces variation with temperature and can be salt bridge.
Electrodes {glass electrode} can pair with calomel electrodes or silver/silver chloride electrodes. Glass electrodes have silver and silver chloride in 0.1 M hydrochloric acid, in thin glass membrane. Temperature, acidity, and sodium contamination affect it. It requires storage in 0.1 M potassium chloride. It requires cleaning.
Reference electrodes {hydrogen electrode} can use platinum electrodes, with hydrogen gas at one atmosphere and hydrogen ion at one-molar concentration. Hydrogen electrodes can have platinum-surface changes and can vary hydrogen-gas pressure and hydrogen-ion concentration, so they are hard to control.
Silver and silver chloride reference electrode {silver electrode} has saturated silver chloride on silver surrounded by saturated potassium-chloride solution as salt bridge. It is stable at high temperature. It can be internal reference for glass electrode.
5-Chemistry-Analytical Chemistry
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Date Modified: 2022.0225