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Eudiometer
Brief description
A eudiometer is a laboratory instrument designed to measure changes in the volume of a gas mixture resulting from physical or chemical reactions, particularly combustion sparked by an electric charge. Invented in the late 18th century and refined by Marsilio Landriani, Joseph Priestley, and Alessandro Volta, the eudiometer played a pivotal role in the chemical revolution. It provided early scientists with the quantitative means to analyze atmospheric air purity, discover the composition of water, and establish the stoichiometry of combustion reactions by detonating measured gases inside a sealed graduated glass vessel.
Use / Function
- Atmospheric Gas Analysis: Measuring the relative proportion of oxygen (“vital air”) and other gases in ambient air samples.
- Gas Combustion Stoichiometry: Determining the exact volumetric combining ratios of reactive gases (such as hydrogen and oxygen forming water).
- Respiration and Air Purity Studies: Assessing ventilation quality, respiration byproducts, and the goodness of air in confined living spaces or mines.
- Scale: Essential precision apparatus for chemical research laboratories, mining safety inspections, and early metallurgical testing.
Operating principle
- Liquid Displacement and Gas Trapping: The graduated glass tube is sealed at one end, filled completely with a confining liquid (water or mercury), and inverted in a trough containing the same liquid. The gas sample is introduced through the open bottom, displacing the liquid.
- Atmospheric Pressure Leveling: The liquid level inside the tube is leveled with the outer bath to ensure the trapped gas sample is measured at ambient atmospheric pressure.
- Sealed Electrical Ignition: Platinum or gold electrode wires hermetically sealed into opposite sides of the closed end of the glass tube provide an internal spark gap.
- Controlled Combustion: A spark generated from an electrostatic machine or induction coil jumps across the electrodes, igniting the combustible gas mixture inside the column.
- Volume Contraction Measurement: As the reaction occurs (e.g., $2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}$), condensable water vapor collapses liquid volume. Liquid rises inside the tube, and the resulting volume deficit directly reflects the proportion of reacted gases.
How to create it
- Form the Heavy Glass Tube: Blow a thick-walled, heavy-gauge borosilicate or soda-lime glass tube closed at one end and open at the other, designed to withstand internal detonation shocks.
- Insert and Seal Sparking Electrodes: Heat the closed upper end of the tube and pierce two opposing small openings. Insert platinum wires with a small internal air gap (2 to 3 mm) and fuse the glass around the wires to create an airtight seal.
- Calibrate and Scribe Volumetric Scale: Calibrate the tube by introducing known increments of mercury or water from an accurate pipette, etching millimeter and cubic centimeter divisions along the exterior wall using hydrofluoric acid or diamond-point scribing.
- Prepare the Confining Trough: Construct a sturdy wood or iron pneumatic trough with viewing windows and support clamps to hold the eudiometer tube securely vertical over mercury or water.
- Assemble Sparking Connections: Attach insulated copper lead wires to the external terminals of the platinum electrodes for connection to a Leyden jar, electrostatic generator, or Ruhmkorff induction coil.
- Operating Verification: Fill the tube with mercury, invert into the trough, admit calibrated ratios of hydrogen and atmospheric air, spark the electrodes, and record volume contraction after cooling to ambient temperature.
Materials needed
- Essential: Heavy-walled Glass for the explosion tube; noble Metal (platinum foil and wires) to withstand corrosive reaction gases and high spark temperatures; conductive Wire for external electrical connections; and high-density Mercury (or clean Water) as the confining pneumatic seal.
- Tools: Glassblower’s torch, diamond scriber, graduated pipettes, pneumatic trough, retort stand, electrostatic spark source (Leyden jar or induction coil).
- Substitutes: Water can be used as the confining liquid for insoluble gases, though mercury is preferred because it does not dissolve carbon dioxide or oxygen.
Variants and improvements
- Volta’s Eudiometer: The foundational electric spark design utilizing platinum wires and a Leyden jar discharge to ignite combustible gases.
- Bunsen’s Absorption Eudiometer: Incorporates chemical absorbing reagents (such as pyrogallol for oxygen or caustic potash for carbon dioxide) introduced via pellets into the mercury column to absorb specific gases without combustion.
- Ure’s Syphon Eudiometer: A U-shaped graduated glass tube where an air cushion in the open limb dampens explosive pressure waves, preventing glass breakage during violent detonations.
- Winkler Industrial Gas Burette: Modernized industrial apparatus adapting eudiometric volumetric absorption principles for flue gas analysis in chemical factories and coal plants.
Limits and risks
- Vessel Explosion Hazard: Sparking flammable mixtures (especially hydrogen and oxygen at stoichiometric 2:1 ratio) generates high shockwave pressures; defective or thin glass will shatter violently into sharp fragments.
- Mercury Toxicity: Using mercury as a confining liquid generates toxic heavy-metal vapors; handling requires strict ventilation and spill prevention measures.
- Thermal Contraction Delay: Combustion generates intense local heat that temporarily expands the remaining gas; volume measurements must only be recorded after the gas has completely cooled back to ambient room temperature.