A eudiometer is a graduated glass tube used to measure the volume of gas produced or consumed in a chemical reaction. It works by collecting gas through water displacement, allowing precise volume readings from markings printed along the tube. You’ll most commonly encounter eudiometers in chemistry labs, where they’re used to study gas behavior, verify chemical formulas, and calculate fundamental constants like the ideal gas constant.
How a Eudiometer Is Built
A standard eudiometer is a long tube made of borosilicate glass, sealed at one end and open at the other. The closed end contains two platinum electrodes, which can deliver an electric spark to ignite gas mixtures inside the tube. Graduation markings run along the length of the glass, letting you read gas volume directly. Common sizes include 50 mL tubes marked in 0.1 mL increments (about 22 inches long) and 100 mL tubes marked in 0.2 mL increments (about 26 inches long).
The open end of the tube sits submerged in a water bath or trough. This creates a seal that traps gas in the closed upper portion while still allowing pressure to equalize through the water column. The whole setup relies on the principle of communicating vessels: gas pressure inside the tube pushes down on the water, and atmospheric pressure pushes up from the open trough, letting you calculate the true pressure of the collected gas.
How Gas Collection Works
The basic procedure starts with filling the eudiometer completely with water and inverting it into a water-filled trough. A chemical reaction generates gas, which bubbles up into the closed end of the tube, displacing water downward. As gas accumulates, the water level inside the tube drops, and you read the volume directly from the graduated markings at the gas-water boundary.
A common classroom experiment uses a small strip of magnesium metal reacting with hydrochloric acid. The reaction produces hydrogen gas, which rises into the eudiometer. Once the reaction finishes, you record the volume of hydrogen from the tube’s scale, note the water temperature and atmospheric pressure from a barometer, then use those values to solve for the gas constant in the ideal gas equation (PV = nRT). Comparing your experimental result to the known value tells you how accurate your measurements were.
Because water vapor is always present inside the tube alongside the collected gas, you need to subtract the vapor pressure of water at your measured temperature to get the true pressure of just the gas you’re studying. This correction is a standard part of any eudiometer calculation.
Where the Name Comes From
The eudiometer was invented around 1775 by the Italian scientist Marsilio Landriani, who named it after the Greek word “eudios,” meaning “goodness of the air.” Landriani designed it specifically to measure how healthy a sample of air was, essentially testing its oxygen content. A few years earlier, in 1772, Joseph Priestley had discovered that nitric oxide could react with the oxygen in ordinary air and remove it. By mixing a known volume of nitric oxide with an air sample inside a sealed tube over water, the oxygen would be consumed and converted into water-soluble nitrogen compounds that dissolved away. The remaining volume told you how much oxygen had been present.
This early version worked differently from modern eudiometers. Instead of collecting gas from a reaction, it removed gas from a sample and measured the shrinkage. Later designs added platinum electrodes so that hydrogen and oxygen mixtures could be ignited by a spark inside the tube. Measuring the volume before and after the combustion revealed exactly how much of each gas had been consumed, giving a direct readout of oxygen content.
What Eudiometers Are Used For
In chemistry education, eudiometers are a hands-on way to connect abstract gas laws to real measurements. Students collect a known gas, measure its volume, temperature, and pressure, then plug those numbers into equations to calculate values like the universal gas constant. The percent error between their result and the accepted value becomes a lesson in experimental precision.
Beyond the classroom, eudiometers have practical research applications. Materials scientists use them to measure how quickly metals corrode by collecting the hydrogen gas released when a metal sample reacts with a surrounding fluid. In one common setup, metal samples are sealed in a solution, and the hydrogen that evolves over days or weeks is funneled into a eudiometer. The total volume of gas collected directly indicates how much metal has dissolved, giving a corrosion rate without needing to weigh the sample repeatedly.
Eudiometers also appear in combustion analysis, where an unknown gas mixture is sparked inside the tube and the volume change reveals the mixture’s composition. If you introduce a hydrocarbon gas along with excess oxygen, ignite it, and measure how much the total volume decreases, you can work backward through stoichiometry to determine the molecular formula of the original gas. This technique is especially useful for identifying simple hydrocarbons and confirming the purity of gas samples.
Reading and Calculating Results
Volume is read directly from the eudiometer’s graduated markings in milliliters, then converted to liters for most gas law calculations. Pressure comes from a laboratory barometer, typically in millimeters of mercury. Temperature is measured at the water bath in Celsius and converted to Kelvin by adding 273.15. You also need to account for the height difference between the water level inside the tube and the water level in the trough, since that column of water exerts its own pressure on the trapped gas.
With all four variables collected (pressure, volume, temperature, and moles of gas from stoichiometry), you can solve the ideal gas equation for any unknown. The most common exercise solves for R, the gas constant, with an accepted value of 0.08206 L·atm/mol·K. Students then calculate their percent error to evaluate how well they controlled their measurements. Common sources of error include misreading the meniscus on the water level, not allowing the gas to reach room temperature before recording, or failing to subtract water vapor pressure.
Eudiometer vs. Gas Syringe
A gas syringe is another tool for measuring gas volume, and it works by letting gas push a plunger along a graduated barrel. The key difference is that a gas syringe collects gas dry, while a eudiometer collects gas over water. This means eudiometer measurements always require a water vapor correction, but the water seal also prevents gas from leaking out, which can be an issue with loose-fitting syringes. Eudiometers are generally better suited for reactions that produce gas slowly over long periods, like corrosion studies, because the water seal holds indefinitely. Gas syringes are more convenient for quick, short-duration reactions where you want a fast reading without dealing with water chemistry.