The Orsat gas analyser is a classic piece of laboratory equipment used for the volumetric analysis of various gaseous mixtures, most commonly flue gases from combustion processes. Developed in the late 19th century by French chemist H. Orsat, it provides a method for determining the percentage composition of specific gases such as carbon dioxide (CO₂), oxygen (O₂), and carbon monoxide (CO) within a gas sample. The principle of operation relies on the selective absorption of different gases by specific chemical reagents, followed by the measurement of the resulting volume changes.
Principles of Operation
The Orsat analyser operates on the principle of volumetric absorption. A precisely measured volume of the gas sample (typically 100 mL) is sequentially passed through a series of absorption pipettes, each containing a reagent designed to selectively absorb a particular component gas. After each absorption step, the remaining volume of the gas mixture is measured in a calibrated burette. The decrease in volume observed after passing through a specific reagent corresponds to the volume of the gas component absorbed by that reagent. Nitrogen (N₂) or other inert gases are typically determined by difference, assuming the total volume equals 100%.
Components
A standard Orsat apparatus consists of several key components:
- Measuring Burette: A calibrated glass tube, typically with a 100 mL capacity, graduated to 0.1 mL. It is usually water-jacketed to maintain a constant temperature, minimizing volume changes due to thermal expansion or contraction.
- Absorption Pipettes: A series of two to four U-shaped or similar glass vessels, each containing a specific liquid chemical reagent. These pipettes are designed to maximize the contact surface area between the gas and the reagent.
- Manifold: A series of glass tubes with stopcocks connecting the measuring burette to each of the absorption pipettes and to the gas inlet and outlet.
- Leveling Bottle (or Aspirator Bottle): A bottle containing a confining liquid (usually water acidified with sulfuric acid to prevent CO₂ absorption, or a saturated salt solution) that is connected to the bottom of the measuring burette. By raising or lowering this bottle, the gas sample can be drawn into the burette, pushed into the absorption pipettes, and the volume can be read at constant pressure.
- Gas Inlet and Outlet: Connections for introducing the sample gas and expelling residual gases.
Reagents
The most common reagents used in an Orsat analyser, and the gases they absorb, are:
- Potassium Hydroxide (KOH) solution (typically 25-30%): Absorbs Carbon Dioxide (CO₂).
- Reaction: CO₂ + 2KOH → K₂CO₃ + H₂O
- Alkaline Pyrogallol solution: Absorbs Oxygen (O₂). This solution contains pyrogallic acid dissolved in potassium hydroxide.
- Reaction: Pyrogallol + O₂ (in alkaline medium) → Oxidized products
- Acidic Cuprous Chloride (CuCl) solution: Absorbs Carbon Monoxide (CO). This solution is typically prepared by dissolving cuprous chloride in hydrochloric acid.
- Reaction: CuCl + CO → CuCl·CO (a complex is formed)
The order of passing the gas through the reagents is crucial: CO₂ must be absorbed first, followed by O₂, and then CO. This is because some reagents can absorb more than one gas (e.g., alkaline pyrogallol will absorb CO₂ if not already removed, and cuprous chloride can react with oxygen).
Procedure
- Purging: The apparatus is first purged with the sample gas to remove any air or previous sample.
- Sampling: A precise volume (e.g., 100 mL) of the gas sample is drawn into the measuring burette.
- CO₂ Absorption: The gas is then passed into the first pipette containing KOH solution. After sufficient contact time, the gas is returned to the burette, and the new volume is read. The decrease in volume represents the percentage of CO₂.
- O₂ Absorption: The remaining gas is then passed into the second pipette containing alkaline pyrogallol. After absorption, it's returned to the burette, and the volume difference indicates the percentage of O₂.
- CO Absorption: Finally, the gas is passed into the third pipette containing acidic cuprous chloride. The resulting volume change gives the percentage of CO.
- Nitrogen (N₂) and Other Inerts: The remaining volume in the burette, if any, is assumed to be nitrogen and other inert gases (e.g., argon), determined by subtracting the absorbed volumes from the initial sample volume.
Applications
Historically, Orsat analysers were widely used in:
- Combustion Control: Monitoring flue gas composition in boilers, furnaces, and internal combustion engines to optimize fuel efficiency and reduce pollutant emissions.
- Industrial Processes: Analyzing gas mixtures in various chemical and metallurgical industries.
- Mine Safety: Checking for dangerous levels of CO and other gases in mine atmospheres (though often supplanted by more robust and real-time detectors).
- Educational Settings: As a teaching tool to demonstrate basic gas analysis principles.
Advantages and Disadvantages
Advantages:
- Simplicity and Portability: Relatively simple in design, robust, and does not require electricity, making it suitable for field use.
- Low Cost: Inexpensive to purchase and operate compared to modern electronic analysers.
- Direct Measurement: Provides a direct volumetric measurement of gas components.
Disadvantages:
- Manual and Time-Consuming: The analysis is labor-intensive and requires a skilled operator.
- Limited Accuracy: Accuracy can be affected by temperature fluctuations, reagent degradation, and operator skill, generally 0.1% to 0.5% by volume. Not suitable for trace analysis.
- Reagent Degradation: Reagents have a limited lifespan and can become exhausted or degrade, especially alkaline pyrogallol which reacts with atmospheric oxygen.
- Hazards: Involves handling corrosive and potentially toxic chemical reagents.
- Lack of Continuous Monitoring: Provides only a snapshot analysis, not continuous monitoring.
Evolution and Modern Alternatives
While the Orsat analyser was a cornerstone of gas analysis for much of the 20th century, its use has significantly declined with the advent of more advanced, accurate, and automated analytical techniques. Modern alternatives include:
- Gas Chromatography (GC): Offers high accuracy, separation of complex mixtures, and automation.
- Non-Dispersive Infrared (NDIR) Analysers: Used for CO₂, CO, and hydrocarbons.
- Electrochemical Sensors: Commonly used for O₂, CO, and other toxic gases in portable detectors.
- Paramagnetic Oxygen Analysers: Provide accurate and continuous oxygen measurements.
Despite being largely superseded, the Orsat gas analyser remains a historically significant instrument and can still be found in some educational laboratories or for basic, quick checks where high precision is not critical.