In the process of experimentation, we often encounter challenges related to units and conversions of gas concentration, gas yield, and reaction rate. Due to the frequent issue of non-uniform description units between experimental requirements and the standard parameters provided by the equipment, this episode will guide you to understand various common gas units encountered in photocatalysis experiments.
Firstly, let's review some high school chemistry knowledge.
mol, mmol, and μmol
Mole (mol), abbreviated as mol, is the unit of amount of substance, and each 1 mol of microscopic substance contains approximately 6.03 × 1023 particles.
1 mol = 1000 mmol = 1000000 μmol
mol, g, μmol, mg
mol, L, μmol, mL
Now, let's begin the formal explanation for this episode.
When conducting experiments related to gases, standard gases such as CO and CH₄ standard gases, H₂ standard gases, etc., are commonly used. Many standard gases are mixtures, such as the standard gases for CO and CH₄, which are mixtures of CO, CH₄, and Ar. The units for annotating concentration are varied, with common ones on gas cylinders being volume concentration (V/V) or ppm, as shown in Figure 1.
Figure 1. Standard gas cylinder label
Using percentage to represent gas content is not precise enough. In most experiments and industrial fields, mass concentration units (e.g., mg/m3) are used to indicate gas concentration. So, how do we calculate mg/m3 when the unit on the gas cylinder is ppm?
First, let's understand the definition of ppm. Ppm is the abbreviation for Parts Per Million, a unit of volume fraction concentration, representing the percentage of solute mass to the total solution mass, also known as the percentage concentration. Similar units include ppb (Parts Per Billion).
For gases, ppm generally refers to molar fraction or volume fraction.
For solutions, ppm generally refers to mass concentration.
The specific conversion relationships are as follows:
The general gas concentration refers to mass concentration c, in units of mg/m3, and mass concentration is related to the temperature and pressure of the gas, which is usually not directly measurable. It needs to be converted based on the measurement results to a standard state (0°C, 101.325 kPa, or 760 mmHg). The measured concentration is usually volume concentration, often expressed in ppm. To obtain mass concentration c, it needs to be converted according to the following formula:
Where M is the molecular weight of the gas; T is the temperature of the gas (in degrees Celsius, denoted as ℃); P is the atmospheric pressure (in pascals, denoted as Pa; 1 Pa is equivalent to 100 kPa); 1.01325 is the value of standard atmospheric pressure, i.e., standard atmospheric pressure is 1.01325 Pa; 273 is the conversion factor between absolute temperature (Kelvin) and Celsius temperature; 273 K = 0°C. The pressure correction formula is based on the gas state equation, and the temperature unit in the state equation is Kelvin, so it is necessary to convert Celsius temperature to Kelvin temperature.
Since most parameters are often calculated under standard temperature and pressure, the simplified relationship can be obtained as follows:
Using mass concentration units (mg/m3) as the representation of the concentration of mixed gases makes it convenient to calculate the true amount of pollutants. However, mass concentration is related to the temperature and pressure conditions of the gas being detected, and its value will vary with changes in temperature, air pressure, and other environmental conditions. Therefore, in actual measurements, the temperature and atmospheric pressure of the gas need to be simultaneously determined. On the other hand, using ppm to describe gas concentration adopts volume ratio, and this issue does not arise.
The laboratory methane gas is labeled as 10%LEL. What does it mean in ppm?
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%VOL, %LEL, and %UEL
%VOL is the percentage of gas volume, where VOL stands for Volume. It mainly refers to how much gas content is in 100 volumes.
%LEL stands for the Lower Explosive Limit, which is the minimum volume concentration of flammable gas in the air that will explode when exposed to an open flame. %LEL is the percentage of the explosive lower limit, divided into one hundred units, with one unit as 1%LEL.
%UEL is the abbreviation for Upper Explosive Limit, which is the maximum concentration of flammable gas in the air that will explode when exposed to an open flame.
When the concentration of flammable gas is too low or too high, it may not pose a significant risk. However, when mixed with air at a certain ratio, combustion or explosion may occur. Below the explosive lower limit, the concentration of flammable gas in the mixture is insufficient to cause combustion or explosion; above the upper limit, the concentration of oxygen in the mixture is insufficient to cause combustion or explosion.
The conversion between %VOL and %LEL involves first determining the explosive lower limit value of the flammable gas. When the concentration of flammable gas in the air reaches its explosive lower limit, the explosive hazard of the flammable gas environment in that space is considered 100%.
For example, the %LEL of H₂ is 4%VOL, meaning that it will explode when its volume percentage in the air reaches 4%VOL when exposed to an open flame. Therefore, considering 4%VOL as 100% hazard, which is termed 100%LEL, 4%VOL = 100%LEL. Consequently, 1%VOL = 25%LEL.
Conversion between ppm and VOL ↑
Therefore, if the laboratory methane gas is labeled as 10%LEL, what does it mean in ppm?
Since ppm cannot be directly converted to LEL, it is necessary to first convert LEL to VOL and then convert VOL to ppm. The explosive lower limit of CH₄ is 4%VOL, and 10% to 25% of the explosive lower limit is generally used as the alarm value. Therefore, for 10%LEL of methane gas, the corresponding relationship is as follows:
10%LEL = 10% × 4%VOL = 0.4%VOL = 4000 ppm
This means that the laboratory CH₄ gas labeled as 10%LEL is equivalent to 4000 ppm.
In common situations, the volume concentration of O₂ is represented by VOL, while the volume concentration of flammable gases such as CH₄ is represented by %LEL. Other toxic gases such as SO₂, H₂S, formaldehyde, etc., use ppm to represent volume concentration values.
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The μGAS1000 Trace Gas Reaction Evaluation System has the remarkable feature that its computer-side software can integrate control over the system, gas chromatograph, and vacuum pump. Through the controllable sampling structure, it achieves fully automatic online sampling and injection. The upper computer software of The μGAS1000 Trace Gas Reaction Evaluation System uses built-in calculation methods to read the test data from the gas chromatograph and generates reports through the software interface, calculating gas yield, reaction rate, and other data.
Figure 2. Software interface of The μGAS1000 Trace Gas Reaction Evaluation System
Figure 3. Experimental data exported by The μGAS1000 Trace Gas Reaction Evaluation System
Through this scenario-based learning process, we sincerely wish all the researchers on the scientific research path to smoothly solve the unit conversion challenges related to gas concentration, gas yield, and reaction rate during scientific experiments and achieve fruitful results!
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