STP is a standard set of conditions used in gas calculations in chemistry and engineering. The values depend on the desired applications, industries or academic settings. The most common set of standards historically was 32°F and 1 atmosphere, but currently, 1 bar is accepted by IUPAC. The ideal gas law can be used to calculate molar volume of an ideal gas at STP. Different industries may have different STP values depending on regional conditions and personal preferences.
In chemistry, physics, and engineering, the term “standard temperature and pressure” (STP) refers to a standard set of conditions for a gas as a basis for calculations. There are several definitions for standard temperatures and pressures and the values of these references will depend on the desired applications, industries or academic settings. Historically, the most common set of standards was a temperature of 32 degrees Fahrenheit (0 degrees Celsius) and a pressure of 1 atmosphere, or 101,325 kilopascals (kPa), although a pressure of 1 bar, or 100 kPa, is currently accepted by the European International Association of Pure and Applied Chemistry (IUPAC). The standard pressure reflects the accepted mean pressure of the atmosphere at sea level, and the standard temperature reflects the freezing point of pure water at that pressure.
Standard conditions are most often used for gas calculations in chemistry and engineering. For example, the molar volume of an ideal gas at STP can be calculated using the ideal gas law. In engineering, standard conditions could be used to calculate gas mass flows based on the standard volume flows of flow meters.
The ideal gas law shows the relationships between pressure (P), volume (V), and temperature (T) for an ideal gas. It can be expressed as PV=nRT, with n referring to the number of moles of gas and R referring to the ideal gas constant. Using the current standard temperature and pressure as defined by the IUPAC, 1 mole of an ideal gas will have a volume of 1,385.9 cubic inches (22,711 liters).
Standard temperature and pressure could actually refer to many different conditions used by various industries. For example, an engineering firm might decide to use 1 atmosphere and 77 degrees Fahrenheit (25 degrees Celsius) as the STP to better reflect the environmental conditions of the area. The Society of Petroleum Engineers, for example, defines standard conditions as 59 degrees Fahrenheit (15 degrees Celsius) and 1 bar. When working with engineering projects, it is important for a person to understand the standards expected by the designer.
Differences in accepted values for standard temperature and pressure may depend on regional conditions. Higher pressures and temperatures make more sense for laboratories and industries located in areas prone to long periods of heat. Lower pressures could be used for laboratories at high altitudes. Of course, differences in accepted values could also be due to the personal preferences of chemists, lab technicians and engineers.
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