The ratio of the mass of a substance to its volume or the mass of its unit volume is called density. It is different for all substances and depends on their nature and composition. For example, for fresh water it is approximately 1.003 g / cm3, and for sea water - 1.025 g / cm3. Also, this physical quantity obeys the additivity rule. That is, the density of a multicomponent mixture (such as gasoline, for example) will be equal to the sum of the products of this characteristic of each component by its mass fraction: Ο mixtures = βΟi β’ ci. The calculation method can be used with a known composition, as well as in cases where it is not necessary to determine the density of gasoline with high accuracy. Another disadvantage of the calculation method is the complexity and high cost of determining the mass fraction of each component. It requires expensive chromatographic equipment and highly qualified personnel, since each brand of oil product is characterized by wide variability of sets of a large number of individual, mainly organic substances (hydrocarbons).
The qualitative and quantitative composition that determines the density of gasoline varies and depends on the feedstock and the technological method of its processing. Get different brands by direct distillation of oil, reforming, isomerization, alkylation, catalytic cracking. Gasoline mainly consists of a mixture of paraffins, naphthenes and olefins. Currently, many countries have introduced restrictions on the content of aromatic hydrocarbons and olefins. This situation is caused by environmental requirements and the need to reduce carbon formation, which occurs due to the presence of benzene and ethylene compounds in the fuel, they often contain gasoline. The density of products whose octane is increased due to aromatic substances is usually higher than that of fuels with a lower content of benzene and its homologs.
In addition, the finished product may contain special additives. They also affect the density of gasoline. For example, oxygen-containing compounds such as ethanol can increase the completeness of fuel combustion and reduce the concentration of carbon monoxide in the exhaust gases. The regulatory document and technological regulations of the enterprise determine those components that can be added to the fuel. Currently, several documents are in force in our country that standardize the quality of motor fuels for vehicles with gasoline engines.
Interstate standard GOST 2184-77 was reissued in February 2003 with amendments 1-6 from 1981, 1983, 1984, 1990, 1997 and 2002, as well as with two amendments from 2000 and 2001, it is recognized by 10 states - former republics USSR. In the Russian Federation, its requirements apply only to the unleaded product of the A-76 brand. The density of gasoline at 20 Β° C is not regulated by this GOST, but its determination is mandatory before shipment to the consumer. For this, areometric and pycnometric test methods are used, which are described by GOST 3900-85.
The state standard of our country GOST R 51105-97 (changes No. 1 and No. 2 published in 1999 and 2000) has been applied since 01/01/1999 and applies to unleaded automobile gasolines, which are divided into brands: Super 98, Premium 95, Regular 92, Regular 91, Normal 80. The density of gasoline 92 and other products with an octane rating of 91 to 98 is normalized at 15 Β° C and ranges from 725 to 780 kg / m3. For a product of the Normal 80 brand, it can vary from 700 to 750 kg / m3. This characteristic is determined using a hydrometer according to the test method according to GOST R 51069-97.
Another state standard - GOST R 51886-2002 - was put into effect in the Russian Federation from 07/01/2002, and from 01/01/2008 amendment No. 1 was issued to it. The document complies with the European normal EN 228-2004, it applies to unleaded gasoline of Super Euro brands -98 and Premium Euro-95. They are characterized by an octane rating of 98 and 95, respectively. The density of gasoline of both grades at 15 Β° C is in the range 720β750 kg / m3 and is determined according to GOST R 51069-97 by the areometric method.
What caused the need to determine the density of motor fuel? Firstly, this value indirectly characterizes the composition of the product and helps to identify it. For example, if the measured density is 0.980 kg / m3 or 0.650 kg / m3, then we can say with confidence that this is not gasoline, but some other product. The density of gasoline is also widely used in practice: knowing the volume of a product in a large tank that cannot be weighed, its mass is calculated by multiplying the density by the volume. This method is called indirect, the requirements for it contain GOST R 8.595-2004. Measure the temperature and level of the product in the tank and calculate its volume. Point samples are taken in accordance with the requirements of GOST 2517-85, combine them and determine the density according to GOST 51069-97 or according to GOST 3900-85. The measurement results lead to standard conditions (temperature 15 or 20 Β° C), only after that these values ββare used to calculate the mass of the product in the tank.