Specific gravity may be defined as the relative weights of equal bulks or volumes of different bodies, water being the recognized standard for solids and liquids, and hydrogen or air the standard for gases. Density should not be confused with specific gravity. They may have the same values and are frequently used interchangeably. Specific gravity is relative, while density is weight of a unit volume. In taking the specific gravity of solids or liquids, the object sought is always the weight of a volume of water equal to that of a volume of the substance, the specific gravity of which we desire, whether that substance be soluble, insoluble, lighter or heavier than water. Then the weight of the substance divided by the weight of an equal volume of water gives the specific gravity. The specific gravity of liquids is usually taken with a specific gravity flask (pyenometer) or a hydrometer. The volume of all bodies varies with change in temperature. It is therefore necessary that determinations should be made at a stated temperature. For scientific purposes 4° C. is selected, because it is the temperature at which water has its greatest density. For other reasons working-room temperature is to be preferred. If a liquid be measured at a reduced temperature and weighed at a higher temperature, expansion occurs and part of the liquid is forced through the capillary tube in the cork and lost by evaporation before an accurate weighing can be made. Another source of error is the condensation of moisture from warm air upon a cold surface. The United States Pharmacopeia has adopted 25° C. as the temperature for taking specific gravity except in the case of a few liquids with a particularly low boiling point. Great Britain has adopted 15.6°; Germany and France, 15° C. In ordinary work barometric pressure is ignored. It is assumed to be normal, 760 mm. Usually the substance and the water are weighed at the same temperature, but as this is not always the case it is best to express the temperature thus: The numerator indicates that the substance was weighed at 25° and compared with an equal volume of water weighed at the temperature indicated by the figures in the denominator.
Key Takeaways
- Specific gravity is a measure of relative weight, using water as a standard for solids and liquids.
- Density should not be confused with specific gravity; density refers to the weight per unit volume.
- Temperature significantly affects measurements, so it's crucial to take readings at consistent temperatures.
Practical Tips
- Use a pycnometer or hydrometer to measure the specific gravity of liquids accurately.
- Ensure that both the substance and water are measured at the same temperature for accurate results.
- For practical purposes, working-room temperature (around 25°C) is often sufficient.
Warnings & Risks
- Measuring substances at different temperatures can lead to inaccurate readings due to expansion or condensation.
- Ignoring barometric pressure may introduce errors in specific gravity measurements.
Modern Application
While the chapter focuses on pharmaceutical applications, understanding specific gravity remains crucial for modern survival scenarios. Accurately measuring liquid density is vital for water purification and identifying safe drinking sources. The principles of temperature control and precise measurement are still applicable today.
Frequently Asked Questions
Q: What standard should be used when determining the specific gravity of a substance?
Water is typically used as the standard for solids and liquids, while hydrogen or air serves as the standard for gases.
Q: Why is it important to measure substances at consistent temperatures?
Temperature variations can cause expansion or contraction in materials, leading to inaccurate specific gravity readings. Consistent temperature ensures reliable measurements.
Q: What are the preferred temperatures for measuring specific gravity according to different countries?
The United States Pharmacopeia prefers 25°C, while Great Britain uses 15.6°C and Germany and France use 15°C as standard temperatures for taking specific gravity.