The Use of Benzoic Acid in Bomb Calorimeters

Benzoic Acid is a colourless crystalline solid and a simple carboxylic acid. The name is derived from gum benzoin, which was for a long time its only known source. Benzoic Acid occurs naturally in many plants and it serves as an intermediate in the biosynthesis of many secondary metabolites. Salts of Benzoic Acid are used as food preservatives and benzoic acid is an important precursor for the industrial synthesis of many other organic substances. The salts and esters of benzoic acid are known as benzoates. - Wikipedia

Although benzoic acid has many uses outside of bomb calorimetry, we will touch on the subject of its use in calorimetry. Benzoic acid is relatively non-toxic and occurs naturally in plants.

Appreciable amounts of it has been found in most berries (around 0.05%) and is formed in apples after infection with a specific fungus.

LABORATORY USE

Due to its constant value, it is the ideal substance for calibration in bomb calorimeters. For this purpose, we press the powdery substance into tablets of approximately 0.5 grams and 1.0 grams, with a calorific value of approximately 26.454KJ/g.

CALIBRATION OF OXYGEN BOMB CALORIMETERS

The calibration of an oxygen bomb calorimeter system (also called standardization) is done using a tablet of benzoic acid. DDS Calorimeters manufacture our own benzoic acid tablets which are suitable for use in all our oxygen bomb calorimeter systems. During this calibration process, a known amount of heat is introduced into the calorimeter by the combustion of a benzoic acid tablet. This produces the energy equivalent of the calorimeter for a specific temperature rise.

Before an unknown sample can be tested, the heat capacity or energy equivalent (EE or E) of the bomb calorimeter must be determined. The value resulting from the burning of the calibration standard Benzoic Acid represents the heat capacity of the calorimeter (including the bomb vessel, insulating polystyrene or ATC and all other parts of the calorimeter system). Energy equivalents are determined at regular intervals by burning a sample of the standardization material with a known heat of combustion (Benzoic Acid at 26.454KJ) under controlled and reproducible operating conditions.

DESIRABLE PROPERTIES FOR BENZOIC ACID

Benzoic Acid tablets used for combustion calorimetry should adhere to the following requirements:

• It must be readily available in pure form
• It must be stable
• It must not be hydroscopic
• It must not be volatile
• It must be easily brought into a form suitable for combustion
• It should offer no unusual difficulties during combustion in the bomb
• The value of the heat of combustion is characterized with suitable accuracy

While DDS Calorimeters Benzoic Acid Tablets adhere to the above requirements, it is also used for the following applications:

• It is advantageous for tests involving materials that are difficult to handle or ignite and assists with the burning of the sample during a determination.

BENZOIC ACID SAFETY & PRECAUTIONS

• SWALLOWED - Accidental ingestion of material may be harmful; ingestion of more than 150g may be fatal or may produce serious damage to the health of the individual. Hazardous in case of ingestion.
• EYE CONTACT - There is evidence that material may produce eye irritation in some people and produce eye damage 24 hours or more after instillation. Severe inflammation may be expected with pain. Hazardous in case of eye contact (irritant).
• SKIN CONTACT - The material may cause moderate inflammation of the skin either following direct contact or after a delay of some time. Open cuts, abraded or irritated skin should not be exposed to this material. Examine the skin prior to the use of the materials and ensure that any external damage is suitable protected. Hazardous in case of skin contact (irritant).
• INHALED - Benzoic acid in large doses is toxic to lungs, the nervous system, and mucous membranes. Repeated or prolonged exposure to Benzoic Acid can produce target organ damage.