investigating the amount of ascorbic acid present in fruit

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Violet J. Lule

Investigating the Amount of Ascorbic Acid Present in Fruits:


To investigate the amount of ascorbic acid present in samples of fruit.


What is Ascorbic Acid and Why It Is Needed:

Ascorbic acid is another name for vitamin C and is an essential vitamin needed for everyday life. Vitamin C is also soluble in water, which is important to humans since we are unable to synthesize our own ascorbic acid within the liver. It is also an antioxidant (which can help prevent cancer) and is added to foods to protect their colour and aroma. Furthermore ascorbic acid is used to prevent the oxidation of fats and oils by atmospheric oxygen, resulting in the development of rancid flavours. In addition ascorbic acid is vital for the production of collagen (intermolecular substance that gives bones, teeth and cartilage their structure). Other reasons why it is needed include:

  • Synthesis of bile acids
  • Maintaining skin elastic
  • Assists in iron absorption
  • Improves resistance to infection

Deficient Amount of Ascorbic Acid:

An insufficient diet of ascorbic acid should be best avoided to refrain from these symptoms such as:

  • Fatigue
  • Insomnia
  • Loss of appetite
  • Minor capillary bleeding

In a very severe case:

  • Scurvy (in which the legs are shaped in an abnormal way).

Using Titration to find out the  Known Amount of Vitamin C:

The titration method can be used to find out the amount of ascorbic acid that is present in samples of fruit and vegetables. Once a permanent colour change has happened, this is when end point has been reached. The solution will go from a colourless to a dark blue–black colour, when it it is titrated against a known concentration of N -Bromosuccinimide solution. The presence of the blue-black is due to the KI and starch solution reacting together, the KI acts as an indicator by having the presence of iodide and acetic acid acts as a 'source of acidic solution'. Once you have noticed a blue-black colour that is when you will know there is presence of ascorbic acid in the sample of fruit or vegetable.


Equation for the Reaction:

C6H8O6 + C4H4BrNO2 C6H6O6 + C4H5NO2 + HBr

This equation shows that 1 mole of ascorbic acid will be titrated against 1 mole of N-Bromosuccinimide to make 1 mole of dehydroascorbic acid, succinimide and hydrogen bromide.


In this experiment the 4% KI and 1% soluble starch will act as indicator. The 1% soluble starch will react with the 4% KI to form a dark blue-black complex. When this colour change happens, this is when you have reached end point.

Health & Safety:

Hazards of N-Bromosuccinimide Solution (NBS):

NBS solution is very dangerous if you are not protected from it. NBS solution can cause irritation to the eyes and skin if split and also more harmful effects such as burning pain in the nose and throat, coughing & shortness of breath. The procedures I took to protect myself from this happening were to:

  • Wear safety goggles in case some of this harmful solution came into contact with my eyes. The safety goggles will prevent the NBS solution from entering my eyes.
  • Wear protective gloves to prevent skin exposure, this decreases the risk of the NBS solution from coming into contact with my skin. Also I will be wearing a lab to make sure that it covers the exposed areas of my skin; this will prevent irritation of the NBS solution if it was split on the skin.
  • Wipe the table with a table cloth if any of the NBS solution spilled onto the table. This will help prevent any indigestion and inhaling taking place. Also I will open the windows to prevent the vapour becoming an irritant to the respiratory tract.


If any is spilt onto my eyes, skin, swallowed or inhaled; these are the precautions I should take:

Eyes: Immediately flush eyes with plenty of water for at least 5 minutes, occasionally lifting the upper and lower eyelids. Then get medical aid immediately.

Skin: Immediately cleanse skin with plenty of water for at least 15 minutes while removing the contaminated clothing and footwear. Get medical aid if irritation develops or persists.

Ingestion: If the victim is conscious and alert, give 2-4 cupfuls of water or milk. Then get medical aid immediately.

Inhalation: Remove from exposure and move to fresh air immediately. If the victim is not breathing, give artificial respiration. If the victim is finding it difficult to breath, give the victim oxygen, afterwards get medical aid.

Hazards of Sulphuric Acid (H2SO4):

Sulphuric acid is highly corrosive to the skin. Also its vapours are corrosive to the respiratory tract and can cause fluid build up on the lungs which could be fatal. Because of this, I will take safety procedures to prevent this from happening:

  • Sulphuric acid can cause severe burns if splashed onto the skin. To prevent this from happening I will wear a lab coat and protective gloves at all times. Also if any sulphuric acid spills onto the table I will clean it up quickly, this will reduce the chances of it harming me or someone.
  • Furthermore sulphuric acid can cause severe burns or prolonged and permanent damage to the eyes. To prevent this from happening I will wear safety goggles at all times to prevent H2SO4 from getting into my eyes.
  • To prevent the vapour from H2So4 becoming an irritant I will open the window to ensure that the room is well ventilated.

Hazards of Acetic Acid:                                                                        

Acetic acts as an irritant to the eyes skin, nose and throat. It can also cause skin burns. To prevent this from happening I will:

  • Wear safety goggles at all times so that I prevent the acid becoming an irritant to my eyes.
  • Also I will wear protective gloves and a lab coat at all times to prevent myself from any skin burns. Also I will make sure to wipe the table with a cloth if any spill.

Hazards of Potassium Iodide (KI):

The hazards of KI are not as harmful as the acids I previously mentioned. Although if inhaled or splashed onto the face, this may cause irritation to the lungs or eyes. To prevent this from happening I will:

  • Make sure that the windows are open to keep the room well ventilated and thus preventing inhalation of KI.
  • Also I will wear safety goggles at all times to prevent the KI from going into my eyes if I was to accidentally splash it.

Hazards of Starch Solution:

The hazards of starch solution involve irritation and watering to the eyes. In addition if this is ingested this can cause nausea, to decrease the chances of this happening I will:  

  • Wear safety goggles at all times to prevent the irritation and watering of my eyes.
  • Also I will make sure to open the window so that he room is well ventilated and thus preventing watering of my eyes.


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Preparing the 5% H2SO4:

  1. Wear safety goggles and a lab coat for the protection of harm from acids.
  2. Pour 50cm3 of distilled water into a 100cm3 glass measuring cylinder.
  3. Then pour in 5cm3 of H2SO4.
  4. Fill up the 100cm3 glass measuring cylinder to the top with distilled water.
  5. When filling up the 100cm3 glass measuring cylinder, make sure that you have good eye-level and that the dip is accurately on the meniscus line.
  6. Pour the mixture into a 250cm3 conical flask (making sure that to tap cylinder so that ...

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