Determination of vitamin c concentration by titration calculations

Foundations of Chemistry Laboratory Manual VITAMIN C DETERMINATION 3F

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EXPERIMENT 3F Determination of Vitamin C Concentration (This experiment is done individually.)

Useful background reading (this is not compulsory but may be helpful):

Tro, 4th and 5th Edition: Sections 7.9, 13.6 (repeat), 2.5 (repeat) – Introduction and all Questions

What is the relevance of this prac…?

The last quantitative techniques experiment demonstrated the importance of acquiring

laboratory skills. The better your laboratory skills, the more accurate and precise your

results and therefore the more useful and significant they are. This may seem fairly

unimportant in a first year practical but our environment, our health and our safety depend

every day on the skills of scientists in the medical, transport, food production and energy

sectors, just to name a few.

Scientists depend on their analytical skills for their livelihood and society depends on these

skills in order to live their day-to-day lives. This experiment provides a real-world example of

how laboratory skills are used in industry. These exact quantitative techniques were used to

discredit GlaxoSmithKline – the major company responsible for manufacturing the popular

fruit drink, Ribena.

Learning objectives (remember these are different to the scientific objectives):

On completion of this practical, you should have:

 Become more proficient with the technique of titration (pipetting and buretting).

 Become aware that chemical reactions are grouped into categories that have similar

chemical features. Two such examples are acid-base reactions and redox reactions.

 Successfully prepared a standard solution using correct technique.

Introduction

You have already performed titrations in your last practical session. This is a common

analytical technique that can be used for many analyses. In the last experiment the titration

was used with an acid-base reaction – a solution of base (sodium hydroxide) of known

A BIG Question

How do we unravel the causes of

disease?

Scientists all over the world are trying to determine

what causes many diseases and how to cure them –

cancer, AIDS, diabetes and neurological conditions

are just a few, high profile examples. Their research

(and therefore their ability to help people) is entirely

dependent on how careful and precise their

experimentation is. This practical provides another

chance for you to master some common laboratory

skills.

Foundations of Chemistry Laboratory Manual VITAMIN C DETERMINATION 3F

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concentration was used to determine the concentration of a solution of sulphuric acid. Acids

and bases are found everywhere from food, to our bodies to cleaning products.

There are too many individual chemical reactions for chemists to be familiar with all of

them, so they are divided into categories of reaction that have specific features. For

example, all acid-base reactions like the one between sulphuric acid and sodium hydroxide,

involve an acid which donates a proton to a base. This means that if a chemist knows what

category a reaction falls into they can make predictions about it even if they are unfamiliar

with the specific reacting molecules.

Another category of chemical reaction is the “redox” category. Redox reactions involve one

reactant losing an electron (or electron density) to another reactant. You will learn about

this category of reaction in Semester 2. Just like with acid-base reactions, titrations can be

used with redox reactions. In this experiment you will perform a redox titration to

determine the concentration of Vitamin C and two solutions.

Vitamin C, or ascorbic acid (C6H8O6), is an essential nutrient. Its structure can be seen in

Diagram 2.1 below. Many animals can produce it within their bodies, but humans must

obtain it through their diet. Vitamin C deficiency can lead to skin, bone and teeth issues and

a serious deficiency can cause death. Some sources of Vitamin C are fruits and vegetables

(including citrus fruits, green chili peppers and broccoli), their juices and Vitamin C tablets.

Diagram 2.1 The chemical structure of essential nutrient Vitamin C, one natural source and

synthetically produced Vitamin C tablets.

In determining the amount of Vitamin C in samples, the redox titration is more useful than

an acid-base titration as natural sources are comprised of other acids and bases that may

affect the results.

In this experiment, the redox reaction is between iodine and Vitamin C – the Vitamin C loses

electron density and the iodine accepts it. The overall equation is shown below:

C6H8O6 + I2  C6H6O6 + 2I- + 2H + Equation 2.1

Foundations of Chemistry Laboratory Manual VITAMIN C DETERMINATION 3F

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The endpoint is shown by an indicator, in this case a solution of starch. Iodine, I2, and starch

form a blue-black product when they react. When there is still Vitamin C in the reaction

vessel, it will react with the iodine, I2, and convert this to the product I- which does not react

with the starch indicator. However, once the Vitamin C has all been removed by reaction,

the excess iodine, I2, will react with the starch indicator and cause the solution to change

colour.

Although you will be doing a redox titration rather than an acid-base titration, the method

and technique are exactly the same. You will use the pipetting and titration skills learnt in

the last prac.

In Part One you will perform a titration using a solution of iodine and a standard solution of

Vitamin C.

Chemistry connections…

Recall from Experiment 0 that a standard solution is one of known concentration. The

concentration of standard solutions is usually known because the chemist has made the

solution themselves using volumetric glassware. For example, in your last practical you used

a standard solution of sodium hydroxide (exact concentration provided on the dispenser)

and whilst you didn’t originally know the sulphuric acid concentration, that solution too had

been prepared volumetrically (you checked your titration skills at the end by discovering the

actual concentration at the servery).

In this experiment you must first prepare the Vitamin C standard solution very accurately so

you can use it to confirm the concentration of the iodine solution by titration in Part One.

This procedure is referred to as standardisation – you are said to standardise the iodine

solution. The accurate Vitamin C standard solution is prepared by dissolving pure Vitamin C

(ascorbic acid, a white powder) in water using a volumetric flask.

In Part Two once you have confirmed the iodine solution concentration you can use it to

determine the unknown concentration of Vitamin C samples. These could include Vitamin C

tablets or fruit or, in this case, apple juice. This is the method by which the concentrations of

many species of interest in many different samples may be determined.

The apple juice container advertises how much Vitamin C is present in the drink. You should

be able to determine whether this assertion is correct.

Chemistry connections…

In 2004, two school students in New Zealand performed redox titrations on several brands

of juice and cordial to determine their Vitamin C content. In their school laboratory they

discovered that the cordial drink Ribena did not contain the advertised Vitamin C content.

The manufacturer GlaxoSmithKline ended up in court facing charges of over two million

Australian dollars because of their find.

Foundations of Chemistry Laboratory Manual VITAMIN C DETERMINATION 3F

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In general; as long as you know the concentration of one reactant (in this case the iodine

solution) you can use it to determine the concentration of the other (in this case the Vitamin

C content of commercial sources).

Experimental

Remember: drawing a diagrammatic representation of what you will do in your experiment

can help you to visualize what you’ll need to do during the session and also to not

accidentally miss crucial steps.

Before commencing work you should watch the video, ‘Solution Preparation’. You will need

to equip yourself with headphones and wait until your colleagues are ready before asking

your demonstrator to start the video.

Reminder - each student should work individually. It is important that you work carefully in

order to gain worthwhile experience from this class.

Hazardous substances

Iodine I2 toxic, irritant

Starch (C6H10O5)n irritant

PART ONE VITAMIN C (ASCORBIC ACID) STANDARD SOLUTION PREPARATION AND IODINE SOLUTION STANDARDISATION

PROCEDURE

1 The standard solution of Vitamin C (ascorbic acid) has a desired concentration of 0.005mol/L. Using the molar mass, M, of ascorbic acid and the two equations – n=cxV and n=m/M – calculate the mass of ascorbic acid required to make up 0.25L (250mL) of solution. (Hint: refer to Experiment 0 Introductory Experiment, Question 3 if you need a refresher on how to do this type of calculation. Be sure to check your units.) Check your calculated mass with your demonstrator before proceeding.

Chemistry connections…

Notice that there are no longer questions worth marks in your Report Book about solution preparation calculations (unlike experiments 0 and 1F). Calculations and then solution preparation are standard skills that all chemists are assumed to have. They are therefore never discussed in scientific articles (beyond stating a standard solution’s concentration) and consequently will not appear in your Report Books anymore. Remember that the molar mass and two equations described above can be used to determine the mass of ANY compound in a solution of ANY concentration and volume. In a regular scientific article the experimental procedure would simply state “A 1L solution of 0.005mol/L ascorbic acid was prepared”, so you need to be becoming familiar with the method/calculations you’d require to prepare this yourself.

Foundations of Chemistry Laboratory Manual VITAMIN C DETERMINATION 3F

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2 Weigh out this calculated mass (you only need to weigh the amount approximately but record the weight exactly) then deliver it to a 250mL volumetric flask using a funnel (be sure to get all of the ascorbic acid into the flask, otherwise the concentration will not be correct - rinsing the funnel into the flask with some deionised water may help).

3 Add approximately 100mL of de-ionised water to the volumetric flask. Swirl the flask until all of the Vitamin C is dissolved.

4 Fill the remaining volume to the aliquot line with de-ionised water (do not do this if there is un-dissolved solid present still).

5 Replace the stopper and invert the flask a number of times to ensure that the solution is homogenous and its concentration is uniform.

Chemistry connections…

If the solid dissolves in the water at the very bottom of the volumetric flask and you do not invert it, what affect might this have on the titrations you do? Would the results from these titrations be reliable?

6 Clean two 250 mL beakers carefully and rinse them at least twice with de-ionised water. Shake them to remove most of the water. The remaining water must be removed by drying with paper towel.

Chemistry connections…

Using your knowledge from the last titrations prac can you explain why the beakers must be dry and have no water present in them before use?

7 In one of the clean and dry 250 mL beakers place approximately 150 mL of iodine solution. (It is the concentration of this solution that you are trying to determine. Ie you are about to “standardise” this iodine solution.) In the other beaker pour a similar quantity of Vitamin C solution you prepared in Step 1. (It helps to label your solutions!)

8 Rinse a 50 mL burette well with de-ionised water.

9 Rinse the burette at least twice with the iodine solution, fill the burette (to near the top), wipe the outside with paper towel and mount it on a burette stand at your bench. Make sure the space in the burette beneath the tap also has solution in it because the volume graduations on the side account for this.

Foundations of Chemistry Laboratory Manual VITAMIN C DETERMINATION 3F

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Chemistry connections…

In the last titrations prac the solution of known concentration was placed in the burette and the “unknown” was placed in the conical flask beneath it. Does it matter which solution goes where?

10 Clean and rinse a 250mL conical flask with deionised water and, using a volumetric pipette, transfer to it a 25.00mL aliquot of the Vitamin C standard solution from the beaker (note: you will always pipette from this beaker and NOT from the volumetric flask).

Chemistry connections…

A chemist will never pipette directly from the volumetric vessel that contains a standard solution. The point of a standard solution is that it has a precisely-known concentration that you can use to your advantage (in this case performing a titration to calculate an unknown concentration). In these experiments the results are therefore only as reliable as the standard solution. Why is it not good practice to pipette directly from the volumetric flask?

11 Add 10 drops of starch indicator to the solution in the conical flask.

12 Titrate the solution in your flask. Like in the last experiment, it is generally worthwhile to do the first titration quickly at the cost of accuracy to determine the approximate end-point for the next more accurate runs. This is referred to as the “Rough” run. Record all your titre values in Table 2.1. Note: the end-point is reached at the first sign of blue colour that remains after 20 seconds of swirling the conical flask.

13 Repeat the titration until you have a minimum of two concordant titres (readings that differ by less than 0.1mL). Top up the iodine in the burette as required.

Question 1 – iodine solution standardisation

a) Calculate the mean (average) volume of the titre values you have chosen. Justify any

exclusions you have made.

b) What is the number of moles of Vitamin C present in the 25.00mL you pipette into

each conical flask?

c) Using the balanced Equation 2.1, how many moles of iodine, I2, must have been

present in the amount of iodine solution you titrated?

d) Given this number of moles and the average titre value, what is the concentration of

your iodine solution?

Foundations of Chemistry Laboratory Manual VITAMIN C DETERMINATION 3F

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PART TWO APPLE JUICE INVESTIGATION

PROCEDURE

1 Thoroughly clean and dry a 250mL beaker and transfer to it roughly 100mL of apple juice. Note the Vitamin C concentration advertised with the nutritional information on the apple juice bottle.

2 Clean and rinse a 250mL conical flask and pipette into it a 25.00mL aliquot of the apple juice.

3 Add 10 drops of starch indicator to this conical flask.

4 Ensure there is a sufficient volume of iodine solution in your burette and that it is set up correctly still from Part One.

5 Perform the titrations until you have a minimum of two concordant titres. Record all your titre values in Table 2.2.

Question 2 – Apple juice investigation

a) Calculate the number of moles of iodine, I2, that was involved during the redox

reaction. (Hint: you are calculating n because you know c and V. What equation

should you use? Refer to Experiment 1F Quantitative Techniques for help.)

b) The equation for the redox reaction between iodine and Vitamin C is provided again

below. Using this balanced equation, how many moles of ascorbic acid (Vitamin C) in

the apple juice reacted with the I2 on average in each titration?

C6H8O6 + I2  C6H6O6 + 2I – + 2H+

c) Given the number of moles of ascorbic acid and original pipette volume, what is the

concentration of Vitamin C in the apple juice you tested?

d) The concentration you calculated in Part c) is in mol.L-1. Convert your concentration

from Part c) to g.L -1

(hint: What equation relates number of moles and mass?).

e) Convert the concentration from Part d) to milligrams per litre (mg.L-1)

f) Now convert the concentration from Part e) to milligrams per 100 ml.

g) How does your experimentally determined Vitamin C concentration compare with the

value given on the juice bottle?

h) List the experimental errors that could lead to a discrepancy between the determined

and the advertised value.

i) Considering your experimentally determined value and the possible sources of error,

make a comment about the accuracy of the advertised amount of Vitamin C present.

Foundations of Chemistry Laboratory Manual VITAMIN C DETERMINATION 3F

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Information Sheet

HARMFUL

(can affect health if exposed to large doses

or to low doses over a long period of time)

IODINE SOLUTION

IDENTIFICATION

Name Iodine

Structure I2

PHYSICAL DESCRIPTION AND PROPERTIES

Description brown solution

Boiling Point not available

Melting Point not available

Vapour Pressure not available

Flammability non combustible.

Density not available

Solubility Soluble: water

Reactivity Considered stable.

HEALTH HAZARD INFORMATION

Major hazards

Inhalation or skin contact can cause sensitisation. Cumulative effects may result following exposure

(limited evidence).

Toxicity

Inhalation: Is not thought to produce adverse health effects or irritation of the respiratory tract. Not

normally a hazard due to its non volatility.

Eye contact: May irritate the eye.

Skin contact: May not thought to produce adverse health effects or irritation of the skin.

Foundations of Chemistry Laboratory Manual VITAMIN C DETERMINATION 3F

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Swallowing: Has NOT been classified as “harmful by ingestion”. May irritate the digestive tract,

causing nausea and vomiting.

FIRST AID INFORMATION

Eyes: Hold the eyelid wide open, wash the eye for at least 10 minutes with flowing water.

Lungs: Remove patient to fresh air.

Mouth: Immediately give a glass of water.

Skin: Wash off skin with running water and soap.

DISPOSAL OF SMALL AMOUNTS/SPILLAGES BY DEMONSTRATORS

Mop spill area with water.

Other Information The full Material Safety Data Sheet for this chemical is available from Chemwatch, on-line

at:

http://www.adelaide.edu.au/hr/ohs/legislation/chemwatch

http://www.adelaide.edu.au/hr/ohs/legislation/chemwatch
Foundations of Chemistry Laboratory Manual VITAMIN C DETERMINATION 3F

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Information Sheet

(can affect health if exposed to large doses

or to low doses over a long period of time)

STARCH SOLUTION

IDENTIFICATION

Name starch

Structure (C6H10O5)n

PHYSICAL DESCRIPTION AND PROPERTIES

Description cloudy, colourless solution

Boiling Point not available

Melting Point not available

Vapour Pressure not relevant

Flammability non combustible.

Density not available

Solubility Soluble: water

Reactivity Considered stable.

HEALTH HAZARD INFORMATION

Major hazards

Inhalation or skin contact and/or ingestion may produce health damage (limited evidence).

Cumulative effects may result following exposure (limited evidence). May produce discomfort of the

eyes (limited evidence).

Toxicity

Inhalation: Inhalation of vapours or aerosols generated by the material may be damaging to the

health of the individual.

Eye contact: May cause irritation and damage of the eye in some persons.

IRRITANT

Foundations of Chemistry Laboratory Manual VITAMIN C DETERMINATION 3F

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Skin contact: Skin contact with the material may be damaging to the health of the individual.

Swallowing: Ingestion of the material may be damaging to the health of the individual.

FIRST AID INFORMATION

Eyes: Wash the eye immediately with fresh flowing water.

Lungs: Remove patient to fresh air.

Mouth: Do NOT induce vomiting.

Skin: Wash off skin with running water and soap.

DISPOSAL OF SMALL AMOUNTS/SPILLAGES BY DEMONSTRATORS

Mop spill area with water.

Other Information The full Material Safety Data Sheet for this chemical is available from Chemwatch, on-line

at:

http://www.adelaide.edu.au/hr/ohs/legislation/chemwatch

http://www.adelaide.edu.au/hr/ohs/legislation/chemwatch
Foundations of Chemistry Laboratory Manual VITAMIN C DETERMINATION 3F

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METTLER ANALYTICAL BALANCE

TITRATION END-POINT

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