Introduction: (Initial Observation)
Vitamin C is an important anti-oxidant, helps protect against cancers, heart disease, stress, it is part of the cellular chemistry that provides energy, it is essential for sperm production, and for making the collagen protein involved in the building and health of cartilage, joints, skin, and blood vessels. Vitamin C helps in maintaining a healthy immune system, it aids in neutralizing pollutants, is needed for antibody production, acts to increase the absorption of nutrients (including iron) in the gut, and thins the blood. Just to mention its most important functions.
Because of importance of vitamin C for our health, I want to see which foods are a better source of vitamin C.
Gather information about vitamin C and it’s chemical structure. Read books, magazines or ask professionals who might know in order to learn about the physical or chemical properties of Vitamin C. Such physical and chemical properties may be used to measure the amount of vitamin C in fruits. Keep track of where you got your information from.
Following are some sample information that you may gather:
Vitamin C (ascorbic acid) is not stored by the body, so you need some each day.
Good sources of vitamin C are fresh fruits and vegetables. Especially good sources are oranges, grapefruits, strawberries, cantaloupe, watermelon, tomatoes, cabbage, green peppers, broccoli, collards and other greens.
What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.
The purpose of this project is to find out which fruits contain the most amount of vitamin C.
Other questions that can be studied in this project are:
- How does the amount of vitamin c change when fruits get older?
- How does storage temperature affect the amount of vitamin c in fruits?
- How does cooking affect the vitamin c in foods?
When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.
The independent variable also known as manipulated variable is the type of fruit (apple, orange, peach,….)
The dependent variable is the amount of Vitamin C in fruit. (Percent by weight)
Controlled variables are experiment procedures and conditions including temperature and light. We control such variables to make sure that they are not having any effect on our experiment results.
Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis.
Following are two sample hypothesis:
- Fruits with sour taste contain the highest rate of vitamin C. My hypothesis is based on my gathered information about the chemical structure of vitamin C. Vitamin C is an acid (Ascorbic Acid) and tastes sour.
- Citrus fruits contain the highest rate of vitamin C. My hypothesis is based on my gathered information that orange, grapefruit are rich in vitamin C.
Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”
In the following experiments the chemical procedure of titration will be used to test for Vitamin C. Titration involves adding a test liquid, drop by drop, to an indicator solution that undergoes a series of color changes as Vitamin C is added to it. You may work with a partner recording the number of drops or number of milliliters required to change the color of the indicator. The point at which the indicator becomes colorless is called the end point.
Vitamin C is an anti-oxidant so it can void the effects of a strong oxidizer such as iodine. Iodine gives a dark blue color to starch solution. Vitamin C is able to reverse this color change and remove the dark blue color. In this experiment we test the effect of vitamin C on starch-iodine solution and find a relation between our starch-iodine solution and the amount of vitamin C that can titrate this solution to the end point. The objectives of this experiment are:
- practicing the chemical procedure of titration.
- learn that the end point of titration will be a colorless solution.
- Using a standard vitamin C solution to standardize the indicator solution.
Make the cornstarch-iodine solution. Mix 2 tablespoons of cornstarch in 500 ml of water to make a cornstarch solution. Filter the starch solution through 2 to 4 of the thicker brand paper coffee filters until you have a clear liquid. The solution may be clear to slightly cloudy, but should not be milky white. Now add tincture of iodine by drops with constant stirring until the solution turns a deep, dark blue. Your indicator solution is ready now. This indicator solution will be enough for this experiment and all other experiments. (If you add too much iodine, the solution will become brownish.)
Now practice the technique of titration and end point using the vial of 10 ml indicator and your standard vitamin C solution.
- Dissolve a 500-mg Vitamin C tablet in 500 ml of water to make your standard (1 gram per liter) solution.
- Pour 10 ml of the blue indicator solution into a clear glass or plastic vial or container.
- Using a clean pipette, drop one drop at a time of standard solution into the indicator solution.
- Swirl the container. Record how many milliliter of standard solution is added to the indicator until the indicator is loosing its blue color*. This is the end point.
- Record the amount of standard solution used in this titration and call it X. In other words X ml standard solution is used to titrate 10 ml of blue indicator.
* Or until the blue color of indicator disappears.
(Note that each ml of your standard solution contains 1 mg of vitamin C. Now you know how much vitamin C can discolor a 10 ml vial of indicator; So, your indicator solution is standardized and can be used for other experiments.)
Now that we have a standard indicator solution and know how much vitamin C is needed to discolor that, we can use it to test the amount of vitamin C in other fruits or fruit juices. This method will not work with fruits that have a dark color juice such as blue or red.
- Prepare and filter the juice of any fruit that you want to test. Juice extractors used to make carrot juice or apple juice may be used for vegetables too. If you don’t have a juice extractor machine you may use a grater and then filter the juice using a cotton cloth.
- Pour 10 ml of the indicator solution into a clear glass or plastic vial or container.
- Using a clean pipette, drop one drop at a time of juice into the indicator solution.
- Swirl the container. Continue adding drops of juice and swirling until the indicator will fully lose its blue color. This is the end point. Record the milliliters of juice used in this titration and call it Y.
- To calculate the amount of vitamin C per ml of juice divide X by Y.
R = Milligrams of vitamin C per ml of fruit juice = X / Y
- Record your results on a data sheet. (How many ml of juice)
- Repeat the above procedures for the standard Vitamin C solution and the juice of every other fruit that you want to test.
Your data table may look like this:
|Fruit name||R||Fruit Weight||Juice volume|
|Standard Vitamin C solution||1|
* Make sure you “swirl” the cornstarch-iodine solution after the addition of each drop of juice. (A swirl is when you hold the container at the top and circle the bottom of the container to stir the liquid.)
* Make sure that all bottles are labeled.
* Use a new container of solution for every titration test
* You can use your data table to make a bar chart.
What is R and how can we use it?
R is the amount of vitamin C in milligrams in one milliliter of the liquid. If you know R for orange juice, you can multiply it by 200 to see how much (in milligrams) vitamin C is in one cup of orange juice. Assuming that one cup is about 200 ml.
What is the “control” for this experiment?
As your control you can keep some of the indicator solution on the side and do nothing with that. The “control” will show that the discoloration of the indicator solution did not happen by itself or by an unknown factor. So the discoloration is caused by the fruit juice that you are testing.
In my first experiment, I used 7 ml of my standard solution to remove the blue color of 10 ml indicator. So X=7.
In the second experiment I used 12 ml of fruit juice to remove the blue color of 10 ml indicator. So Y=12.
R = Milligrams of vitamin C per ml of fruit juice = X / Y = 7 /12 = 0.58 mg/ml
Critical Thinking Questions:
1. Discuss the results of your experiments. Possible questions may include: How many drops or how many ml did it take to titrate the indicator with the Vitamin C solution? How many drops or how many ml did it take to titrate the indicator with orange juice?, and How many drops or how many ml did it take to titrate a juice with very little Vitamin C?
2. What is the relationship between the number of drops of juice needed to titrate the indicator solution and the amount of Vitamin C in the juice? (The fewer drops required to titrate the indicator, the greater the amount of Vitamin C in the liquid.) (This may be confusing, but more experimenting and other activities should reinforce this relationship and make it less confusing.)
How can I test other foods?
Make solutions for other food either by squeezing the juice directly from the foods (such as with oranges and lemons), or by blending with water and filtering the pulp of the food through coffee filter paper. Place each food sample in a different, labeled container.
Materials and Equipment:
- a variety of fruits, vegetables, and potatoes (Try to include a mix of foods that do and do not have Vitamin C such as oranges, apples, strawberries, bananas, lemons, tomatoes, etc.)
- a variety of juices, including juices with and without Vitamin C (avoid using red or purple colored drinks – see the procedure)
- iodine solution (iodine tincture)
- One 500mg vitamin C tablet
- 1 ml pipettes
- white construction paper
- data sheet
- coffee filter paper
Results of Experiment (Observation):
Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.
Since different fruits contain different amounts of juice, you need to calculate how much (by weight) vitamin C is in each fruit. To be able to do this, weigh the fruit before getting the juice. Then measure the volume of juice and find the ratio of juice in fruit by dividing the juice volume by the fruit weight. Finally do your experiment and calculate R for each fruit juice. R is the amount of vitamin C per ml of fruit juice. Multiply R by the percent of juice in fruit to calculate the amount of vitamin C in 1 gram of fruit.
Summary of Results:
Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.
It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.
Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.
Related Questions & Answers:
What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.
If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.
If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.
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