Q: Instead of the Lugol’s solution or the iodine solution for starch test, can I substitute a pure potassium iodine solution?
A: Quite simply, no. The iodine needs to be elemental (i.e. not an ion and not part of another compound), or else the proper reactions won't take place, and your titration won't work.
Q: When I make the starch indicator solution, it does not really dissolve, but is clumpy and/or milky instead. What should I do?
A: The starch solution should be clear and may need to be heated more. You could try continuing to heat the starch solution in a pan on the stove until the solution becomes clear, just before it boils.
Q: Why do I have to make a fresh vitamin C standard solution and titrate it every day?
A: Since the calculations in this project are based on the data you get from the vitamin C standard solution titration, it is very important that these data are accurate. Because of this, there are two main reasons why making and titrating a fresh vitamin C standard solution increases the accuracy of your measurements:
- When exposed to air, vitamin C will oxidize, or break down; after any vitamin C solution is exposed to air it will have a lower, unknown concentration the next day.
- If you test orange juice samples over multiple days, you must make and titrate a fresh vitamin C standard solution each day or else you cannot compare data from one day to the next. By titrating a fresh vitamin C standard solution each day you take measurements and by observing the variability in the amount of iodine needed to titrate the vitamin C standard solution, you can better compare your orange juice titration results to the day-to-day variations in your vitamin C standard titrations.
A: When you first seen the color change, you should stop the buret from dripping, pick up the flask, swirl it, and put it back down. Wait for 30 seconds. If the color goes away, then continue with the titration, but if the color stays, then stop the titration. The color change indicates that all of the substance being titrated (vitamin C, or ascorbic acid) has reacted with the titrating substance (iodine titration solution). Specifically, when there is no ascorbic acid left, the iodine can react with the starch, causing the solution to change color. Therefore, the first time color lasts after adding a drop of iodine solution and swirling it is the first time when there is no more vitamin C in the solution, and this is when you should stop the titration. Be careful to ensure that the color is lasting and isn’t temporary by always swirling the solutions well.
Q: Why should I add the iodine titration solution drop by drop?
A: The titration reaction is very sensitive. One drop of iodine solution can be enough to drive the reaction to completion (if it were near completion before the drop was added). If more than one drop is added at a time, the data will not be as accurate as it could be.
Q: Why should I swirl the flask each time I add the iodine titration solution?
A: Mixing the flask is important to ensure that the solutions have completely reacted with each other. When adding the iodine solution, you may see a temporary color change that goes away when you swirl the flask. The color change that is important for this experiment is the one that is not temporary and does not go away after mixing the solutions.
Q: What if my solutions never change color?
A: There are a number of reasons your titration may not work. Three of the most common problems that can lead to a solution not changing colors in a titration are:
- Using the wrong substances. Make sure that you use Lugol's iodine solution or another iodine solution sold as a starch indicator (not a pure potassium iodine solution) and a true, soluble starch. (If you purchased the Science Buddies kit, this problem would not apply. The materials in the kit are correct for this Project Idea.)
- Using incorrect concentrations. If your vitamin C standard solution is too concentrated or your iodine titration solution is too dilute, or your starch indicator solution is not between 0.5 to 1.0%, it may require much more than 50 mL of iodine solution to titrate the standard solution and/or orange juice. The most common problem here is an overly dilute iodine solution; sometimes, the Lugol's iodine solution sold in stores is already dilute, and you don't need to dilute it more.
- Using too much orange juice. If there is a large amount of vitamin C in the orange juice that is being tested, it may require more than 50 mL of iodine titration solution for it to change color. You could try using a smaller volume of orange juice.
A: To accurately read the volume of a liquid in the buret, read where the bottom of the meniscus is. The meniscus is where the surface of the liquid forms a curve. For example, in the picture below, based on reading from the bottom of the meniscus, the volume in the buret is about 14.58 mL.
(Image: G. Carboni, 2004)
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