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HomeResearch fun Chemical experimentsdetermining the amount of vitamin C in Apple



The answer to this question can be found in the directory. But it says about Apple in General, and how much vitamin C in this Apple that you're going to eat?

The determination of vitamins a difficult business. But vitamin C - ascorbic acid - it is possible to determine in the home.

You may have decided that since vitamin C is an acid, to determine the quantity you need with alkali. It would be good... But in our case, this analysis is not suitable. In fruits, in addition to ascorbic, there are many other organic acids: citric, malic, tartaric, and other, all of them come with the alkali in the reaction of neutralization. Hence, the alkali will not help.

We will use the characteristic feature of ascorbic acid - ease of its oxidation. Of course, you know that during storage and cooking to lose a lot of vitamin C. this is Due to the fact that the molecule of ascorbic acid is unstable, it is easily oxidized even the oxygen in the air, turning into another acid, dehydroascorbic, which has no vitamin properties. We use to analyze an even stronger oxidizing agent is iodine.

Will provide ourselves with iodine solution of known concentration. You can take pharmacy alcohol solution of iodine (iodine tincture) with iodine concentration of 5%, i.e. 5 g in 100 ml. Next prepare the starch solution: dissolve 1 g in a little cold water, pour out a glass of boiling water and boil another minute. This solution is suitable for experiments during the week.

You are now ready to determine the vitamin. But before proceeding to the analysis, will do a dry run on pure ascorbic acid.

Take 0.5 g pharmacy ascorbic acid (without glucose), dissolve it in 500 ml of water and collect 25 ml of solution. Add about half a Cup of water - the exact number is irrelevant and another 2-3 ml of starch solution. Now gently, drop by drop, add from pharmacy pipette solution containing iodine, constantly shaking the contents (easier to do it in a conical flask, Carefully consider drops and observe the color of the solution. As soon as the iodine will oxidize all of ascorbic acid, following his drop, proreagirovali with starch, stain solution in blue. This means that our operation is the titration is finished.

But how do you know how much we spent iodine tincture? Drops - not units... In chemical laboratories have special burette with graduations in milliliters. We will use another, quite accurate, method, although more long. Using the same pipette, count the number of drops contained in the pharmacy bottle of iodine (it usually accommodates 10 ml). Do not worry - all the work will take several minutes. Knowing the volume of a drop, it is possible to accurately determine the volume of iodine solution used for titration of ascorbic acid. And now, using equation (suggest you write it yourself), you can check how much the tablet was ascorbic acid. And it is possible to solve the inverse problem: knowing the amount of acid, to determine the concentration of iodine tincture - is it really five percent?

This is an easy way analysis chemists often used for the determination of iodine and other oxidants. He called ascorbimetric.

Now proceed to the solution of our main tasks is to determine the amount of vitamin C. the Concentration of the iodine solution known to us: 1 ml 5%solution with 35 mg of ascorbic acid (by the way, that is how much of it is contained in one tablet of multivitamin).

Let's start with orange or lemon juice (just squeezed or canned). Measure 20 ml of the juice and dilute with water to volume of about 100 ml. Pour a little of the starch solution, and then, as before, add dropwise a solution of iodine until a steady blue color, does not disappear within 10-15 C. However, now in solution is significantly less ascorbic acid than in the experiment with the tablet, and iodine will titration is also less. If the content of vitamin C is very small, it may happen that it will take just 1-2 drops of iodine tincture; however, our analysis will, of course, very approximate. To be more precise, it is necessary either to take a lot of juice or dilute iodine tincture. Chemists (and all lovers of delicious juice) will prefer the second way. In the analysis of fruit juices is convenient to dilute iodine water 40 times, you will get a 0.125 %solution, 1 ml of which corresponds 0,875 mg of ascorbic acid. Do not forget that the surface tension of water more than alcohol, and, therefore, water droplets are much larger. Then have to re-calculate the volume of one drop.

In a school lab or in chemical circle the desired solution of iodine can be prepared more precisely: 1 l it must contain 1.27 g of iodine. But the solubility of iodine in water at room temperature is very small (only 0.3 g/l), so the solution will have to add a little potassium iodide or sodium iodide, in the presence of which the solubility of iodine in water is greatly increasing. One of these salts is certainly in pharmacy iodine solution.

Let's proceed now to the analysis of the apples. Here we meet with the same difficulty: apples contain an enzyme ascorbicacid, in the presence of ascorbic acid is rapidly oxidized in air. To avoid this, the analysis should be carried out in an acidic environment.

Thin knife stainless steel cut and C weighted Apple sample in the form of slices, peeled to the core with seeds. Vitamin C is distributed in the thickness of the Apple unevenly, and we want to make the analysis not some one zone, and Apple in General. This slice move in a porcelain mortar with diluted hydrochloric acid and thoroughly grind with the pestle. Add the starch solution and titrate the mixture is diluted with a solution of iodine. The sample mass can be found by difference: will weigh the Apple to analysis, as a whole, and then again, without the slice.

But accurate whether our way? Because juice contains not only vitamin C, but also other organic substances, which can also react with iodine (albeit slower than ascorbic acid). And yet the method is sufficiently accurate. The following results were obtained when one of the definitions.

From bought in a drugstore hips cooked broth - 25 grams of fruit on 0,5 l of water. By the iodine method vitamin C content, calculated on the dried fruit, 3.8% (according to reference data on dry matter rosehips contains 4.5% of vitamin C), freshly pressed orange juice was 0.05% vitamin and canned - 0,075% (according to reference data -0,04%). If we consider that in various fruits even one kind of vitamins, usually fluctuates, it should be recognized that the method gives a fairly complete picture of the content of vitamin C.

So, you learned how to quickly and accurately determine ascorbic acid. Now before you open a vast field of activity. You can explore a variety of vegetables and fruit (juice which is not painted itself red); you can know whether the vitamin from the variety. Remember only that the dissolved ascorbic acid in air is oxidized, experiments should be conducted with fresh juice.

Here are a few topics for research. Locate the content of ascorbic acid in freshly-picked fruits and those that kept a week, a month, six months. Sometimes it is said that iron knife cannot cut fruits - vitamin C is destroyed. Check whether this is the case, holding the juice in the iron bowl. (By the way, if you will compare the content of the vitamin in fresh juice and canned, keep in mind that the latter may contain specially added ascorbic acid.) Finally, make sure how the content of vitamin C in the juice by heating and themselves make of these experiments conclusions.

O. Holguín. "Experiments without explosions"
M., "Chemistry", 1986


Your comments:

2014-02-17 16:44:39
Not with a knife to cut the Apple. you cannot connect the SS iron, so ill be identified ascorbic acid!
2013-10-16 15:37:16
a little starch water is what? and every time one number you need to add?
Your name (nick):
Enter the result of the calculation

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