How much vitamin C in an apple? Fun chemistry experience at home

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How much vitamin C in an apple? Chemical experiments

Entertaining experiments in chemistry

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The answer to this question can be found in the handbook. But it talks about an apple in general, and how much vitamin C is in this particular apple that you are going to eat?

Determining vitamins is a tricky business. But vitamin C - ascorbic acid - can be determined at home.

Perhaps you have decided: since vitamin C is an acid, then its amount must be determined using alkali. It would be nice... But in our case, such an analysis is not suitable. In fruits, in addition to ascorbic, there are many other organic acids: citric, malic, tartaric and others, they all enter into a neutralization reaction with alkali. So lye won't help.

We will use the characteristic feature of ascorbic acid - the ease of its oxidation. Of course, you know that a lot of vitamin C is lost during storage and cooking. This is due to the fact that the ascorbic acid molecule is unstable, it is easily oxidized even by atmospheric oxygen, turning into another acid, dehydroascorbic acid, which does not have vitamin properties. We use an even stronger oxidizing agent, iodine, for analysis.

Stock up on a solution of iodine of a known concentration. To do this, you can take a pharmacy alcohol solution of iodine (iodine tincture) with an iodine concentration of 5%, i.e. 5 g in 100 ml. Next, prepare a starch solution: dilute 1 g of it in a small amount of cold water, pour it into a glass of boiling water and boil for another minute. This solution is suitable for experiments within a week.

Now everything is ready for the determination of the vitamin. But before proceeding to the analysis, let's practice on pure ascorbic acid.

Take 0,5 g of pharmacy ascorbic acid (without glucose), dissolve it in 500 ml of water and take 25 ml of the solution. Add about half a glass of water - the exact amount does not matter - and another 2-3 ml of starch solution. Now, carefully, drop by drop, add a solution of iodine from a pharmacy pipette, constantly shaking the contents (it is more convenient to do this in a conical flask). Carefully count the drops and watch the color of the solution. As soon as iodine oxidizes all ascorbic acid, the next drop of it, having reacted with starch, will color the solution blue. This means that our operation - titration - is over.

But how do we know how much we used iodine tincture? Drops are not units of measurement... In chemical laboratories there are special burettes with divisions - immediately in milliliters. We will use another, quite accurate, method, although longer. Using the same pipette, we calculate how many drops are contained in a pharmacy bottle of iodine (it usually holds 10 ml). Don't worry - the whole job will take a few minutes. Knowing the volume of one drop, one can quite accurately determine the volume of the iodine solution used for titration of ascorbic acid. And now, using the reaction equation (we suggest you write it yourself), you can check how much ascorbic acid was in the tablet. Or you can solve the inverse problem: knowing the amount of acid, determine the concentration of iodine tincture - is it really five percent?

This simple method of analysis is often used by chemists to determine iodine and other oxidizing agents. It's called ascorbinometry.

Now let's start solving our main task - determining the amount of vitamin C. We know the concentration of an iodine solution: 1 ml of its 5% solution corresponds to 35 mg of ascorbic acid (by the way, this is how much it is contained in one pill of multivitamins).

Let's start with orange or lemon juice (freshly squeezed or canned). Measure out 20 ml of juice and dilute with water to a volume of approximately 100 ml. Pour in a little starch solution, and then, as before, add iodine solution drop by drop until a stable blue color appears, which does not disappear within 10-15 s. True, now there is much less ascorbic acid in the solution than in the experiment with the tablet, and less iodine will also be used for titration. If the content of vitamin C is very low, it may happen that only 1-2 drops of iodine tincture are needed; in this case, our analysis will, of course, be very approximate. To make the result more accurate, you must either take a lot of juice, or dilute the iodine tincture. Chemists (and all lovers of tasty juice) will prefer the second way. When analyzing fruit juices, it is convenient to dilute iodine with water 40 times, and this will give a 0,125% solution, 1 ml of which corresponds to 0,875 mg of ascorbic acid. Just remember that the surface tension of water is greater than that of alcohol, and therefore the water droplets are much larger. So, you have to re-calculate the volume of one drop.

In a school laboratory or in a chemical circle, the required iodine solution can be prepared more precisely: 1 liter of it should contain 1,27 g of iodine. But the solubility of iodine in water at room temperature is very low (only 0,3 g / l), so a little potassium iodide or sodium iodide will have to be added to the solution, in the presence of which the solubility of iodine in water greatly increases. One of these salts is certainly in the pharmacy solution of iodine.

We now turn to the analysis of apples. Here we will meet with the following difficulty: apples contain the enzyme ascorbic oxidase, in the presence of which ascorbic acid is rapidly oxidized in air. To prevent this from happening, the analysis must be carried out in an acidic environment.

Using a thin stainless steel knife, cut a sample from a pre-weighed apple in the form of a slice, from the peel to the core with seeds. Vitamin C is unevenly distributed in the thickness of an apple, and we want to analyze not just one zone, but the apple as a whole. Transfer this slice to a porcelain mortar with dilute hydrochloric acid and rub it thoroughly with a pestle. Add starch solution and titrate the mixture with dilute iodine solution. We determine the mass of the sample by the difference: we weigh the apple before analysis, as a whole, and then again, without a slice.

But is our method accurate? Indeed, the juice contains not only vitamin C, but also other organic substances that can also react with iodine (although more slowly than ascorbic acid). And yet the method is quite accurate. Here are the results obtained with one of the definitions.

A decoction of 25 g of fruit per 0,5 l of water was prepared from rose hips purchased at a pharmacy. According to the iodine method, the content of vitamin C, based on dry fruits, was 3,8% (according to reference data, the dry matter of rose hips contains 4,5% of vitamin C), Freshly squeezed orange juice contained 0,05% of the vitamin, and canned - 0,075% (according to reference data, -0,04%). If we take into account that in different fruits, even of the same variety, the content of vitamins, as a rule, fluctuates, then it should be recognized that the method gives a fairly complete picture of the content of vitamin C.

So, you have learned to quickly and fairly accurately determine ascorbic acid. Now a huge field of activity opens before you. You can explore a variety of fruits and berries (the juice of which is not colored red by itself); you can find out if the vitamin content depends on the variety. Just remember that dissolved ascorbic acid oxidizes in air, experiments should be carried out with freshly squeezed juice.

Here are some research topics. Find the content of ascorbic acid in freshly picked fruits and in those that were stored for a week, month, six months. Sometimes they say that you cannot cut fruit with an iron knife - this destroys vitamin C. Check if this is the case by holding the juice in an iron bowl. (By the way, if you compare the vitamin content in fresh and canned juice, keep in mind that the latter may contain specially added ascorbic acid.) Finally, observe how the content of vitamin C in juice changes when it is heated and draw conclusions from these experiments yourself .

Author: Olgin O.M.

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