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Why a standard solution with a low concentration should be used: Exploration through vitamin C titration


Chungnam Samsung Academy

At Chungnam Samsung Academy, chemistry club Ignis members conducted a vitamin C titration experiment using the beverage Vita500, along with an iodine standard solution and starch. We varied the iodine standard solution concentration for each experiment to 0.0025M, 0.005M, and 0.01M and compared the appropriate error rate according to the standard solution concentration.

Below are concepts necessary for the experiment.

- A standard solution is a solution used in titration in which the concentration is accurately known.

- The mole (mol) is a unit used to express the amount of a substance composed of very small particles in terms of the number of particles. 1 mol represents the amount of substance made up of ‘6.02 × 10²³’ particles, known as Avogadro’s number. 

- This experiment is a redox titration experiment. Here, an oxidation-reduction reaction is a chemical reaction in which an oxidation reaction that gives electrons and a reduction reaction that receives electrons occur simultaneously between substances. All redox reactions occur simultaneously, not just in one direction.

- Titration is a method of quantitative analysis to determine the concentration of a solution of unknown concentration using a solution of known concentration, that is, the standard solution mentioned above.

The experiment was conducted by the following method.


  • Starch

  • Iodine solution

  • Vitamin C drink (Vita500)

  • Stirrer

  • Glass rod

  • Electronic scale

  • Magnetic bar

  • 100mL beaker

  • 500mL beaker

  • 1L volumetric flask

  • 500mL volumetric flask,

  • Funnel

  • 100mL graduated cylinder

  • 25mL graduated cylinder

  • 250mL erlenmeyer flask

  • Burette

  • Dropper

  • Pipette

  • Powder paper

  • Micro spoon.

Experimental process

1. Manufacture of 0.05 M starch indicator solution

1) Put 50 ml of distilled water and 0.85 g of starch in a 100 ml beaker.

2) Starch is insoluble in distilled water, so it is heated using a stirrer and magnetic bar.

2. Preparation of a standard solution of 500mL 0.0025M, 0.005M, and 0.01M iodine

1) Add 20 ml (0.01 M) of iodine solution to a 250 ml beaker, add a small amount of distilled water, and mix with a glass rod.

2) This solution is placed in a 1L volumetric flask using a funnel.

*Rinse the beaker and glass rod several times with distilled water.

3) Fill the indication line of the volumetric flask with distilled water with the dropper.

4) Complete a 1 L 0.01 M iodine standard solution.

5) The above solution is divided into 500 mL (0.01 M), 250 mL (0.005 M), and 125 mL (0.0025 mL), respectively, in a 500 mL volumetric flask.

6) Fill the indication line of the volumetric flask with distilled water with the dropper.

7) Complete a standard solution of 500 mL 0.01 M, 0.005 M, and 0.0025 M iodine.

3. Dilution of vitamin drink

1) Put 10 ml of vitamin drink (Vita 500) into a 100 ml graduated cylinder.

2) Add 90 ml of distilled water to the graduated cylinder and mix with a glass rod. (Scale with a dropper)

4. Configuring appropriate devices and conducting appropriate experiments

1) Twenty ml of the three dilutions were measured with a 25ml graduated cylinder and transferred to a 250ml Erlenmeyer flask, followed by 3ml of the starch indicator prepared in No. 1 as a dropper.

2) Add an iodine standard solution to the burette.

*Fill the standard solution to the end of the burette and send it down once, replace it with a new beaker and refill it.

3) After placing a magnetic bar in the erlenmeyer flask and placing it on the stirrer, operate the stirrer.

*At this time, the heat part of the stirrer is not activated.

4) Place a stirrer with the erlenmeyer flask placed under the burette, open the burette cock, and drop the iodine standard solution little by little.

5) If the solution is blue overall, the volume of the iodine standard solution is obtained after locking the burette cock.

6) A process of 1 to 5 is carried out with 0.0025 M, 0.005 M, and 0.01 M iodine standard solutions, respectively.

It was very common for errors to occur during the experiment; trial and error was needed to produce meaningful results.

Our club identified errors that occurred in the experiment largely in two ways.

1. First, there was error caused by the lack of prior research.

There was a lack of preliminary investigation into starch. We used starch among the reagents, but we did not understand it well, so we tried to dissolve starch without knowing that it was being used in a state where the starch was not completely dissolved when using the starch directive. Because of this, an immense period of time was consumed, and our first experiment did not finish on time.

Additionally, we couldn't accurately determine the amount of the standard solution, iodine solution. We couldn't produce enough iodine solution to conduct an accurate experiment. In the first experiment, the amount of iodine solution we made was 100 mL. However, in the third experiment, the amount of iodine solution was increased tenfold to 1L.

Third, the concentration of the standard solution, iodine, was inaccurate. The experiment was planned and carried out in the first round by making an iodine standard solution separately for each molar concentration. However, when making a standard solution with multiple molar concentrations in an appropriate experiment, where quantification is important, one standard solution must be diluted to reduce the error. Therefore, we modified the method to make a standard solution by pushing and diluting one standard solution.

2. Experimental errors

The first error is the use of beakers in the manufacturing process of indicators. While making starch indicators, beakers were used to measure volume in the manufacturing process of starch indicators. This mistake was changed to use volumetric flask in the second experiment. Even during the use of beakers, there was a problem that the solution concentration was not matched because the solution was not completely transferred.

Second, in the process of determining the size of the magnetic bar in the appropriate process, we made a mistake. Because the magnetic bar's size was too big, the color changing due to the reaction of the sunken starch and iodine solution was obscured. In the third experiment, a smaller erlenmeyer flask and a smaller magnetic bar were used, and the solution was dropped more slowly.

Third, there was a problem that the measuring instrument was incorrectly selected during the vitamin C solution dilution process. The calibration cylinder was used to measure and dilute the amount of Vita 500 in the vitamin C dilution solution. However, we found out that quantitative measurement was important in the appropriate experiment, so we measured the exact amount with a pipette to prepare the vitamin C dilution solution again.

Observation and Measurement Results:


Standard solution of 0.0025M, 0.005M, and 0.01M iodide

Diluted solution of Vita500 and manufacturing of 0.05 M starch indicator solution

Measurement results

When I3- is added at the point where all C6H8O6 (vitamin C) reacts and the redox reaction is completed, it reacts with the starch indicator and turns blue.

Chemical reaction that took place:

  • I2 + I-(from KI) ⇄ I3-

  • Ascorbic acid(C6H8O6) + I3- + H2O → Dehydroascorbic acid(C6H6O6) + 3I- + 2H+

  • KI-I2 : I3- : C6H8O6 = 1 : 1 : 1

Graphing the results of the experiment:

In conclusion, as the concentration of the standard solution used for titration decreases, the error tends to decrease. The vitamin C titration experiments using standard solutions with various concentrations revealed why a standard solution with a low concentration should be used in the titration experiment.


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