2.7: Volumetric Titration-Home

Learning Objectives

• To use titration techniques to find the concentration of an unknown sample 

Theory

           Vitamin C (Ascorbic acid C₆H₈O₆₎ is oxidized by free iodine molecule as follows. This reaction is often used in volumetry to quantify the amount of Vitamin C in a sample. Thiat is what we will see in this titration lab experiment. Since iodine is involved, and  these are redox reactions not neutralization reactions, these titrations are called iodometric titrations. However, in kinetics lab experiments, this reaction is often used as an example where the speed of a reaction is measured or rather calculated. The following equation shows the 1:1 reaction between Vitamin C (Ascorbic acid) and Iodine.

C₆H₈O₆ + I₂ à C₆H₆O₆ + 2I^- + 2H⁺

              Iodine (I₂) solution is a brown solution and dissolved Iodide (I^-) ions are colorless. Free iodine in the presence of starch makes an I₂-Starch complex that is dark blue in color. If there is vitamin C, it will react with I₂. So, there will not be any free iodine to complex with starch to give the blue color. Once the vitamin C is oxidized completely, the free iodine will be available, and it will complex with starch to give the blue color. This is an indication of the completion of vitamin C oxidation. This is called the end-point of the reaction. The amount of iodine used to reach the end-point is recorded. It will be converted to the mole equivalent. Since this is a 1:1 reaction between iodine and Vitamin C as shown in the above equation. The mole of iodine used is equal to the moles of Vitamin C reacted or present in the sample. The moles of vitamin C is then converted into grams or other unit derivatives for comparison.

           The sample that needs to be analyzed is called the analyte. In this case it is Vitamin C samples (tablet or fruit juice). The reagent with a known (pre-calculated) concentration used from the burette or an addition equipment is called the titrant. In our case it is the free iodine solution (Tincture iodine (2%). Since we do not have a burette, we are using a long graduated pipette with a bulb to measure the amount of iodine solution added to reach the blue end-point. Watch the following video demonstration.

             We will do two titrations. The first one is a calibration reaction to calculate the exact concentration (Molarity of Tincture Iodine (2%) solution (titrant) using a known concentration of Vitamin C tablet. This is to confirm the molarity of the titrant before we analyze any radom sample. This improves the accuracy of our results.

            The second is the analytical titration to find out the amount of vitamin C in a juice/cola containing unknown concentrations of vitamin C using a known titrant, Tincture Iodine solution. The volume of iodine required to reach the blue end point will be used to find the concentration of vitamin C sample. If the fruit juice is diluted for titration, the calculated concentration needs to be multiplied back by the dilution factor to get the original concentration. The following video is only for a demonstration purpose. We will be using a slightly different procedure using Vitamin C tablet instead of pure ascorbic acid. We will be adding Iodine solution using a long graduated pipette with bulb instead of a burette. See the procedure given below the video.

 

Procedure

Materials Required

Vitamin C tablets (Nature Best brand) 500 or 1000 mg supplements, 2-3 #, Tincture iodine antiseptic (2%) 30ml, , Starch (corn or potato), 1-2 spoons, Beakers or clear bowls for mixing, graduated cylinder, electronic balance, spoons, long graduated pipette with bulb to be instead of a burette, fruit juice or any beverage with vitamin C.

Note

Wear gloves. Iodine can cause stains.

Procedure and Report Sheet

PART A-Calibration of the Titrant Tincture Iodine Solution (2%)

• The vitamin C tables are not standardized for their ascorbic acid (vitamin C) content compared tp pure ascorbic acid available in a real laboratory. In order to find the concentration or mols of vitamin C in a particular amount of the tablet, it is titrated with a standardized base like NaOH. Since we do not have NaOH. We are going to calculate the millimols/mg of the tablet based on the titration values given in the video. This is why we need to use the same brand of Vitamin C tables as used in this reaction shown in the video. This can be used to find the mols of Vitamin C in our calibration experiment.
• M1V1=mols of NaOH. Since it is a 1:1 reaction between Vitamin C and NaOH, the number of mols of Vitamin C should be equal to the mols of NaOH used. 

  It comes to 0.001milli mol Vitamin C per every milli gram of the Vitamin C tablet. For example, for sample 1

• To prepare a standard solution of vitamin C with 0.002M Concentration for calibration titration
  2000mg Vitamin C tablet X 0.001mmol Vitamin C/mg tablet = 2.0 mmol Vitamin = 0.002 mol Vitamin C. If this is dissolved in 1 Liter it would be a 0.002 M standard vitamin C solution.
  
  0.002 M=0.002 mol/1L        Instead of dissolving 2000 mg tablet in 1 L, we can scale it down by a factor of 10 since we need only 100 ml of the standard solution.
  ie, 200mg tablet in 100 ml water.

• Crush a Vitamin C tablet to a fine powder. Keep a clear plastic cup or beaker  (medium size) on the scale. zero the weight. Measure out exactly 100 mg (0.10 g) of Vitamin C powder into using a spoon.

• Measure out exactly 100.0 ml water using a graduated cylinder, and add into this. Dissolve the solution.

• Prepare a starch solution by dissolving 1/2 a spoon of starch (potato or corn) in 50 ml water in a microwave.

• Add 1 ml starch solution into the Vitamin C solution prepared. 

• Using a graduated pipette, draw 1-5 ml Tincture iodine (2%) is a graduated cylinder.

• Slowly add the iodine into the Vitamin C solution. Swirl the Vitamin C container to mix them. Add the iodine solution drop-by-drop until you see the whole solution turns blue as shown in the video. Keep track of the addition and record how many milliliters of iodine solution was used to reach the blue end-point. Some times you need more than 5 ml. Then draw more iodine solution and add, but keep track of the total milliliters added.

• Use the M1.V1=M2.V2 equation to find out the exact concentration of the iodine solution.
  M1=Concentration of standard Vitamin C solution = 0.002M ; V1-Volume of standard Vitamin C solution =100ml
  M2=Concentration of Tincture Iodine solution (2%) =?  M   ; V2=Volume of Iodine solution used to reach the blue end-point = ____ml (enter the ml added here)
  M2=(M1.V1)/V2 =               M of Iodine solution (titrant)

PART B:Iodometric Titration to Find the Concentration of Vitamin C in a Fruit Juice Sample

• Find a fruit juice/lemonade/soda with Vitamin C in the list of ingredients. Since we do not know the concentration of Vitamin C in the sample, dilute it by a factor 10 to be comparable with the iodine solution concentration.

• Measure out exactly 10 ml of the vitamin C containing beverage into a 100 ml graduated cylinder. Add enough water to make it exactly to 100 ml.

• Add 1 ml starch solution into the Vitamin C solution prepared. 

• Using a graduated pipette, draw 1-5 ml Tincture iodine (2%) is a graduated cylinder.

• Slowly add the iodine into the Vitamin C solution. Swirl the Vitamin C container to mix them. Add the iodine solution drop-by-drop until you see the whole solution turns blue as shown in the video. Keep track of the addition and record how many milliliters of iodine solution was used to reach the blue end-point. Some times you need more than 5 ml. Then draw more iodine solution and add, but keep track of the total milliliters added.

• Use the M1.V1=M2.V2 equation to find out the exact concentration of the iodine solution.
  M1=Concentration of unknown Vitamin C solution = ?? M ; V1-Volume of unknown Vitamin C solution =100ml
  M2=Concentration of Tincture Iodine solution (2%) = (enter the value calculated in step A) M   ; V2=Volume of Iodine solution used to reach the blue end-point = ____ml (enter the ml added here)
  M1=(M2.V2)/V1 =               M of Vitamin C in teh Unknown Vitamin C solution

• Actual concentration of Vitamin C in the beverage solution = 10X M1=                M

Data Analysis

Questions

1. What is the purpose of the calibration titration?
2. Show the work to convert the actual concentration of Vitamin C for the beverage (the last calculation) in M to moles.
3. Convert the above moles of Vitamin C in Q2 to grams.

Contributors and Attributions

Manjusha Saraswathiamma, Minnesota State University, Moorhead has developed this experiment to be performed at home with less hazardous and cost-effective lab supplies. Manjusha would like to acknowledge all the contributors of the YouTube videos embedded on this page.