Experiment: Redox Titration and Determination of Percent Iron (Fe)

20 February 2014

Abstract

The purpose of this experiment was to standardize Potassium Permanganate

(KMnO4) in order to determine the percent of iron in the sample. In the first part of this

experiment we needed to standardize the Potassium Permanganate through a titration

process. After standardizing the KMnO4 it was determined that the average molarity was

0.0158 mol. This information was then used for part two of the experiment to determine

the amount of Iron in the unknown sample. The unknown iron sample was titrated with

the standardized potassium permanganate. Upon completing the second part of this

experiment the calculated average percent of Iron in the sample was 13.66%. Our

hypothesis was accepted because we were able to use titration techniques to both

standardize the potassium permanganate and determine the percent of iron in an unknown

sample.

Introduction

A reduction-oxidation reaction (redox) is a “reaction that involves elements that

transfer electrons to one another.” Ref-2 In a redox reaction there are two different possible

products. A product is said to have been reduced if during the reaction it gained

electrons. A product is said to have been oxidized if during the reaction it had lost the

electrons. “The reactant that has lost electrons during a reaction is said to be the reducing

agent, while the reactant that gains electrons is known as the oxidizing agent.” Ref-2

Because of the laws of conservation, the number of electrons that a product gains must be

the same as the number of electrons that have been lost by the reactant. An important step

during redox reactions is determining the spectator ion. A spectator ion is an element that

does not change its oxidation state in a reaction. Ref-2 This is important because the

oxidation states allow us to determine which element is oxidizing and which element is

reducing. We can determine which element is experiencing either oxidation or reduction

by comparing their oxidation sates before and after the reaction by observing which

element gains or loses electrons Ref-1

The titration technique that took place to standardize a product is a little different

compared to the titration technique used to find the percent of an element. In order to

determine the molarity of the Iron in the sample, we first needed determine the molarity

of KMnO4. In order to do this, the KMnO4 needed to have Sodium Oxalate added to it so

that it could become standardized. Additionally, we did not have to add an indicator

because the Potassium Permanganate acted as its own indicator when the end point was

reached.

During this experiment we hypothesized that we can use a titration technique to

standardize KMnO4. We also hypothesized that the percent of Iron can be determined in

the sample by using a titration technique similar to the ones in previous experiments.

Materials and Methods

Reference “Redox Titration and Determination of % Fe” handout given by Dr. Mare

Cudic.

Data

Table 1: Standardization of KMnO4

Trial 1 Trial 2 Trial 3

Vol Na2C2O4 (mL) 10 10 10

Initial Buret (mL) 8.4 9.8 11

Final Buret (mL) 9.8 11 12.2

Vol KMnO4 (mL) 1.4 1.2 1.2

Moles C2O4 (mol) 5x10-5 5x10-5 5x10-5

Moles KMnO4 (mol) 2x10-5 2x10-5 2x10-5

Molarity KMnO4 (M) 0.01422 0.0166 0.0166

Average Molarity(M) 0.015873

Table 2: Determination of Iron (II) Percentage

Trial 1 Trial 2 Trial 3

Mass of Fe sample (g) 0.3693 0.3241 0.3843

Initial Buret (mL) 12.2 23.4 33.2

Final Buret (mL) 23.4 33.2 45.2

Vol KMnO4 11.2 9.8 12

Moles KMnO4 1.792x10-4 1.568x10-4 1.92x10-4

Moles Fe(II) 8.96x10-4 7.84x10-4 9.6x10-4

Mass of Sample Fe 0.0500 0.0437 0.0536

% Fe in Sample 13.54% 13.48% 13.95%

Average 13.66%

Calculations

Table 1 Trial 1:

Vol KMnO4 = final buret (mL) – initial buret (mL)

o 9.8 – 8.4 = 1.4 mL

Moles of C2O4

o 0.05 x 0.0005 = 5x10-5

Moles of KMnO4 = moles of C2O4 * (4/5)

o 5x10-5 * (4/5) = 2x10-2

Molarity KMnO4 = (moles KMnO4) / vol KMnO4

o 2x10-2 / 0.0014 = 0.0143

Average Molarity = (Molarity Trial 1 + Molarity Trial 2 + Molarity Trial 3) / 3

o (0.0143 + 0.0166 + 0.0166) / 3 = 0.0158

Table 2 Trial 1:

Vol KMnO4 = final buret (mL) – initial vuret (mL)

o 23.4 – 12.2 = 11.2

Moles KMnO4 = average mol. KMnO4 * vol. KMnO4

o 0.0158 * 0.0112 = 1.792x10-4

Moles Fe(II) = moles KMnO4*5

o 1.792x10-4 * 5 = 8.96x10-4

Mass of Sample Fe = mol Fe * mFe

o 8.96x10-4 * 55.85 = 0.0500

% Fe in Sample = (Mass of sample Fe / Mass of Fe sample) * 100

o (0.0500 / 0.3693) * 100 = 13.54%

Discussion

The purpose of this experiment is to standardize Potassium Permanganate so we

can determine the percent of iron in the sample. For the first part of this experiment we

needed to standardize KMnO4. A redox titration technique was performed to find the

molarity of KMnO4. Sodium oxalate was mixed with sulfuric acid and water and then

heated to 60 degrees Celsius so that it can be titrated with the potassium permanganate.

We did not need to add an indicator like phenolphthalein to this experiment because the

permanganate acts as its own. When the endpoint (color change) was reached we

calculated the amount of permanganate that was added so that we can determine the

molarity of KMnO4. After completing this step the average molarity of KMnO4 was

determined to be 0.0158 M. It is important to convert mL to L so that we can get the

proper Molarity for the next step of the experiment. Data table 1 shows the calculations

for the standardization of KMnO4.

After determining the molarity of the KMnO4 we could then begin the titration of

the Iron salt to determine the percent composition of Iron. First we dissolved a sample of

iron salt into water and then we added both sulfuric and phosphoric acid. The reason we

added acid to both parts of the experiment is because we needed to ensure that the

experiment occurs under acetic conditions. The reason we added the phosphoric acid was

because the reaction of the iron ion can hinder our ability to see the endpoint. As the

redox reaction occurs the iron II becomes iron III that has a brown color. This color can

interfere with the pink color of the end point and the phosphoric acid neutralizes the

coloration allowing us to accurately see the endpoint. When the end point was reached,

an average of 11 mL of KMnO4 was dispensed. The average moles of KMnO4 in the three

trials was 1.76x10-4, the average moles of Fe in all three trials was 8.8x10-4. We were then

able to calculate the % Fe in the sample. To do this, the mass of the iron in the sample

was divided by the entire mass of the sample and multiplied by one hundred. The average

percent of Iron in the sample was 13.66%. Data table 2 shows the calculations that we

used to determine the percentage of Iron II in the sample.

Conclusion

The hypothesis that we can use a titration technique similar to the ones in

previous experiments to standardize KMnO4 and to determine the percentage of Iron in

the sample was accepted. This was accepted because we were able to accurately

determine the average molarity of KMnO4. This molarity was then used in determining

the percentage of Iron in the sample that was taken. The average molarity of KMnO4 was

determined to be 0.0158. The average percent of Iron II in the sample was calculated to

be 13.66%. One error that could have occurred during this experiment involved heating

of the mixtures. If the trials had different temperatures this could have influenced how

much of the KMnO4 was absorbed. Another possible error that could have influenced our

results is when it came time to reading the buret. If the amount dispensed was calculated

incorrectly then the results could have been heavily influenced by that.

Literature Cited1 Tro, N. (2010). Principles of chemistry: A molecular approach. (2nd ed., pp.151-158).

Illinois: Pearson.

2 Redox Titration and Determination of Percent Iron www.nova.edu/~giarikos