CHEM 319 Study Guide - Ammonium Sulfate, Ethylenediaminetetraacetic Acid, Cuvette

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29 Oct 2014

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Introduce the student to the fundamental concepts of sample dissolution, derivatization. Harris, d. c. (2007); quantitative chemical analysis 7th ed. ; 26-27, 336-337, 378- Manganese is a minor constituent found in many steels. In this experiment you will be analyzing a steel sample to get the weight percentage of manganese. You will first dissolve the sample in an acidic solution and then oxidize the colorless mn2+ to form the purple. Once this has been done, it is possible to measure the absorbance permanganate ion (mno4 of this solution and use beer s law (a = bc) to determine the concentration of the permanganate ion. This molar concentration can easily be related to weight percentage of the steel sample. You will be calibrating your experiment against a standard steel sample with a known percentage of manganese. You will be using ammonium peroxydisulfate and potassium periodate to oxidize the mn2+.

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INTRODUCTION

Many transition metal cations can have more than one possible charge (Worksheet 1). Differently charged species of the same element can have different chemical and physical properties. Knowing the charge can allow separation of the different ions based on their properties. For example, Cu²⁺ is blue and stable in solution, while Cu⁺ is colorless and reacts with oxygen. Hemoglobin contains iron as Fe²⁺, but it is inactive at binding oxygen when the iron gets oxidized to Fe³⁺. There are many similar situations in which it is important to always properly indicate charges of ions.

This laboratory experiment uses the difference in chemical properties of Fe²⁺, iron(II), and Fe³⁺, iron(III), to quantify the amount of each present in a solution of unknown concentrations of each. You will first determine the amount of iron(II) in your sample by titrating with a reagent that does not react with iron(III). You will then titrate a second sample after converting all the iron(III) to iron(II) in order to determine the total iron present. The percent composition of Fe²⁺ in the unknown mixture will be calculated.

BACKGROUND

The concentrations of both iron(II) and iron(III) ions in a solution can be determined by titration with potassium permanganate. The reaction is an oxidation-reduction reaction, or redox, for short. A brief introduction to oxidation-reduction reactions is given in the background of the âAcids and Basesâ experiment. The permanganate ion effectively oxidizes iron(II) to iron(III) in an acidic solution while being reduced to manganese(II) ion. The balanced net ionic reaction is:

5 Fe²⁺(aq) + MnO4^â(aq) + 8 H⁺(aq) â 5 Fe³⁺(aq) + Mn²⁺(aq) + 4 H₂O(l)

Iron(III) is not oxidized by the permanganate ion, and thus does not react. Thus in order to determine the concentration of iron(III) ions, they must first be reduced to iron(II) ions. Zinc metal is used as the reducing agent, also under acidic conditions.

Zn(s) + 2 Fe³⁺(aq) â 2 Fe²⁺(aq) + Zn²⁺(aq) (reduction of the Fe3+)

Zn(s) + 2 H⁺(aq) â Zn²⁺(aq) + H₂(g) (a side reaction that oxidizes excess Zn)

The Zn2+ formed does not interfere with the subsequent titration because Zn2+ is not oxidized by permanganate. Notice that converting all the iron(III) to iron(II) means that the total iron concentration ([Fe2+] + [Fe3+]) is titrated, not just the iron(II) originally present.

Techniques

This is a lab will utilize several of the techniques introduced in the experiment âLaboratory Techniques and Measurements,â the procedures for which are given in Appendix B. Poor laboratory technique and misuse of the glassware will result in poor data results, which will affect the grade you receive on the laboratory report. In order to use your time efficiently in lab, review how to properly:

rinse glassware prepare a standard solution use a volumetric pipette

weigh by difference set up and use a burette use a graduated cylinder

heat samples on a hot plate

Analysis of Iron by Oxidation-Reduction Titration

This lab uses the common quantitative technique of titration. The only difference between the titration you will be doing in this lab and that done in the Molar Mass of a Known Acid lab is the type of reaction used: this lab will use a redox reaction in place of the acid-base reaction performed in the previous experiment.

In order to determine the concentrations of iron(II) and iron(III) ions in a solution containing both ions, two portions of the same sample must be titrated separately. This first titration will determine only the iron(II) concentration. A second sample will be treated with zinc metal in order to reduce the iron(III) to iron(II). The resulting solution will be titrated with potassium permanganate. The result of this titration will give the total iron concentration in the sample. The difference between the first and second titration is the equal to the iron(III) concentration.

In this titration, potassium permanganate serves as its own indicator. The intense purple color of the permanganate solution becomes colorless when the permanganate is reduced to manganese(II). Thus, as iron(II) is oxidized with the addition of the permanganate solution from the burette it will impart a purple color that will fade. However, once there is no iron(II) left in solution a faint purple color will remain, marking the endpoint of the titration. This color can be slightly altered by the presence of iron(III), which is yellow in solution. Remember that iron(III) is generated by the reaction between iron(II) with permanganate, increasing the yellow appearance of the solution and which can interfere with the detection of the endpoint. Phosphoric acid is added to help minimize this effect.

pre lab questions:

Using the balanced equations given below, determine the overall molar ratio between aqueous oxygen and thiosulfate. Using the known concentration and added volume of sodium thiosulfate solution along with this molar ratio, it is possible to calculate the amount and concentration of dissolved oxygen in the original sample.

Using this information: I used 4.00 g potassium dichromate. The amount of Iron ll is 4.00 gram in 2nd flask. Used grey moose vodka 2.00 ml and filled flask with water. This was the 2% vodka solution. For the experiment I used 5.00ml of the vodka and added 35.00 ml of more water to make it 1/8 of 2% or 0.25% of the original. With this information and the answer's i have placed in the table please help with remainder of questions. Need to work to understand answers so I will get correct on upcoming test. Thanks.

Calculate the concentration of the dichromate ion in the first volumetric flask.

Calculate the concentration of the iron (II) ion in the second volumetric flask.

Experiment 2: Titrate the Vodka Sample

Lab Results

Record the following lab data in the table below. If you had to repeat one of the titrations, disregard the value that was different.

(a) volume of potassium dichromate solution added to the Erlenmeyer flask in mL	5.00ml
(b) coarse titration volume of iron (II) solution range in mL	14.10 -14.15 ml
(c) volume of iron (II) solution delivered from the burette in mL during the first fine titration	14.13ml
(d) volume of iron (II) solution delivered from the burette in mL during the second fine titration	14.11ml
(e) average volume of iron (II) solution used in the fine titrations	14.12ml
(f) the color of the analyte solution at the end point of the titration	purple
(g) the color of the analyte solution after adding the indicator	dark green

Data Analysis

Record and calculate the quantities in the table below using the data from your dichromate titrations. Use an average value for the volume of iron (II) solution used in the titration. If one of your values is very different, and you had to perform the titration three times, disregard the value that was very different when computing the average.

(a) volume of potassium dichromate solution added to the Erlenmeyer flask in mL 5.00ml
(b) moles of dichromate ion added to the Erlenmeyer flask
(c) average volume of iron (II) solution delivered from the burette in mL 14.12ml
(d) moles of iron (II) ions delivered from the burette
(e) moles of excess dichromate ions that reacted with the iron (II) ions (remember that the ratio in which they react is 1 dichromate : 6 iron (II))
(f) moles of dichromate that reacted with the ethanol in the vodka (Subtract excess dichromate ions that reacted with the iron (II) ions from the original moles of dichromate ion present.)
(g) moles of ethanol in the 5 mL diluted vodka sample according to the stoichiometric ratio of 2 dichromate ions to 3 ethanol molecules

The amount of alcohol in a drink is typically reported as percent alcohol by volume. Volume percent or volume/volume percent (% v/v) most often is used when preparing solutions of liquids. Volume percent is defined as:
% v/v = V_solute/V_solution Ã 100
Find the percent alcohol (ethanol) by volume for the vodka used in the lab by following the steps outlined in the table below.

(a) given the molar mass of ethanol of 46.07 g/mol, calculate the mass of alcohol (ethanol) in the tested sample solution
(b) given the density of ethanol of 0.7893g/ml, find the volume in mL of ethanol present in the diluted vodka solution
(c) record the volume of vodka used in the experiment in mL
(d) find the percent alcohol by volume (% v/v) in the diluted vodka solution
(e) the diluted vodka solution was prepared by diluting 2.00 mL vodka to 100.00 mL. Calculate the dilution factor used (N:1)
(f) multiply the percent alcohol by volume in the diluted vodka solution by the dilution factor to obtain the % v/v alcohol in the original vodka solution

Conclusions

The Grey Moose vodka tested in this lab reports a percent alcohol by volume of 40.0% on its label. How does your value compare to the reported one? If the values are different, give one possible experimental error that might have contributed to the difference.

Potassium permanganate is another strong oxidizing substance similar to potassium dichromate. An acidic solution of purple permanganate ions can get reduced to colorless Mn²⁺ ions in the presence of ethanol. Write down the redox reaction between permanganate and ethanol, and balance it using the half-reaction method.

Besides vodka, there are other colorless alcohol-containing beverages that can be titrated following the procedure in your lab. Given the average values for the percent alcohol by volume listed in the table below, which beverage do you expect to use the least amount of iron (II) standard solution during the titration? Assume all lab procedures stay the same.

	% alcohol by volume
White rum	37.0%
Vermouth	18.0%
White whine	12.0%

CHEM 319 Study Guide - Ammonium Sulfate, Ethylenediaminetetraacetic Acid, Cuvette

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