The purpose of this experiment was to find the percentage of iron in an unknown iron oxide sample which could be determined through titration with a standard solution of potassium permanganate. Success in this analysis requires careful preparation and attention to detail. Performing this experiment also provides practice at titrations. The overall reaction is: 8H+(aq) + MnO4-(aq) + 5e- Mn2+(aq) + 4H2O
In acid solutions the permanganate ion undergoes reduction to manganese (2+) ion. Since the permanganate ion is violet and the manganese ion is colorless, the end point in the titrations using potassium permanganate as the titrant can be taken as the first permanent pink color that appears in the solution. This titration involves the oxidation of Fe2+ ions to Fe3+ ions by the permanganate ion, and is carried out in sulfuric acid solution to prevent the air oxidation of the ferrous ion. The end point of the titration is sharpened markedly if phosphoric acid is present. The reason being the Fe3+ ion by itself has a yellow color that can partly mask the pink endpoint, but the addition of phosphoric acid forms a colorless complex with the Fe3+ ion.
Performing this lab requires patience and precision as well as a firm understanding of how to record accurate date (such as properly reading a buret). To complete the lab it is required to know how to convert from grams of a substance to moles of a substance, molarity, mole to mole ratio, calculating percent of a substance in a sample, and percent error. Also know that reduction and oxidation is what occurs in a redox reaction (Reduction is loss of oxygen. Oxidation is gain of oxygen). Procedure: The procedure was the same as the handout with the exception that only 2 titrations were preformed. Data: (handwritten neatly and attached)
Results and Discussion:
The moles and mass of iron in each sample were determined by using the volume of KMn04 required to titrate the unknown sample and the average molarity of the KMnO solution. The mass percent of iron (8.29 or 6.85 in data) in each sample was calculated from the mass of iron present in each unknown sample (0.0849g and 0.07120g) and each sample’s mass (1.023g and 1.040g). The average percent of iron in your unknown sample (7.57%) was calculated by adding the percentages of iron, in each sample (8.29 or 6.85), together and dividing by two (the number of samples). With the average percent of Fe in the samples calculated, then the percent error could be calculated by subtracting the average percent of Fe in the samples from the provided actual percent Fe value of the unknown (9.23% provided by professor) then dividing by the actual percent Fe value and multiplying by 100 (percent error formula shown in theory portion of lab). The results produced a percent error of 17.98%, which is rather high.
A high percent error means that the average percent of Fe in the samples calculated and the actual percent of Fe in the unknown sample (provided by professor) differ by a lot. This means the data produced during the lab was rather inaccurate. What could have been done to produce a lower percent error? One potential source of error could arise if not enough acid was added to the iron solution before the titration was started. This would allow much of the permanganate to go to MnO2(s) rather than to Mn2+. Another error could come from failure to stop the drip of KMn04 early enough which would produce an inaccurate volume of KMn04 used in the reaction based on the buret reading. Simple mistakes could also have been made by inaccurate readings of the buret or beakers.
After performing this experiment it is apparent that the qualitative analysis of substances can be found using the titration technique preformed in lab. The titration method requires two solutions, one of which is of known concentration and the other of un-known concentration. Titration is based on some reaction between both known and unknown solutions such as acid-base reaction or redox reaction. Performing this particular lab also aided with the understanding of redox reactions. Redox reactions are a combination of two reactions; oxidation and reduction reactions. The oxidation reaction involves an increase in increment in the oxidation state whereas in a reduction reaction, the oxidation number of the reactant molecules reduces. The substance that becomes oxidised is known as the reducing agent. On the contrary, the substance that reduces becomes known as the oxidising agent in the redox reaction. A substance which accepts electrons to form an anion displays reduction reaction and vice-versa.
Clark, Jim. ChemGuide. Ed. Jim Clark. N.p., n.d. Web. 7 June 2014. .