Page 1 of 5
I.
Introduction
Gravimetric analysis, which by definition is based upon the measurement of mass, can be generalized into two types; precipitation and volatilization. Precipitation involves a precipitating agent (or precipitant) to form a weighable solid whose mass can be used to calculate the mass of the analyte while volatilization is a method in which mass measurements are combined with the formation or loss of volatile materials. Examples of volatilization are combustion and thermogravimetric analysis. In this experiment, precipitation is the focus of gravimetric analysis. (Hage & Carr, 2011) The basic steps which are carried out in this method are: pretreatment of the sample, precipitation, digestion or Ostwald ripening, filtering, filtering, washing, igniting, weighing. Washing is done remove the species adsorbed b y the precipitate while digestion increases the purity of the final precipitate by allowing the small precipitate particles dissolve and release impurities. Digestion also allows larger particles take those dissolved ions and grow in size. Precipitates must be formed slowly as to ensure purity. This is b ecause solids formed slowly have high degree of order o rder in its structure. There are additional techniques which are us ed to produce best possible quality of precipitates. One of this is by slow addition addition of precipitating reagent with effective stirring. This prevents local high concentrations of the precipitating agent. In addition, many precipitates are more soluble at the lower lower (more acidic) pH values values and so the rate of precipitation is slower, thus it is important to carry out the precipitation process at a pH near the acidic end of the pH range in which the precipitate is quantitative. Some examples of gravimetric methods are the analysis a nalysis of silver by reacting silver ions in water with an excess of chloride ions to form a precipitate of silver chloride, combustion a nalysis in which the mass of the emitted gases during combustion is determined and used to obtain the mass of the analyte. (Hage & Carr, 2011) This experiment will focus on the analysis anal ysis of iron in a sample. It aims to illustrate the principles and proper techniques involved in precipitation and gravimetric analysis and; to demonstrate how percent composition of an analyte is calculated in an unknown sample using gravimetric data. (CHM130L Laboratory Manual, 2013)
Page 2 of 5
II.
Methodology
Determination of the constant mass of 2 sets of crucible and cover was done in preparation for the two trials that was carried out in the gravimetric analysis. This was done by subjecting the crucible and cover to extremely high heat. Heating, cooling, and weighing the crucible and cover were done alternately three times until a mass of not greater than .003 mg from the previous weighing was obtained. After the crucible and cover preparation, 0.677 g of red orange solid sample was obtained. It was then mixed with water and HCl. Insoluble impurities in this mixture were filtered. The filtrate was mixed with HNO3 producing a clear yellow solution. It was then diluted to 200ml with distilled water and was mixed with ammonia until pH of 8 was obtained. The solution containing the reddish brown precipitate that appeared was boiled for about 5 minutes. The precipitate was then allowed to settle before filtering. NH4Cl was used to quantitatively transfer all the precipitate from the beaker to the filter paper. AgNO3 test was omitted due to unavailability of the chemical. Once drained, the filter paper containing the precipitate was kept in the previously weighed crucible and cover. This was heated until complete ignition of iron oxide was ensured. Heating, cooling, and weighing of crucible and cover containing iron oxide were again done alternatively supposedly for 2-3 times until a constant mass was obtained. It was achieved in trial 1, however in trial 2, the crucible cover blew off during the second heating. The mass obtained during the first weighing in trial 2 was the o ne used for the calculation of weight percent of iron in the sample. The weight percent obtained from each trial was averaged and compared by calculating the average deviation.
III.
Results and Discussion
Table 6-1: Determination of constant mass of the crucible and cover Mass of the crucible and cover
Weighing
Trial 1
Trial 2
1
42.0250
42.0610
2
42.0276
42.0629
3
42.0267
42.0618
Page 3 of 5
Table 6-2: Determination of the amount of Iron in the sample Trial 1
Trial 2
0.677
0.639
42.208
42.238
42.207
crucible cover broke
mass of Fe2O3 in the sample
0.180
0.1762
mass of Fe in the sample
0.126
0.123
18.628
19.286
Initial mass of the sample mass of crucible & cover + iron oxide (Fe2O3)
weighing 1 weighing 2
weight % of Fe in the sample average weight % of the sample average deviation
18.957 0.329
Table 6-3: Qualitative analysis of the procedure Step # (6-2.1a)sample from initial weighing (6-2.1b)sample + H20 + HCl (6-2.1c)filtrate + HNO3
Observation moist red orange solid granules
clear yellow solution clear yellow solution
(6-2.2a)diluted solution (filtrate + HNO3) + ammonia
reddish brown precipitate was formed
Page 4 of 5
(6-2.2b)diluted solution (filtrate + HNO3) + ammonia, after digestion
reddish brown precipitate became more distinct, supernatant liquid was colorless
(6-2.3)after 2nd filtration residue
reddish brown gelatinous solid
filtrate
colorless liquid
(6-2.5)residue after
residue is a red brown solid powdery
ignition
substance
Weight percent of iron was calculated using the gravimetric factor 2Fe/Fe2O3. Calculation started with the mass of iron oxide (Fe2O3) obtained multiplied by the gravimetric factor and ratio of molar mass of iron and iron oxide. The obtained value represents the mass of iron in the sample. This was divided by the mass of the sample multiplied by 100 to get the weight percent of iron. Results from trial 1 and trial 2 only had 0.329 average deviation from each other. It can be considered comparatively small as the constant mass requires only 0.0 03 difference. However, the average weight % of iron from two trials was way far from the theoretical value of 34.4% calculated from a 0.6094 g of FeCl3 given -- almost 50% error.
The error obtained can be considered primarily random since both trials almost have the same result. This is also true when the results were compared to the results obtained by other groups, however there are also systematic error which could have been avoided if the recommended pH after the addition of ammonia was followed; digestion was done for about 25 minutes; and by transferring quantitatively. There are other factors which can be accounted for the loss of some of the mass of the analyte during the procedure and those could have been eliminated by filtration of gelatinous precipitate while the solution is still hot. This is to keep impurities from getting trapped in the gelatinous precipitate once the solution gets cold. Another important technique which was followed during the experiment is the use of ashless filter paper. This do es not produce pulp or tiny particles of paper which could add up to the weight of the precipitate. Another useful aid but must be done at a specified length of time is digestion. Digestion or
Page 5 of 5
Ostwald ripening is a technique for obtaining prec ipitates with both large and pure particles by heating a precipitate in its original temperature that is near the boiling point of the solution. This heating alters the solubility product for the precipitate so that more can dissolve in the solution. Because solubility is a dynamic equilibrium, a solid forms at the same rate as dissolution occurs. The result is a situation in which small particles tend to dissolve and release ions. At the same time, larger particles tend to take some of these released particles and grow in size (Hage & Carr, 2011). Heating the solution for a longer period of time could make the solution boil and eventually causing disorder in the precipitate particles which is not favorable for crystal growth.
IV.
Summary and conclusion
Gravimetric analysis is a long and tedious method but very useful in determining the mass of the analyte in a sample using the mass of the precipitate formed. The accurac y of the result depends on the quality of the final precipitate thus it is very importance to observe the p roper techniques in each step. One must always consider the correct pretreatment procedure for the sample as to ensure optimum amount of analyte in the solution, slow formation of precipitate as to avoid peptization or formation of colloid, digestion and washing of precipitates to remove impurities from coprecipitation, and careful treatment of the solid during and after ignition. The mass of the precipitate can be used to calculate for the mass of the analyte by using a gravimetric factor (GF).
V.
References
(n.d.). Retrieved June 2013, from http://www.chem.tamu.edu/class/majors/tutorialnotefiles/gravimetric.htm CHM130L Laboratory Manual. (2013).
Hage, D., & Carr, J. (2011). Analytical Chemistry and Quantitative Analysis. New Jersey: Pearson Prentice Hall. Hargis, L. G. (1988). Analytical Chemistry: Principles and Techniques. New Jersey: Prentice Hall.