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L. H. Kalbus and R. H. Petrucci California State College San Bernardino. 92407 and G. E. Kalbus California State University Long Beach. 90840
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A Multifaceted Experiment for Quantitative ~nalysis Back titration, hydroxide determination, and selection of an indicator
One important topic considered in a quantitative analysis course is the selection of an indicator for specific acid-hase titrations. Two methods to accomnlish this are usuallv discussed. One is to ralculate the which will exist at the eauivalence . ooint..and the other. if calculations are too diffic&, involves experimental deterkination and plotting of the titration curve. In this experiment thesrudent is presented with the problem of selectinr! n suitable indicator for the determination of the hydroxide in basic copper chromate. A sample is weighed out, and a measured excess of standard hydrochloric acid is added. The acid reads with two species, the hydroxide ion and the chromate ion present in the basic copper chromate. A back titration, followed with a p H meter, is performed using a standard solution of sodium hydroxide. The p H curve is plotted and two breaks are obtained as shown on the graph. The excess HCI and the first hydrogenof the chromic acid are neutralized a t the first break. The second hydrogen of the chromic acid is neutralized and the precipitation of the basic copper chromate is completed at the second break. The p H curve for the back titration of potassium chromate (which serves as a blank) is also plotted on this samegraph. From the observation of the p H which exists a t the first break, the student can select an appropriate indicator to use for this determination. In the actual experiment, a blank sample containing exactly the same amount of chromate as the unknown is also titrated. From the difference in titrant volume required to reach the first endpoint for the chromate blank and for the sample, the percent of hydroxidecan becalculated. Prepare and dry basic copper chromate (2CuCrOa 3Cu(OH)z).' Weigh out about 0.2 g and transfer to a 150-ml beaker. Pipet 50 ml of 0.1000 M HCl into the beaker containing the sample and stir until dissolved. Using a p H meter, titrate potentiometrically with 0.1000 M NaOH. Plot the curve and determine the p H a t the first equivalence point.
Consult the appropriate references and select a suitable indicator to use in this titration. (Methyl orange is satisfactory as an indicator.) Weigh to the nearest 0.1 mg approximately 0.2 g of basic copper chromate and transfer to a 250-ml beaker. Pipet 50 ml of 0.1000 M HC1 into the beaker and stir until the sample is dissolved. Add the selected indicator and titrate with 0.1000 M NaOH to the endpoint. Dilute the sample that has just been titrated to 150 ml. Add 5 ml of concentrated H&04 and 5 ml of concentrated H3P04 and titrate potentiometrically with approximately 0.1 N standard ferrous ammonium sulfate solution. Calculate the percent chromate in the basic copper chromate sample. I'renare a blank solution by weighing out the exact amount of pure potassium chromate reqLired to provide the same amount of chromate as the unknown and transfer to a 150-ml beaker. Pipet 50 ml of the standard acid solution into the beaker, dissolve the sample, and add the indicator. Titrate with 0.1000 M NaOH to the endpoint. The percent hydroxide is calculated using the following equation (mle~..r - mls,,~.)
X
(M base) X (0.017011X (100)
g samvle ~
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(The derivation of this equation makes a good student exercise.) As an alternative to using a KzCr04 blank exactly equivalent to the unknown, the HCI may be standardized in terms of ml HClIg Cr04. This may he accomplished as follows (a) Weigh out any exact amount (0.24.5 g) of pure K2Cr01, followed by the hack titration procedure described above. (b) Calculate the number of ml HCI consumed by the KPCIOI.(Since the strengths of the acid and base are identical, this is simply equal to 50.00 ml- ml base in back titration.) Express this result in the form ml HClIg CrOc (c) After the chromate has been determined in the unknown sample, calculate the ml HC1 consumed by the chromate using the factor determined in (b). (d) Determine the total ml HCI consumed by the unknown (again, 50.00 ml - ml base in back titration). (el Subtract the volume of HCI in (c) from (d). This difference is equivalent to the amount of hydroxide in the sample. The instructor may prepare student unknown samples by mixing varying known amounts of pure potassium chromate and basic copper chromate.
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Titration curves.
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"IO I O I X I Y ".OH
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Based on a paper presented at the 166th National Meeting of the American Chemical Society, Chicago, August 1973. 'Kalbus, L. H., and Petrucci, R. H., J. CHEM. EDUC., 46, 776 (1969).
Volume 53, Number 11, November 1976 / 719
