Iodometric Determination of Copper PDF

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IODOMETRIC DETERMINATION OF COPPER Experiment 1

Emily Jaramillo & Shivarpit Dua Professor Kenneth Yamaguchi Chem 2205: Analytical Chemistry Lab September 19, 2019

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I.

Introduction Iodometry or iodometric titrations use a redox reaction where an oxidizing analyte is

added to excess Iodide (I-) to form Iodine (I2) quantitatively. In this experiment, iodometric titration using a standardized 0.1 M sodium thiosulfate solution is utilized to determine the percent of copper in an unknown brass sample. To do so, a specified unknown brass sample is digested to liberate copper into its soluble cupric ion (Cu2+) form. Then, excess iodide is added in the form of potassium iodide to promote the solubility of iodine and the formation of the CuI precipitate along with liberated iodine. The reaction is as follows: 2 Cu2+ (aq)+ 4 I- (aq)  2 CuI(s) + I2 (aq) The liberated iodine is then titrated using standard sodium thiosulfate and yields the subsequent reaction: I2 (aq) + 2 S2O3-2 (aq)  2 I-(aq) + S4O6 -2(aq) It is important to note that a starch solution along with sodium thiocyanate is added before the titration to clearly indicate the endpoint and to prevent the absorption of iodine onto copper iodide. Once the titration is complete the volume of sodium thiosulfate is then used to stoichiometrically find the mass and percent of copper in the unknown brass sample. Ultimately, the values found for percent of copper in four samples can be statistically analyzed by finding the standard deviation, confidence interval, and employing the Grubb’s test for outliers.

II.

Experimental

3 

Preparation of a 0.1 M Sodium Thiosulfate Solution  Not Performed



Standardization of a 0.1M Sodium Thiosulfate Solution:  Not Performed



Determination of Copper in Brass 1. Weigh four samples of about 0.140 grams of the unknown Brass sample (A-102) into 250 Erlenmeyer flasks. 2. Add approximately 5 mL of sulfuric acid to each sample and a stir bar. Cover with a watch glass. 3. Place flaks on hot plate making sure to heat carefully. 4. If sample does not dissolve after a few minutes add a few more mL of sulfuric acid.  A few mL’s of concentrated Nitric acid was added instead of sulfuric acid to promote the digestion of the brass samples. 5. Remove samples from heat after dissolution is complete and let them cool. After cooling, add 25mL of distilled water.  Samples were placed in ice to speed up the cooling process. 6. Add 4M ammonium dropwise until a pale blue color is noted. 7. Add 5 mL of glacial acetic acid and prepare to titrate each sample. Prepare titration set up as shown in Figure 1. 8. Add about 3 grams of potassium iodide to a sample 9. Titrate with thiosulfate immediately until a light tan color is achieved. 10. Add 5mL of starch solution and 2 grams of sodium thiocyanate.

4 11. Swirl flask gently for a few seconds and complete titration by adding thiosulfate dropwise. 12. Once the endpoint is reached the solution should exhibit a white or slightly gray precipitate and the blue coloration of the solution should disappear. 13. Repeat steps 8-12 for the remaining samples.

III.

Results Analysis A. Data Table 1: Determination of Copper in Brass of Four Samples Sample

mL of

Moles of

Moles of

Grams of

Grams of

Percent of

s

Sodium

Sodium

Copper

Copper

Unknown

Copper

1 2 3 4

Thiosulfate 13.1 mL 19.1 mL 21.3 mL 22.4 mL

Thiosulfate 0.00131 mol 0.00191 mol 0.00213 mol 0.00224 mol

0.0832 g 0.1214 g 0.1354 g 0.1424 g

Brass 0.140 g 0.143 g 0.142 g 0.145 g

59.5% 84.9% 95.3% 98.2%

Calculations

0.00131 mol 0.00191 mol 0.00213 mol 0.00224 mol

5 Sample 1: 13.1 mL Na2 S 2 O 3 ×

( 10001 LmL ) ×(

)

0.1 M Na2 S 2 O 3 =0.00131mol Na2 S 2 O 3 1L

 Note that the net stoichiometry of the reaction is 1:1 because 2 moles of copper requires 2 moles of thiosulfate therefore: 2+¿ 2+¿=0.00131 mol Cu¿ ¿ Moles Na2 S 2 O 3 =Moles Cu 2+¿ ×

g Cu =0.0832 g ( 63.55 1 mol )

2+ ¿=0.00131 mol Cu ¿ Mass Cu 2+¿=

¿

832 g × 100=59.5 % ( 0.00.140 g ) Percent Cu

¿

Sample 2:

( 10001 LmL )× (

1 9.1 mL Na2 S 2 O 3 ×

)

0.1 M Na2 S2 O3 =0.00191 mol Na2 S2 O 3 1L

 Note that the net stoichiometry of the reaction is 1:1 because 2 moles of copper requires 2 moles of thiosulfate therefore: 2+¿ 2+¿=0.00191 mol Cu¿ ¿ Moles Na2 S 2 O 3 =Moles Cu 2+¿ ×

g Cu =0.1214 g ( 63.55 1 mol )

2+ ¿= 0.00191 mol Cu Mass Cu¿ 2+¿=

¿

g ×100=84.9 % ( 0.1214 0.143 g ) Percent Cu

¿

6 Sample 3:

( 10001 LmL )× (

21 .3 mL Na2 S 2 O 3 ×

)

0.1 M Na2 S2 O3 =0.00 213 mol Na2 S 2 O 3 1L

 Note that the net stoichiometry of the reaction is 1:1 because 2 moles of copper requires 2 moles of thiosulfate therefore: 2+¿ 2+¿=0.00131 mol Cu¿ ¿ Moles Na2 S 2 O 3 =Moles Cu 2+¿ ×

g Cu =0.1214 g ( 63.55 1 mol )

2+ ¿=0.00213 mol Cu ¿ Mass Cu 2+¿=

¿

1214 g ×100=95.3 % ( 0.0.140 g ) Percent Cu

¿

Sample 4: 22.4 mL Na2 S 2 O 3 ×

( 10001 LmL ) ×(

)

0.1 M Na2 S2 O 3 =0.00 224 mol Na2 S 2 O 3 1L

 Note that the net stoichiometry of the reaction is 1:1 because 2 moles of copper requires 2 moles of thiosulfate therefore: 2+¿ 2+¿=0.00224 mol Cu¿ ¿ Moles Na2 S 2 O 3 =Moles Cu 2+¿ ×

g Cu =0.1424 g ( 63.55 1 mol )

2+¿=0.00 224 mol Cu ¿ Mass Cu 2+¿=

¿

1424 g ×100= 98.2% ( 0.0.140 g ) Percent Cu¿

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B. Statistical Analysis of Data Mean:

∑ X i = 84.9 % +95.3 %+98.2 % =92.8 % X´ = n 3 Standard Deviation: X X i−¿´ ¿ ¿2 ¿ ¿ n−1 ¿ ∑¿ ¿ s=√¿ Confidence Interval: CI = X´ ± t s √n

95% ( 4.303) s (4.303 )(6.99 ) CI = X´ ± =92.8 ± =92.8 ± 17.37 √n √3

Range: 75.43% - 110.17% 90% CI = X´ ±

(2.920)s (2.920)( 6.99) =92.8 ± =92.8 ± 11.78 √n √3

Range: 81.02%-104.58% Grubb’s Test:

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∑ X i = 59.5 %+ 84.9 %+95.3% +98.2 % =84.48 % X´ = n 4 X X i−¿´ ¿ ¿2 ¿ ¿ n−1 ¿ ∑¿ ¿ s=√¿

|questionable value− ´X | |59.5−84.48 %|

G calc =

s

=

17.52

=1.43

Gtable for 4 samples at 95% CI: 1.463 G calc < G table

 Because the calculated G value is less than the tabulated G value the questionable value can be kept, however, if it is known that the value was deviated due to a systemic or random error during the experiment it should still be discarded as an outlier.

C. Discussion



Preparation and Standardization of a 0.1 M Sodium Thiosulfate Solution This portion of the experiment was not performed by the students. Due to its nature,

preparing and standardizing sodium thiosulfate would have posed as a source of potential error when calculating the percent of copper and would have required too much time. 

Determination of Copper in Brass

9 In this portion of the experiment one deals with the utilization of a redox titration to determine the percent of copper in an unknown sample of brass. To begin, four samples of unknown brass sample A-102, all within the range of 0.140-0.145 grams, where weighed and placed in beakers labeled one through four. Subsequently, 5 mL of 9M sulfuric acid were added to digest the brass sample and promote the dissolution of the cupric ion into a solution. A stir bar was also placed into all the beakers and then they were quickly covered with a watch glass and heated via a hot plate. It is important to note that the samples did not dissolve at first which may have been caused by uneven heat distribution. For this reason and to have enough time to titrate the samples, a few milliliters concentrated nitric acid was added. After the addition of the nitric acid, the color of the solution quickly changed from a pale turquoise to a more vibrant turquoise and the dark brass samples began to disappear and leave only a white residue at the bottom of the beaker. Once fully dissolved, the beakers where carefully removed from the hot plate and placed in an ice bath to facilitate the cooling process. Following cooling, 25 mL of distilled water were carefully added to each beaker and then very carefully 4M ammonium was introduced in a dropwise fashion to all the beakers until a pale blue/dark blue almost indigo color was observed. This change in coloration was a visualization of the neutralization of the acid in each of the beakers. Because the volume of nitric acid added to each beaker whilst digesting the samples was not defined, each sample required variable quantities of ammonium to reach the desired coloration. For this reason, litmus paper was implemented to gauge if the sample solutions’ pH values were within an acceptable range between weak acid and neutral to continue with the titration process. At this point, each sample was ready for final titration after adding 5 mL of glacial acetic acid to remove the copper (II)ammonia complex. Next, 3 grams of potassium iodide were added to the beaker with the first

10 sample turning the pale blue solution to an orange colored one. Adding iodide in excess then allowed for the formation of the iodine which was then titrated using 0.1M sodium thiosulfate solution producing a light tan color in the beaker. 5 mL of starch solution along with 2 grams of sodium thiocyanate were added to create a sharper endpoint color and to displace any absorbed iodine. Note that when starch was added the solution turned to a blue/black coloration. The sample was then transferred to an Erlenmeyer flask to facilitate titration and sodium thiosulfate was added dropwise via a burette. At this time, a precipitate of gray/ white coloration began to settle at the bottom of the Erlenmeyer flask signaling the endpoint of the titration. The final volume of the sodium thiosulfate solution was noted. Lastly, the same steps, beginning with the addition of potassium iodide were followed to titrate the next three samples. To calculate the percent copper after having titrated each sample the initial volume of sodium thiocyanate solution found within the burette subtracted from the final volume of the sodium thiocyanate. This resulting volume was then multiplied by the concentration (0.1M) to yield the mols of sodium thiocyanate. It is important to note that sodium thiocyanate and copper have a 1:1 stoichiometric relationship as 2 moles of copper are required for every 2 moles of thiosulfate as shown by the reaction: 2−¿ −¿ →2 Cu I ( s)+S4 O6¿ (aq) ¿ 2−¿ +2 I (aq ) +2

¿

2 C u( aq ) +2 S 2 O 3(aq) The moles of copper were then multiplied by the Molar mass of copper (63.55 g/mol) to find the grams of copper in the sample. The percent of copper was then found via the following equation: % Copper ∈Sample=

calculated grams of copper ×100 mass of original sample

11 After all the percentages where calculated as shown in Table 1 a statistical analysis was performed. To perform this analysis correctly a Grubbs test was performed to eliminate any outliers from the data. The Grubbs test concluded that the value for the percent of copper found in sample 1 could be retained, however, if a value is known to be deviated due to a random or systemic error whilst running the experiment it is automatically discarded. This was the case for Sample 1, as less 4M ammonium was added to this sample before titration causing the volume of sodium thiosulfate solution needed to reach the endpoint to be much smaller that the other samples. The burette was also not filled to the 1mL mark with sodium thiosulfate solution and therefore an incorrect reading of the volumes could be a potential source of error when causing a smaller percent of copper. Once the outlier was removed, the percentages for samples 2,3, and 4 where relatively precise as is shown in Table 1. The mean for the percentages was 92.8% and was then used to find the standard deviation for the values and was calculated to be 6.99. This standard deviation was not relatively high and shows that the dataset was not too deviated from the mean. Furthermore, the confidence interval (CI)for the mean was also calculated to find the amount of uncertainty. At a 95% CI the values would be certain from the mean of 92.8% with a margin of error of ±17.37 % giving a range of 75.43% - 110.17%, however, at a 90% confidence interval the values would deviate from the mean with a value of

± 11.78% giving a range of

81.02%-104.58%. These values are representative of the percent of copper found in the unknown sample we used (A-102) and show that this sample had a very high percentage of copper. Possible sources of error in this experiment include the uneven heat distribution in the beginning during the sample dissolution, variance of ammonium added to neutralize the acid which would change the pH of the resulting solution, as well as the over addition of sodium

12 thiosulfate. A high volume of thiosulfate would then yield a “fake” high percentage of copper in the brass sample as they are one to one stoichiometrically.

IV.

Conclusion Overall the experiment demonstrated that in real life analysis of samples both sample

preparation and the analytical method used to quantify the analyte need to be done with relative precision as any errors can completely deviate the final values of analyte. More specifically, having had to remove the outlier in our reconciled data shows that more than three samples are required to yield less variance within results. From this experiment I feel the objective of familiarizing with Iodometry was met as the reduction of iodine to iodide was extremely visual during the titration process with the tan/yellowish color of iodine changing drastically to a white/grey precipitate. Moreover, the statistical analysis of our values showed that there was precision as the standard deviation was relatively low (6.99) and the values were all within the 95% confidence interval range. To conclude, .

References Harris, D. C., & Lucy, C. A. (2016). Quantitative chemical analysis. New York, NY: Macmillan Higher Education / Worth Publ. Yamaguchi, K (n.d). Chem 2205. Analytical Chemistry Laboratory Manual. New Jersey City University.

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