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The Standardization of EDTA and the Determination of a Mg2+/Ni2+ Ion Mixture Composition using EDTA Andy Nguyen Department of Chemistry Georgia State University April 18, 2018

Introduction Ethylenediaminetetraacetic acid or EDTA is an acid that contains exceptional properties that are used for medical, industrial, and analytical purposes. Here it is used to determine the concentrations of two unknown metal ion mixtures in a solution. In total, EDTA has six hydrogen ions (H+) and two of those come from the amine group and the rest are from the carboxylic group. EDTA has a large metal binding constant because it is a synthetic acid and under a special pH, it always has a 1:1 ratio with the metal ions in solution (Harris,2010). However, since the binding constant has such a high value, it is difficult to determine an accurate result and the break in the titration curve is the key to determining the binding of the metal ions(Berman, 1991). The purpose of this experiment is to observe and analyze how EDTA reacts when it is titrated into a solution that contains Nickel (Ni 2+) and Magnesium (Mg2+). To do so, EDTA must be standardized from NaOH and then the EDTA is then used to determine the composition of the unknown Mg2+/Ni2+ solution mixture(Norton, 2007).

Procedures The machine used to perform the titration is the SI Analytics Titroline 7000 in conjunction with the Titrisoft Software. For the standardization of EDTA, the NaOH that was used was already prepared and used in previous experiments and has a concentration of 0.145M.

Preparing Solution A1 w/0.05M EDTA: About 18.6g of EDTA was weighed and dissolved in a 1L beaker filled with DI water. A bit of heating was required to dissolve all the EDTA but once complete, the solution was transferred to the 1L beaker and DI water was filled until there was 1000mL total solution. Preparing Solution A2 w/0.025M EDTA: Using solution A1, 50.0mL of A1 was pippeted into a 100mL volumetric flask. The flask was then filled with DI water until the 100mL line to make the solution approximately 0.025M EDTA. Preparing Solution A3 w/0.025M EDTA and MgSO 4•7H2O: About 0.6162g of MgSO4•7H2O salt was weighed to make solution A3. A3 is made in the same manner as A2 except in this scenario, MgSO4•7H2O salt was added to the solution. Preparing 0.05M Mg2+ solution: Approximately 6.973g of MgSO 4•7H2O was weighed and dissolved in a 250mL beaker with DI water to have a total of 250mL of solution. The total solution is 250mL with a 0.05M Mg2+ concentration. Preparing 0.05M NI2+ solution: Approximately 3.100g of Ni(Ac) 2 was measured and was mixed with DI water in a 250mL beaker to make a total of 250mL Nickel solution with a concentration of 0.05. Preparing the 2.5:7.5 ratio of Mg 2+ and Ni2+ solution: A volumetric pipet was used to transfer 25mL of the prepared Mg 2+ solution and 75mL of the prepared Ni 2+ solution into a 100mL volumetric flask. The flask was then mixed to ensure that the solution was evenly mixed together.

Titration of Mg2+ with the Standardized EDTA (0.05M): Using a 10mL volumetric pipet, 10mL of Mg2+ solution was transferred into a tall 200mL beaker. For the first trial, the Mg 2+ was mixed with an acetate buffer (pH=4.7) and in the second trial, the Mg 2+ was mixed with an ammonium buffer (pH=9.3). In each trial a small amount of Murexide indicator was added to show color change and both solutions were titrated using the Standardized EDTA. Titration of Ni2+ with Standardized EDTA (0.05M): For the titration of Ni 2+, the procedures are the same as the titration of Mg2+ with standardized EDTA. Titration of the 2.5:7.5 mixture of Mg2+ and Ni2+ solution: Using the prepared solution of 25mL Mg2+ and 75mL Ni 2+, six total titrations of this solution was performed. Three titrations were done using an acetate buffer and the other three were done using an ammonium buffer. To prepare the solution for titration, 10mL of the mixed solution was pipetted into a 200mL tall beaker and about 100mL of the appropriate buffer was added along with a small pinch of Murexide indicator.

Results Table 1. Data from the three trials of the standardization of EDTA from NaOH. By using the NaOH that was previously made, it is used to react with EDTA. Since the concentration of NaOH is already known, the concentration of EDTA can be found. Trial # 1 2 3 Avg.

M of NaOH (mol/L) 0.145 0.145 0.145

Veq of NaOH (mL) 8.936 8.770 8.656 8.787

V of EDTA (mL) 4.468 4.385 4.328 4.394

M of EDTA (mol/L) 0.0518 0.0509 0.0502 0.0510

Titration of EDTA and MgSO4 Salt w/NaOH 14 12 10

pH

8 6 4 2 0

0

2

4

6

8

10

mL of NaOh

12

14

16

18

20

Figure 1. The overlap of the data from table one for all three trials of titration. From the figure, the equivalence point is determined to be around the high 8mL area. Table 2. Data for the titration of Ni 2+ metal ion with an acidic (acetate) buffer and a basic (ammonium) buffer. Results Veq(mL) mV

Acetate Buffer (pH=4.75) 9.374 936

Ammonium Buffer (pH=9.25) 8.792 864

Ni2+ Titration with Acetate and Ammonium Buffer 940 920

mV

900 Acetate Buffer pH=4.75 Basic Buffer pH=9.25

880 860 840 820

0

5

10

15

20

25

mL of EDTA

Figure 2. Graph of the data for the titration of Ni2+ under acidic and basic buffers. Notice how in the basic buffer, there is a drop in mV around the 8-9ml area. This is where the equivalence point is.

Table 3. Data for the titration of Mg2+ in Acidic and Basic buffer solutions. The Veq for the acidic solution is not available because EDTA cannot bind with Mg2+ in an acidic solution. The reason for this is explained in the discussion section. Results Veq(mL) mV

Acidic Buffer (pH=4.75) N/A N/A

Basic Buffer (pH=9.25) 14.28 865.4

Titration of Mg2+ with EDTA 980 960 940 920

mV

900

Acidic Buffer pH=4.75 Basic Buffer pH=9.25

880 860 840 820 800

0

5

10

15

20

25

mL of EDTA

Figure 3. Graph for the titration of Mg2+ with EDTA. Notice in the acidic buffer, the line goes up instead of downwards. This indicates that the EDTA is not reacting with the Mg2+ metal.

Table 4. Data for the three trials of 2.5:7.5 mixture of Mg 2+ and Ni2+ respectively. Titration was done with three trials of an Acidic buffer with a pH of 4.75.

V of Ni2+(mL)

Veq(mL)

M of Ni2+(mol/L)

Trial#

M of EDTA (mol/L)

1

0.0510

7.294

10.0

0.037

2

0.0510

6.930

10.0

0.035

3

0.0510

6.944

10.0

0.035

7.056

10.0

0.036

Avg.

Titration of 2.5:7.5 Mixture of Mg2+/Ni2+ with EDTA in Acidic Buffer 1600 1400 1200

mV

1000 Acidic Buffer Trial 1 pH=4.75 Acidic Buffer Trial 2 pH=4.75 Acidic Buffer Trial 3 pH=4.75

800 600 400 200 0

0

5

10

15

20

25

mL of EDTA

Figure 4. Graph of the three trials of the known mixture of Mg 2+/Ni2+ in acetate (acidic) buffer with a pH of 4.75.

Table 5. Data for the three trials of 2.5:7.5 mixture of Mg 2+ and Ni2+ respectively. Titration was done under basic conditions with a pH of 9.25 along with EDTA. Trial# 1 2 3 Avg.

M of EDTA (mol/L) 0.0510 0.0510 0.0510

V of Mg2+(mL) 10.0 10.0 10.0 10.0

Veq(mL) 8.960 10.28 10.25 9.83

M of Mg2+(mol/L) 0.017 0.009 0.016 0.014

Titration of 2.5:7.5 Mixture of Mg2+/Ni2+ with EDTA in Basic Buffer 1800 1600 1400 1200 Basic Buffer Trial 1 pH=9.25 Basic Buffer Trial 2 pH=9.25 Basic Buffer Trial 3 pH= 9.25

mV

1000 800 600 400 200 0

0

5

10

15

20

25

mL of EDTA

Figure 5. Graph of the three trials of the titration of the known mixture of Mg2+/Ni2+ in ammonium (basic) buffer with a pH of 9.25.

Table 6. Data for the titration of EDTA with Unknown Mg 2+/Ni2+ mixture #48 in Acidic Buffer with pH of 4.75. Trial # 1

M of EDTA (mol/L) 0.0510

Veq Acidic (mL) 12.176

V of Mixture (mL) 10.0

M of Ni2+(mol/L) 0.0621

2 3 Avg.

0.0510 0.0510

12.536 11.996 12.236

10.0 10.0

0.0639 0.0612 0.0624

Titration of Unknown Mixture #48 of Mg2+/Ni2+ with EDTA in Acidic Buffer 1600 1400 1200

mV

1000 Acid Buffer Trial 1 pH=4.75 Acidic Buffer Trial 2 pH=4.75 Acidic Buffer Trial 3 pH=4.75

800 600 400 200 0

0

5

10

15

20

25

mL of EDTA

Figure 6. Graph of the data for all three trials of the titration of the unknown mixture #48 in Acidic conditions with a pH of 4.75.

Table 7. Data for the titration of EDTA with Unknown Mg 2+/Ni2+ mixture #48 in Basic Buffer with pH of 4.75. Trial # 1 2 3 Avg.

M of EDTA (mol/L) 0.0510 0.0510 0.0510

Veq Basic (mL) 20.30 20.32 20.48 20.37

V of Mixture (mL) 10.0 10.0 10.0

M of Mg2+(mol/L) 0.041 0.040 0.043 0.041

Table 8. Data for the %composition of the unknown mixture of Mg 2+/Ni2+ #48. Metal Mg2+ Ni2+

Percent Composition (%) 39.7 60.3

Titration of Unknown Mixture #48 of Mg2+/Ni2+ with EDTA in Basic Buffer 1400 1200 1000

mV

800

Basic Buffer Trial 1 pH= 9.25 Basic Buffer Trial 2 pH= 9.25 Basic Buffer Trial 3 pH=9.25

600 400 200 0

0

5

10

15

20

25

30

mL of EDTA

Figure 7. Graph of the data for all three trials of the titration of the unknown mixture #48 in Basic conditions with a pH of 9.25.

Discussion EDTA is capable of binding to almost any metal ion and the reason it can do this is because is a chelating effect that explains the strong chemical interactions within EDTA that form a stable complex with metal ions. In this scenario, the molarity of EDTA was found using the known concentration of a 0.145M of NaOH solution. The formula used to fine this is M 1 V 1= M 2 V 2 Where M2=EDTA and M1V1 is the Concentration of NaOH multiplied by the volume of the equivalence point in the titration. Once calculated, the average concentration of EDTA was found to be 0.0510M and this number would be used to find the percent composition and concentration of the Mg2+/Ni2+ mixtures. To find the concentration of the Ni 2+ ion, the graph is extremely important because the strong drop in the ion during the titration of the basic buffer. The sudden drop indicates a clear and distinct interaction between the EDTA and the nickel ion. Even though the drop in the Acidic buffer is not as strong, there is still a drop so a reaction is still occurring. In the titration with the Mg2+ with the acidic and basic buffers, only the basic buffer showed a slight drop whereas the acidic buffer had no drop at all. This is due to the fact that EDTA will only bind with Mg 2+ in basic solution.

A mixture of Mg2+/Ni2+ with a ratio of 2.5:7.5 which is 25% of Mg 2+ and 75% Ni 2+ was created to continue the experiment with EDTA and determining the unknown mix of metal ions later. In this scenario, the [Ni 2+] was calculated using the titration of the acidic buffer and the [Mg2+] is calculated using the data from the basic buffer. Since Mg 2+ does not bind to EDTA in acidic conditions, it is logical to assume that the only thing that EDTA will bind to in the solution will be Ni 2+ and that result will then give the [Ni 2+]. The concentrations of each ion was multiplied with the concentration of EDTA and then divided by the 10.0mL which is the mixture volume. The individual concentrations were then divided by the total concentrations and multiplied by 100 to get the percent composition of each metal ion. The results are that the percent composition of Mg2+ is 28% while Ni2+ is at 72% which is very close to the initial ratio of 25% and 75% respectively. These values prove that the mixture and technique for determining the percent composition is valid. By using the same technique that was used to determine the known percent composition, it is possible to determine an unknown composition mixture of the two metal ions. The unknown mixture #48 was mixed with an acetate and ammonium buffer and three titrations with each buffer was performed. Using similar methods as the previous experiment, the concentrations of the Nickel ion was first found through the titration of the acetate buffer and then that information was used to determine the concentration of the Mg 2+ buffer. For each concentration, the average values were used to determine the final composition which turns out to be 39.7% of Mg2+ and 60.3% of Ni2+.

Conclusion

To start off the entire experiment, EDTA needed to be standardized by using the NaOH that already had a known concentration. Once that was completed, EDTA was then used to titrate with a Mg2+ and Ni 2+ ion in acidic and basic solutions. The reason for this step is to discover the conditions of formation for both metal ions. Through data analysis, it was concluded that Mg2+ would not bind to EDTA under acidic conditions while Ni 2+ is able to bind to EDTA under both acidic and basic conditions. This is crucial information as it will allow for the calculations of the percent composition of the unknown mixture of Mg 2+/Ni2+ later. The titration of a mixture with known ratios of both the metal ions is then performed under basic and acidic conditions. The reason for this section is to verify if the method for determining the percent composition is legitimate. As the ratio is already known, the experiment was performed and then the concentrations of each metal is compared with the total concentration. The results of this experiment verified that the technique for determining the concentrations of Mg 2+ and Ni2+ was valid and can be used to determine the unknown ratio mixture of the two metal ions. Through the same methods as the known ratios of metal ion mixture, the percent composition for the unknown #48 was determined to be composed of 39.7% Mg 2+ and 60.3% Ni2+. Overall the results of the experiment performed demonstrates that each metals react with the EDTA differently under specific pH conditions and that information can be used to determine the composition of the mixture.

Reference Berman, R. a. (1991). Flow Optrodes for Chemical Analysis. roc. SPIE 1368, Chemical, Biochemical, and Environmental Fiber Sensors II.

Harris, D. C. (2010). Quantitative Chemical Analysis 8th Ed. In D. Harris. New York: W.H Freeman and Company. Norton, D. (2007). Chemistry 4000/6000 Laboratory Manual and Course Materials. Atlanta, GA: Department of Chemistry. GSU; Print....