Title Quantitative Analysis of a Solution Containing Cobalt and Copper
Name Manraj Gill (Lab partner: Tanner Adams)
Abstract In this series of experiments, we determine the concentrations of two metal species, copper and cobalt, in a mixture of the two by initially using an ion exchange column to separate them from the mixture. We consequently use two different techniques (complexometric titration and atomic emission spectroscopy) that allow us to quantify the amount of each metal present individually. This step-by-step analysis not only goes through the basis for the experimental approach but also is a breakdown of the measurements that leads us to the final results: the Co:Cu ratio in the unknown mixture.
Purpose In this series of three experiments (Parts I – III), we aim to initially separate a solution containing a mixture of Cobalt, Co (II), and Copper, Cu (II), ions in an unknown ratio and in unknown concentrations. The end goal of the entire experiment is to determine how much Co and Cu were originally present in the sample we obtained in Part I. But to do so, as explained in further detail in the ‘Theory and Methods’ section below, we initially separate the two ions (Part I) based on their unique chemical properties and then we attempt to quantify the separated ions through two different methods (Parts II, III). The overall purpose is an analysis of the efficiency of the separation and quantification approaches used to measure the constituent compounds in a mixture. This “efficiency” can be determined by comparing the obtained measurements to the known amounts that were used to make the solution.
Theory and Methods (Part I) In order to initially separate the two ions from each other and obtain a purified amount of each of Cu and Co, we pass the unknown solution through an Ion Exchange Chromatograph. The principle of Ion Exchange Chromatography is based on utilizing an ion exchange resin. In this case, we use an anion exchange resin, namely, AG 1-X8. AG (analytical grade) 1-X8 is a strongly basic anion exchange resin with quaternary ammonium function groups attached to the styrene divinylbenzene copolymer lattice. (1) 1-X8 specifically has a high amount of resin cross-linkage making it suitable for exchange with and separation of low molecular weight inorganic anions! (1)
The basis for using an anion exchange is to exploit the different number of chlorine ions (Cl-) that can be bound to the two different metal atoms. And then, the consequently different affinities to the Cl- bound metals to the resin serve to provide enough of a separation between the two if the resin vessel is tall enough. The strength with which an ion is held in the resin increases with increasing charge on the ion. Therefore, the more Cl- bound, the longer it stays in the resin.
Now, to understand the theory of different amounts of chlorine ions being bound to the two metals, we look at the stability constants for metal-chloro complexes and find that, at specific concentrations of chrloide ion, there are different fractional distribution curves for the copper-chloride complexes and cobalt-chloride complexes. Therefore, we use the specific chloride concentration that yields in the metals being bound to significantly different number of chloride ions. This is characterized by looking at the stepwise stability constants. That is, the stability of the complex upon addition of another chloride. The equation can be written as follows:
n+ - m+ M + bCl ⇌ MClb
In the equation, the b value is the number of Cl- ions and the M is representative of either Cu or Co. The corresponding equilibrium constants as b varies from 0 (no chloride ions bound) to 4 (4 chloride ions bound) allows us to extrapolate the likelihood of the complex being present at specific concentrations of chloride.
The graphs on the left plot out the distribution of a complex (based on the b values) at a range of chloride concentrations. And as stated previously, the property we want to exploit is the different number of chlorides that can be bound to the two metals at the same concentration of chloride ions. As marked on the plots, at pCl- = 0.5 (yellow line), [Cl-] = 3.0M, the copper complexes are present in highly chloride bound states (b = 2, 3, 4) whereas the cobalt complexes are less likely to be bound to chloride ions (b = 0, 1). Therefore, at this concentration of chloride ions, the copper will be retained more strongly in the resin while the cobalt will “run through” faster.
Application of Theory (Part I) Based on the approach described above, we were able to use 3M HCl in the resin and separate the mixture of Co (II) and Cu (II) in the unknown sample. We had Unknown #2 and from 3 milliliters of the unknown solution, collected 3 samples each of copper and cobalt.
Theory and Methods (Part II) In this second part of the series of experiments conducted, we focus only on the cobalt solution that was purified from Part I. The focus is on quantifying the amount of cobalt that was initially present in Unknown #2. To do this, we titrate the 3 separated cobalt solutions to determine with some level of statistical certainty, the amount of cobalt that was initially present.
Acid-base titrations are done by neutralizing an acid with a base (or vice-versa). Potentiometric titrations are performed by measuring changes in voltage as compounds precipitate out. However, in the case of titrating out metals such as cobalt, we perform a complexometric titration. In a complexometric titration, the metal ions react with chelating agents to create a complex and an evident color change can be used to determine the end-point of a complexometric titration. (2) In the determination of metal ions, polydentate ligands are used. A polydentate ligand has more than 2 donor atoms that are used to bind to a central metal ion or atom. (3) The process of polydentate ligands binding to the metal ion and forming a complex (chelate) is called chelation. (3) And in such a case, the polydentate ligand is called a chelating agent. In this experiment, we use a chelating agent called ethylenediaminetetraaceic acid (EDTA). It is a polydentate ligand because it can form up to six bonds with the metal ion upon chelation.
However, a “regular” titration whereby the metal solution is titrated with the solution of a known concentration of EDTA is difficult to perform. This is because a “regular” or direct titration requires fast course of reaction. That is, the complex formation must be fast enough. (2)
Therefore, we perform a “back” titration to measure the amount of cobalt present. In this case, we add a known amount of EDTA of a known concentration to our cobalt solution. The added moles of EDTA are in excess of the predicted moles of metal in the solution. And the solution is mixed very well to ensure that complete chelation is allowed for. We then “back-titrate” by gradually adding cobalt solution of a known concentration until the indicator changes color. At the end-point/equivalence point (when the color changes), the number of moles of EDTA would be equal to the total number of moles of cobalt present. Therefore, the initial number of moles of cobalt is the difference!
The chemical basis of the indicator used in this reaction is as follows. The compound, thiocyanate ion, is added that reacts with a free cobalt species and turns blue. Therefore, as long as any cobalt solution (from the standard stock) is still chelating and not free to react with thiocyanate, the solution will not turn blue. Upon turning blue, we will know that the number of moles of cobalt has just gone to a level where it is more than the number of moles of EDTA present. The lighter the hue of the blue, the lower the solution is post the equivalence point. Therefore, we attempt to identify the lightest hue of blue so that we are as close to the equivalence point as possible.
Application of Theory (Part II) The concentration of EDTA used is 0.03063M and the concentration of the standard solution of cobalt (used for the titration) is 9.26163mg/mL. However, the stock solution is accurately diluted 10x to obtain a final concentration of ~0.9mg/mL.
Since this dilution was performed and involves some degree of error associated with the volumetric transfer and the use of volumetric flasks, an initial standardization is performed which allows us to accurately know this concentration of the 10x diluted cobalt standard solution! The results from the standardization can be analyzed as follows:
10.00mL EDTA x 0.03063moles/L EDTA x 1L/1000mL = 3.063 x 10-4 moles EDTA
Since 10.00ml of the diluted Co2+ standard was initially added. And since 7.75ml of this was actually added to reach equivalence, to calculate the concentration of the cobalt standard, we perform the following calculations:
(3.063 x 10-4 moles Co2+) / (17.75mL Co2+) = (1.7256 x 10-5 moles/mL Co2+) x (58.93g/mol Co2+) = (1.0169 x 10-3 g/mL Co2+) x 1000mg/g = 1.016916mg/mL
This varies from the 0.926163mg/mL expected. A ~10% difference. But this difference largely comes from the major error associated with not having a quantitative measurement of the equivalence point, but a visual one. Rather than relying on potentiometric voltage changes or pH changes as in the previous titration experiments, we use a visual end-point indicator. That leads to us basing the equivalence point on a hue of blue that might actually be very far past the actual equivalence point.
However, this argument above is the motive behind the standardization. We now base the actual experimentation on the unknown cobalt isolated and visually compare the bluish hue to the one observed in the “test” run. Thereby “normalizing” the obtained values. A sample calculation is now shown that details how we calculate the concentration in the unknown:
10.50mL Co2+ was added to reach equivalence point (or a shade of hue similar to the one obtained in the standardization)…
(10.50mL Co2+) x (1.01691mg/mL Co2+) = (10.6776mg Co2+) x (1mol/58.93g Co2+) x (1g/1000mg) = 1.8119 x 10-4 moles Co2+ added from the standard Co2+
Therefore, the moles of Co2+ in the unknown…
3.063 x 10-4 moles (moles EDTA) - 1.8119 x 10-4 moles (moles Co2+) = 1.2511 x 10-4 moles Co2+ in 1 ml of Unknown #2
This gives us,
1.2511 x 10-4 moles Co2+ x 58.93g/mol Co2+ = 7.3727 x 10-3 g/mL Co2+ = 7.37 mg/mL Co2+ in Unknown #2
Performing similar calculations on the other two titrations, gives us the following result:
Titration Concentration of Co2+ in Unknown #2 (mg/mL) #1 7.37 #2 7.08 #3 7.08
The later 2 titrations reflected the indicator change at the same volume of Co2+ standard addition, i.e. of 10.80mL.
An analysis of this data from the three titrations gives us the concentration of Co2+ in Unknown #2 with its 95% confidence interval = 7.17 +/- 0.29 mg/mL
Theory and Methods (Part III) In this third part of the experiment, we focus on the other metal separated from the unknown solution: Copper. We use atomic emission spectroscopy to determine the amount of copper that was initially present in Unknown #2!
The initial plan was to use a Microwave Plasma (MP) based atomic emission spectrometer (MP-AES) but the samples of copper were analyzed using Inductively Coupled Plasma (ICP) based atomic emission spectrometer (ICP-AES). In regards to spectroscopy, there are multiple methods that can be employed: atomic emission, absorption and fluorescence. The main goal with atomic spectroscopy is the determination of elements present in a sample by analyzing its electromagnetic spectrum. (4) The physical basis behind atomic spectroscopy is the excitation and decay to the ground state of electrons in the atoms. Due to the discrete energy levels that are characteristic of specific atoms, we can determine the atoms present by analyzing the wavelengths detected. (4)
In absorption atomic spectroscopy, the atoms absorb specific wavelengths and we can determine the atoms present by looking for wavelengths not detected. However, this can be hard to quantitatively analyze due to the background levels and the lack of sensitivity. In the case of emission atomic spectroscopy, we excite the atoms (after the liquid sample is atomized) and detect its emission spectrum. The amount of light of a particular wavelength detected allows us to more accurately determine the number of atoms present of a specific species! For these reasons, emission based atomic spectrometers are preferred.
The primary difference between MP and ICP is the means by which the flame is produced. MP employs microwave energy to produce a plasma discharge whereas ICP relies on energy supplied by electromagnetic induction. Therefore, we do not expect many differences in the emission spectrum observed between the two methods as long as the flames generated from both are of an energy high enough to excite the electron transition we are hoping to detect.
This brings to the topic of detection… As described previously, we look for characteristic wavelengths. By doing this, we can distinguish between the two metals that are present in our solution. The specific wavelengths chosen are:
Atom Wavelength (nm) Copper 324.752 Cobalt 238.892
By looking specifically at these wavelengths and measuring the intensity of light observed at these wavelengths, we can quantify the amount of each atom present in the sample. However, to accurately correlate an intensity measurement to concentration value, we need to calibrate the instrument with standard solutions of known concentrations. The concentration of an atom in a sample can then be extrapolated from the intensity measured at that atom’s characteristic wavelength!
Application of Theory (Part III) With a series of samples prepared by the lab staff of concentrations ranging from 1.0 to 5.0 ppm (1ppm = 1mg/L = 1µg/mL) of both copper and cobalt, calibration curves can be created with the intensity measurements at the wavelengths mentioned above:
Copper
8000000 7000000 6000000 5000000 4000000 3000000 2000000 1000000 0 Intensity at 324.752nm 0 1 2 3 4 5 6 [Cu] (ppm)
Cobalt
2500000
2000000
1500000
1000000
500000
0
Intensity at 238.892nm 0 1 2 3 4 5 6 -500000 [Co] (ppm)
Now, to determine the concentrations of copper and cobalt present in the 3 individual separations performed on Sample #2, we obtain the following intensities:
Separation Sample Number Intensity of Copper Intensity of Cobalt 1 2570921.9 -316.5 2 2457675.6 -290.2 3 2587401.5 121.8
Now, to correlate these intensity measurements to concentrations… we rely on the y- intercept (the Intensity measurement using the 0ppm control) and the slope between the intensity measurements of interest. That is, between 1ppm and 2ppm on the standard plot. For copper,
m = (2821097.6 – 1418989.5) / (2 - 1) m = 1402108.1
y = (1402108.1)x + 62320.7 x = (y - 62320.7) / (1402108.1)
[Cu] (ppm) = (Intensity324.752nm + 62320.7) / (1402108.1)
From this relationship it follows,
Sample Number [Cu] (µg/mL) 1 1.789 2 1.708 3 1.801
Which gives us the concentration of Cu2+ in sample with its 95% confidence interval = 1.766 +/- 0.085 µg/mL.
However, due to the incredibly high sensitivity of the ICP-AES, only a very dilute sample is inspected to avoid the inaccuracies from overloading the instrument. One milliliter of Unknown #2 was placed on the ion exchange column for each separation. The separated copper obtained from that separation was diluted 1:100 and again 1:100 before being analyzed. That results in a 1:10,000 overall dilution. This results in the concentration of copper in Unknown #2 = 17.66 +/- 0.85 mg/mL Cu2+
If we perform the same analysis on the Cobalt present… we can initially observe that the values of cobalt are incredibly low. That is, the intensity of light at the wavelength corresponding to a characteristic cobalt atom emission is very low in all 3 separations. However, there is variation between the separations… That is, in the third separation, very slightly more cobalt is detected than in the 1st separation. But that is because we are comparing very minute measurement changes and it most likely does not amount to a considerable variation. The calculation can be performed and can be summed up as the concentration of residual cobalt in a “purified” copper separation of Unknown #2 = 4.22 +/- 9.67 µg/mL Co2+ (calculations not shown, but performed similarly to the calculation above) This is an incredibly small amount, essentially negligible (note the units of µg/mL), and thereby affirms the effectiveness of Part I’s separation of the two metals!
Discussion In regards to a retrospective regard at each part of the experiment, it is immediately evident that the use of the ion-exchange chromatography was incredibly efficient. This was demonstrated by the detection of negligible amounts of cobalt in the purified copper solution. This can be attributed largely not to the use of the AG 1-X8 resin but more so the use of 3M HCl that created a bias for the copper rather than the cobalt in the resin.
In regards to a larger commentary on the EDTA complexometric titration employed in Part II for the analysis of the cobalt metal, the use of a visual equivalence point indicator makes it significantly more difficult to know precisely when equivalence has reached. And despite the use of the standardization techniques in the analysis (as demonstrated in detail in the Application of Theory for Part II), there is inherent inaccuracy in the technique. However, this complexometric titration is the most robust means of detecting metal ions, and the consistency of the results and high agreement (tighter 95% confidence interval) proves the reliability of this technique.
Lastly, the precision in atomic emission spectroscopy has already been discussed in detail and it allowed us to confirm the efficiency of separation. Perhaps this confirmation could have also come from a similar complexometric titration of copper in the purified cobalt. But given the efficiency of separation, such a titration would not be very telling due to the difficulty in distinguishing a potentially small amount of copper residue using an indicator based titration. Additionally, there would be inaccuracies in the final concentration of copper in Unknown #2 because the actual sample that is analyzed using ICP-AES is a 1:10,000 dilution and scaling up such a magnitude brings about associated error.
Conclusion In conclusion, we can say with a high degree of confidence that in Unknown #2, the concentration of Co2+ is 7.17 +/- 0.29 mg/mL and that of Cu2+ is 17.66 +/- 0.85 mg/mL. Giving us a roughly 1 : 2.5 ratio of Co : Cu in Unknown #2! This three step analysis of an unknown solution of multiple metals proved to be a very accurate means of quantitative analysis! Instruments such as atomic emission spectrometers definitely do present an advantage over techniques like complexometric titrations due to the reasons discussed in detail above. And while the technological implementation of such instruments brings about greater accuracy, a proper use of complexometric titration is more rewarding from the perspective of understanding the dynamics at a molecular level! It is this chemical understanding that initially allowed us to exploit the difference between the two metals that made it easy to isolate them from their mixture!
References
1. "Analytical Grade Anion Exchange Resin." Bio-Rad Laboratories, Inc. Life Science Research. Web. < http://www.bio-rad.com/en-us/product/analytical- grade-anion-exchange-resin> 2. "Complexometric Titration." Sigma-Aldrich- Analytical / Chromatography. Web.