Question

Ohta and Tanaka reported on an ion-exchange chromatographic method for the simultaneous analysis of several inorganic anions and the cations \(\mathrm{Mg}^{2+}\) and \(\mathrm{Ca}^{2+}\) in water. \({ }^{28}\) The mobile phase includes the ligand 1,2,4 -benzenetricarboxylate, which absorbs strongly at \(270 \mathrm{nm}\). Indirect detection of the analytes is possible because its absorbance decreases when complexed with an anion. (a) The procedure also calls for adding the ligand EDTA to the mobile phase. What role does the EDTA play in this analysis? (b) A standard solution of \(1.0 \mathrm{mM} \mathrm{NaHCO}_{3}, 0.20 \mathrm{mM} \mathrm{NaNO}_{2}, 0.20\) \(\mathrm{mM} \mathrm{MgSO}_{4}, 0.10 \mathrm{mM} \mathrm{CaCl}_{2},\) and \(0.10 \mathrm{mM} \mathrm{Ca}\left(\mathrm{NO}_{3}\right)_{2}\) gives the following peak areas (arbitrary units). \(\begin{array}{lcccc}\text { ion } & \mathrm{HCO}_{3}^{-} & \mathrm{Cl}^{-} & \mathrm{NO}_{2}^{-} & \mathrm{NO}_{3}^{-} \\ \text {peak area } & 373.5 & 322.5 & 264.8 & 262.7 \\\ \text { ion } & \mathrm{Ca}^{2+} & \mathrm{Mg}^{2+} & \mathrm{SO}_{4}^{2-} & \\\ \text { peak area } & 458.9 & 352.0 & 341.3 & \end{array}\) Analysis of a river water sample (pH of 7.49 ) gives the following results. \(\begin{array}{lcccc}\text { ion } & \mathrm{HCO}_{3}^{-} & \mathrm{Cl}^{-} & \mathrm{NO}_{2}^{-} & \mathrm{NO}_{3}^{-} \\ \text {peak area } & 310.0 & 403.1 & 3.97 & 157.6 \\ \text { ion } & \mathrm{Ca}^{2+} & \mathrm{Mg}^{2+} & \mathrm{SO}_{4}^{2-} & \\ \text { peak area } & 734.3 & 193.6 & 324.3 & \end{array}\) Determine the concentration of each ion in the sample. (c) The detection of \(\mathrm{HCO}_{3}^{-}\) actually gives the total concentration of carbonate in solution \(\left(\left[\mathrm{CO}_{3}^{2-}\right]+\left[\mathrm{HCO}_{3}^{-}\right]+\left[\mathrm{H}_{2} \mathrm{CO}_{3}\right]\right) .\) Given that the \(\mathrm{pH}\) of the water is \(7.49,\) what is the actual concentration of \(\mathrm{HCO}_{3}^{-}\) ? (d) An independent analysis gives the following additional concentrations for ions in the sample: \(\left[\mathrm{Na}^{+}\right]=0.60 \mathrm{mM} ;\left[\mathrm{NH}_{4}^{+}\right]=0.014\) \(\mathrm{mM}\); and \(\left[\mathrm{K}^{+}\right]=0.046 \mathrm{mM}\). A solution's ion balance is defined as the ratio of the total cation charge to the total anion charge. Determine the charge balance for this sample of water and comment on whether the result is reasonable.

Short answer

EDTA prevents metal ion interference; calculate each ion's sample concentration using peak areas and standard values; use pH to find true \( \text{HCO}_3^- \) concentration; ion balance confirms sample consistency.

Step-by-step solution

Understanding the role of EDTA

EDTA is a chelating agent that binds strongly to metal ions such as \( \text{Mg}^{2+} \) and \( \text{Ca}^{2+} \). In the context of ion-exchange chromatography, EDTA is added to the mobile phase to prevent these metal ions from interacting with the stationary phase, which can interfere with the detection of the anions. By forming stable complexes with these metal ions, EDTA ensures more reliable anion detection and prevents peak broadening.

Calculating concentration of ions in standard solution

Use the relationship between peak area and concentration. The formula for ion concentration based on peak areas compared to the standard is:\[C_{\text{sample}} = \frac{\text{Peak Area}_{\text{sample}}}{\text{Peak Area}_{\text{standard}}} \times C_{\text{standard}}\]Apply this formula for each ion to find their concentrations in the river water sample. For example, for \( \text{HCO}_3^- \):\[C_{\text{HCO}_3^-} = \frac{310.0}{373.5} \times 1.0 \, \text{mM}\]

Calculating carbonate species concentration

To find the concentration of \( \text{HCO}_3^- \), use the pH value and the equilibrium expressions for carbonate species. At pH 7.49, most total carbonate is as \( \text{HCO}_3^- \), but a small fraction will be \( \text{CO}_3^{2-} \) and \( \text{H}_2\text{CO}_3 \). Use the Henderson-Hasselbalch equation to adjust the concentration:\[pK_a1 = 6.35, \, pK_a2 = 10.33, \, ext{use ratios to find distribution}\]

Charge balance calculation

First, calculate the charge contributed by each ion in the sample using their concentration and charge magnitude. Then, sum the total positive and negative charges. The ion balance is:\[\text{Ion Balance} = \frac{\sum \text{Cations} \times \text{Charge}}{\sum \text{Anions} \times \text{Charge}}\]Substitute known concentrations for ions like \( \text{Na}^+ \), \( \text{NH}_4^+ \), \( \text{K}^+ \), \( \text{HCO}_3^- \), \( \text{Cl}^- \), \( \text{NO}_2^- \), etc.

Key concepts

Ion detection

In ion-exchange chromatography, ion detection is a critical step that allows for the identification and quantification of ions in a solution. The method relies on the indirect detection of analytes by measuring changes in the absorbance of a substance within the mobile phase. Typically, the ligand used in the process, such as 1,2,4-benzenetricarboxylate, absorbs light at a specific wavelength—in this case, at 270 nm. When anions in the sample interact with the ligand, this interaction alters the absorbance, leading to indirect detection. The resultant absorbance drop correlates with the presence and concentration of the specific anions being analyzed. By comparing these absorbance changes to those from known standards, the concentrations of the ions in the sample can be determined.

EDTA role in chromatography

The use of EDTA in ion-exchange chromatography is crucial due to its chelating properties. EDTA is a well-known agent that strongly binds to various metal ions, including calcium (\(\text{Ca}^{2+}\)) and magnesium (\(\text{Mg}^{2+}\)). During the chromatographic process, these metal ions could otherwise interact with the stationary phase, potentially interfering with the detection of anions and causing broader peaks and less defined chromatographic results. By adding EDTA to the mobile phase, metal ions are bound tightly, preventing such interactions. This not only enhances the reliability of anion detection by stabilizing the system but also helps in maintaining sharp and accurate chromatographic peaks, facilitating better identification and quantification of analytes.

Charge balance calculation

Calculating the charge balance in a solution involves evaluating the total charges contributed by both cations and anions. In a balanced solution, the total positive charge from cations should equal the total negative charge from anions. Calculating this involves:

• Summing up the products of the concentration and charge for each cation and anion.
• Checking if the ratios of these sums approach an equal value, meaning a balanced charge distribution.

To calculate the ion balance as given in the exercise, you simply use the formula:\[\text{Ion Balance} = \frac{\sum \text{Cation Concentrations} \times \text{Charges}}{\sum \text{Anion Concentrations} \times \text{Charges}}\]This calculation helps in verifying the accuracy of your analysis since any significant deviation from unity might indicate errors in concentration determination or contamination in the sample.

Concentration determination

Determining ion concentration in a sample using ion-exchange chromatography involves comparing the peak areas obtained from the sample to those from a known standard. This is done using a simple ratio formula:\[C_{\text{sample}} = \frac{\text{Peak Area}_{\text{sample}}}{\text{Peak Area}_{\text{standard}}} \times C_{\text{standard}}\]This equation forms the basis of quantifying ions, as it relies on peak area values that correspond to the concentration of ions. It's important to analyze each ion separately. By applying this formula across all ions of interest, such as bicarbonate (\(\text{HCO}_3^-\)), chloride (\(\text{Cl}^-\)), and others in the sample, precise concentrations can be calculated, allowing for a comprehensive understanding of the sample's ion composition.