ION CHROMATOGRAPHY
TP Analytical Chemistry II 7 th, December 2012 Table of content: 1 Abstract... 3 2 Introduction... 3 3 Methodology... 3 4 Results... 3 4.1 Determination of the conductivity of the eluent... 3 4.2 Identification of peaks... 4 4.3 Determination of the detection sensitivity... 4 4.4 Analysis of anions in mineral water... 5 4.4.1 Analysis of Evian... 6 4.4.2 Analysis of Aproz... 6 5 Discussion... 6 6 Conclusion... 7 7 References... 7 8 Appendices... 7 8.1 Notebook s scan... 7 2
TP Analytical Chemistry II 7 th, December 2012 1 Abstract The times of retention qualifying each anion were determined with known solutions. Then, the obtained calibration curves for chloride and nitrate were used to analyse the content in anions on two samples of water. The obtained results were: Evian Ion Cl - NO 3 - w [mg/l] 7.52 5.12 0.38 0.09 w [mg/l] 16.36 2.16 Aproz 0.82 0.04 Table 1: Sum up of the results 2 Introduction The analytical method used in this experiment is the ion chromatography. It is based on the different interaction of analysed ions with the mobile and stationary phases. This interaction implies a different retention time for each ion, which is specific to the latter. Thus, knowing retention times for several ions allows us to investigate the analysed ions concentrations in unknown solutions. In this experiment, an anion exchange column is going to be used to investigate the presence of different anions. The suppressor column allows the identification of very small amounts of ions, as it suppresses the conductivity of the eluent. 3 Methodology The methodology given by the protocol [1] was followed. No modifications were made. For the preparation of the solutions, calculations based on the following formula were carried out: With C 1 and C 2 respectively the concentration in anion X -, therefore the concentration of the stock solution in [mol/l] (C X-=C NaX), and the desired concentration in anion after dilution in [mol/l]. "# With C 1 and C 2 respectively the concentration of the stock solution in [ppm] and the desired concentration after dilution in [ppm]. Therefore the volume to take from the stock solution is: "# 4 Results 4.1 Determination of the conductivity of the eluent The mobile phase is a 4.2mM carbonate buffer (ph=10.8) obtained by mixing 1.0 mm solution of NaHCO 3 and 3.2 mm solution of Na 2CO 3. The conductivity is calculated by the following formula: 3
TP Analytical Chemistry II 7th, December 2012 The concentrations of the ions are calculated knowing the constants of carbonate dissociation equilibriums. When mixing NaHCO3 and Na2CO3, the suppressor column exchanges sodium cations for the protons. "#$ " Therefore we are in the presence of the following equilibrium (pka=6.35): " " " From mass balance we know that " and from charge balance, that ". Therefore, when defining x=h+, we deduce: " " "# " "# " " """ """ "" " " " "" " 4.2 Identification of peaks Ion F- Cl - NO 2 - Br - NO 3 - PO 4 3- SO 4 2- Mean tr [min] 3.43 4.93 5.815 7.42 8.17 C in A [mg/l] 0.5 1 1.5 3 Peak height A [µs/cm] 0.74 1.29 0.83 0.38 C in B [mg/l] 1 0.5 1 2 9.9 11.555 1 8 2 0.4 0.45 0.62 2 Peak height B [µs/cm] 1.51 1.68 0.52 0.16 0.74 12 1 0.7 0.26 Table 2: Identification of peaks in solutions A and B 4.3 Determination of the detection sensitivity The calibration curves made with a series of solutions containing different concentrations of Cl- and NO3- are used to determine the value of detection sensitivity. Theoretical detection sensitivity can be calculated as: With Vloop= 20 µl and Φ= 0.6671 ml/min ""# 4
TP Analytical Chemistry II 7 th, December 2012 And the experimental detection sensitivity can be calculated knowing the concentration and peaks area: " Therefore, the slope of a graph plotting the peak area against weight concentration of ions will correspond to the detection sensitivity. "#$%&'()*+,-.% /-- %-- 0-- 2#3456 "#$%&'()* +,#-&('./' - -&- )&- $-&- $)&- 0-&- 0)&- 1-&- /%&-0+1.% Figure 1: Calibration curve for chloride "#$%&'()*+,-.% $0- $--.- /- %- 0-2#3456 "#)&.%-%* +,#-&(('-1 - -&- )&- $-&- $)&- 0-&- /%&-0+1.% Figure 2: Calibration curve for nitrate a th. [S.m 2.s/g] a exp. [S.m 2.s/g] Cl - 2.16E-03 1.48E-03 NO 3-1.07E-03 5.84E-04 Table 3: The detection sensitivity of chloride and nitrate 4.4 Analysis of anions in mineral water The anions content in two mineral water samples was determined, first, qualitatively. Then, a quantitative analysis of chloride and nitrate content was completed. 5
TP Analytical Chemistry II 7 th, December 2012 4.4.1 Analysis of Evian Ion Cl - NO 3 - SO 4 2- mean t R [min] 4.93 8.32 11.61 mean peak area (A) 111.324 29.890 w [mg/l] 7.52 5.12 0.38 0.09 Table 4: Ion content in Evian 4.4.2 Analysis of Aproz Ion Cl - - NO 3 2- SO 4 mean t R [min] 4.92 8.32 11.74 mean peak area 242.039 12.628 w [mg/l] 16.36 2.16 0.82 0.04 Table 5: Ion content in Aproz 5 Discussion The number plates is: " and the equivalent plate height is: Knowing L=15 cm and estimating the w 1/2, the results are based on the A solution: Ion F - Cl - - NO 2 Br - - 3-2- NO 3 PO 4 SO 4 mean t R [min] 3.43 4.93 5.815 7.42 8.17 9.9 11.555 w 1/2 0.2 0.15 0.2 0.6 0.5 0.4 0.45 N 1631 5990 4687 848 1480 3397 3656 H [µm] 92 25 32 177 101 44 41 Table 6: Estimation of column parameters The presence of a suppressor column doesn t influence the number of plates. Decreasing the flow rate or using a longer loop could increase the sensitivity of this technique, but it would increase the time of manipulation. To analyse the cations, one should change the separator column so that the stationary phase would have strong affinity with cations instead of anions. 6
TP Analytical Chemistry II 7 th, December 2012 Finally, we can observe a negative peak at the beginning of each measurement. These peaks correspond to the dead time. 6 Conclusion The errors on the results and the results themselves are acceptable therefore this experiment functioned well. It was easy to determinate the presence of different anions because each anion has a specific retention time, and to make a quantitative analysis of concentration of chloride and nitrate ions in water samples. Unfortunately not all ions can be analysed by this method. In fact CO 3 2- cannot be analysed by suppressor chromatography. It could be analysed instead by titration. 7 References [1] M. Borkovec, TP_IonChromatography, Travaux pratiques de chimie analytique / Chimistes et Biochimistes 3ème année, 2012 [2] M. Borkovec, Error Analysis Introduction, Travaux pratiques de chimie analytique / Chimistes et Biochimistes 3ème année, 2012 8 Appendices 8.1 Notebook s scan 7