In the Classroom edited by
Topics in Chemical Instrumentation
David Treichel Nebraska Wesleyan University Lincoln, NE 68504
Flow Injection as a Tool for Teaching Gravimetric Analysis Raquel P. Sartini, Elias A. G. Zagatto,* and Cláudio C. Oliveira Centro de Energia Nuclear na Agricultura, Universidade de São Paulo, Box 96, Piracicaba SP 13400-970, Brazil; *
[email protected] Undergraduate classes on classical gravimetric determinations are usually tedious, and students are little motivated to learn gravimetry in view of the awkward steps of filtration, drying, and weighing (1) and the long time required for analysis. In addition, the technique of gravimetry is less exploited for routine analysis in industrial, agricultural, and biomedical laboratories than instrumental techniques that, although faster, may yield biased results. Considering the intrinsic ability of flow-injection systems to manage solutions (2), their ease of implementation (3), the inherent high sample throughput, and the possibility of obtaining the analytical signal without the need to reach chemical equilibrium, the development of flow-injection systems with gravimetric detection becomes a very attractive alternative. The feasibility of gravimetry in flow-injection analysis was originally demonstrated in a model system for determination of barium after its precipitation as oxalate (4). The system involved a partial and reproducible step of precipitate drying. Precipitation, filtration, drying, and weighing were automated, but the resulting instrumental setup was complex. Considering that high-density precipitates can be weighed inside a flowing stream, a simpler system exploiting Archimedes’ principle was recently proposed (5). The strategy permitted the development of a flow-injection procedure for the determination of total reducing sugars (6 ) in syrups and molasses. A drying step was not required and a degassing unit was placed in the analytical path because the Fehling reaction carried out at about 95 °C led to formation of air bubbles. The resulting setup was then somewhat complex. In view of their complexity, neither of the aforementioned systems (4, 5) is suitable for teaching purposes. Precipitation of phosphate with ammonium and magnesium ions under slightly alkaline conditions has often been employed for teaching purposes. An ignition step to convert phosphate to pyrophosphate is required to avoid problems related to uncertainties in the water of crystallization in the precipitate (1). By exploiting this reaction, a simple flowinjection system with pedagogical value can be designed, since the precipitation takes place at room temperature, the density of magnesium ammonium phosphate is suitable, and the crystals formed are easily solubilized. The aim of the present work was to develop a flowinjection system for demonstrating a gravimetric procedure in an instrumental fashion. Phosphate determination in fertilizer extracts was selected because of its practical relevance. Students are thus presented with a modern analytical approach for chemical analysis, namely flow-injection gravimetry.
Experimental Procedure
The Flow-Injection System The system comprised an Ismatec IPC-12 peristaltic pump with Tygon and Acid-flex pumping tubes, a manually operated injection valve (7), and a Mettler Toledo AB 204 analytical balance with the filtration unit hanging under its plate. Balance The filtration unit (Fig. 1) was similar to that already described 5 (5) and included a minifilter 1 made from sintered porous glass. Transmission lines, coiled reactors, and sampling loops (100 cm, about 500 µL) were made 2 from 0.8-mm i.d. polyethylene tubing and the coil B2 was made 3 from 1.0-mm i.d. PTFE tubing with a winding diameter of 1.5 cm. The manifold was designed in accordance with the manual pro4 cedure (1). Because the alkalinity resulting from a sudden ammonia addition would lead to precipita- Figure 1. The filtration unit. tion of the magnesium hydroxide, 1: inlet stream; 2: glass, the reaction was initiated in a less 0.5-mm wall thickness; 3: alkaline medium. Reagent R1 was sintered glass with 0.106 then prepared to contain ammo- µm porosity; 4: outlet stream; 5: steel wire connium chloride, hydrochloric acid, nected to the lower side of and magnesium chloride. A con- the balance plate. tinuous increase in alkalinity was attained later by the confluence of an ammonia solution. This was required because the increase in pH improved the formation of precipitate. The flow-injection system is diagramed in Figure 2. In the sampling position (Fig. 2a), sample and reagent R1 are aspirated to fill sampling loops a and c, which define the injected volumes. By switching the valve sliding bar to the alternative position (Fig. 2b), the selected volumes are simultaneously intercalated into two merging carrier streams C. At confluence x, the established sample and reagent zones merge and mixing is improved inside coil B1. Then reagent R2 is added (point y) and phosphate is precipitated inside the coil B2. The precipitate (magnesium ammonium phosphate) is accumulated into the minifilter and the mass variations are continuously monitored. When a stable measurement is observed, the actual mass value is recorded. The injection
JChemEd.chem.wisc.edu • Vol. 77 No. 6 June 2000 • Journal of Chemical Education
735
In the Classroom
valve is then switched back to situation in Figure 2a and the acidic solution inside loops b is injected to solubilize precipitate towards waste (W). When baseline is restored, the next sample is injected.
Solutions Reagent R1 was prepared by dissolving 20.33 g of MgCl2⭈6H2O and 26.75 g of NH4Cl in about 200 mL of a 0.01 mol L᎑1 HCl solution and adjusting the volume to 250 mL with the acidic solution. R2 was a freshly prepared 5.0% (w/v) NH 3 solution. Distilled water was used as carrier streams and a 2.5 mol L᎑1 HCl solution (reagent R3) was used to dissolve the precipitate. The standard stock solution, 10.0% w/v P, contained 219.68 g of KH2PO4 in 1000 mL of water. The solution was stable for a week when kept cool. Standard solutions (0.10– 1.00% w/v P) were prepared daily in 2.5% w/v (NH4)2C6H6O7 plus 1.25% w/v Na2H2EDTA. The procedure for sample preparation was similar to that recommended by AOAC (8). The extracting solution was prepared by dissolving 50.0 g of (NH4) 2C6H6O7, and 25.0 g of Na2H2EDTA in 1.5 L of distilled water, adjusting the pH to 7.0 with aqueous (1+1) NH3, and adjusting the volume to 2.0 L with water. About one gram of ground and sieved (sieve #20) fertilizer was accurately weighed and transferred to 125-mL polyethylene flasks containing 50 mL of the extracting solution. The flasks were closed and vigorously agitated in a shaker for 60 min, let stand for residue settlement, and decanted. Alternatively, the samples could be filtered through a Whatman #1 filter paper to speed up the samplepreparation step.
IC
A a
W C
S
b R3 c
V C
x
B1
y
B2
AB
R1
b V
R2
W
IC
B W C
S a R3
b V C
x
B1
y
B2
AB
R1 c V
b
R2
W
Figure 2. Flow diagram of the system in the (A) sampling and (B) alternative positions. IC: injection valve, with the alternative position specified by the dashed area; S: sample, aspirated at 3.9 mL min᎑1; R1: 0.4 mol L᎑1 MgCl2⭈6H2O, 2.0 mol L᎑1 NH4Cl, 0.01 mol L᎑1 HCl at 0.4 mL min᎑1; R2: 5.0% w/v NH3 at 0.4 mL min᎑1; C: water carrier streams at 2.5 mL min᎑1; R3: 2.5 mol L᎑1 HCl aspirated at 3.9 mL min᎑1; a, b, c: 100-, 200-, 100-cm sampling loops; B1: 100-cm coil; B2: 200-cm coil; AB: analytical balance; x, y: confluences; V: recuperation vessel; W: waste.
Results and Discussion
736
20
d c 15
mg
System Dimensioning The manifold was dimensioned with a symmetrical configuration because sensitivity was not a limiting factor. Total flow rate was chosen as a compromise between sampling rate, washing efficiency, mixing conditions, and hydrodynamic pressure (2). Injected volumes were 500 µ L. Coil B1 was 100 cm long and it improved the overlap between sample and R1 zones. Varying the length of B2 within the range of 100–400 cm did not markedly affect the analytical signals; it was set as 200 cm. Reagent concentrations were not critical. The magnesium concentration, however, should be ≥ 0.4 mol L᎑1 in reagent R1 so that calibration curves are linear. For lower concentrations, relative supersaturation (1) was not suitable and linearity deteriorated (Fig. 3). Similarly, concentration of reagent R2 was not critical, and variation between 3.0 and 7.0% w/v NH3 did not alter the results. Surfactant addition was not required, as precipitate did not adhere to the tube inner walls. Concentration of reagent R3 was varied between 1.0 and 4.0 mol L᎑1 HCl and any value ≥ 2.5 mol L᎑1 was suitable. The selected reagent concentrations are given in Figure 2. Influence of temperature was investigated by immersing the reactor B2 in a thermostatic water bath and varying the temperature between 10 and 40 °C. When it decreased from 25 to 10 °C, the analytical signal corresponding to 0.4% w/v
b 10
a 5
0
0.0
0 .3
0 .6
0 .9
%P Figure 3. Influence of MgCl2 concentration. Curves a, b, c, d refer to 0.1, 0.2, 0.3, 0.4 mol L᎑1 MgCl2 in R1 reagent.
Journal of Chemical Education • Vol. 77 No. 6 June 2000 • JChemEd.chem.wisc.edu
In the Classroom
Table 1. Results from Flow Gravimetr y and an Official Method (8 ) P2 O5 (wt % ± SD) Sample No. Flow Gravimetry Spectrophotometry a
1
25.5 ± 0.4
24.4 ± 0.2
2
9.2 ± 0.4
10.0 ± 0.2
3
26.2 ± 0.4
25.8 ± 0.2
4
50.5 ± 0.7
47.3 ± 0.2
5
25.6 ± 0.7
28.6 ± 0.1
6
16.7 ± 0.7
15.0
7
42.9
b
43.2 ± 0.4
8
55.0 ± 0.7
54.9 ± 0.3
9
53.5 ± 0.4
53.7 ± 0.3
10
15.5 ± 0.6
17.1
b
b
aData bSD
are based on three replications. estimated as
