In order to follow the guidelines set by the European Higher Education Area (EHEA) regarding the use of problem-based learning in the analytical chemistry laboratory curriculum at the undergraduate level, an interpretative lesson about the relevance of monitoring and maintaining the extractant pH in bioaccessibility tests of trace elements in soil environments—harnessing the single-extraction procedure as endorsed by the EU Standards, Measurement, and Testing (SM&T) program using 0.43 mol/L AcOH as leaching medium—was taken as a tutorial example. The idea behind is to supplement laboratory assays combining acid-base reaction tests with potentiometric detection. Based upon constructivism active learning, students should observe the increase of extractant pH upon dissolution of soil carbonates whereupon they should design appropriate analytical procedures supported by software algorithms to sustain the nominal extractant pH for reliable measurements throughout the extraction test, which, according to SM&T, lasts 16 h.
From an educational viewpoint, the pH-stat methodology  is to be described first, namely, the addition of minute aliquots of a strong acid to the extraction medium to compensate for the neutralization of the acetic acid by dissolved carbonates, followed by the presentation of the hardware and software employed in the tutorial example: (i) Eutech PC2700 pH-meter, (ii) CAVRO XP3000 syringe pump, (iii) magnetic stirrer, (iv) extraction vessel, (v) 3 mol/L HCl reservoir, and (vi) CocoSoft for controlling all of the instrumentation. Figure 2 shows a diagrammatic description of hardware connections and the information flow path between hardware and Cocosoft.
A monitoring algorithm is first taught and provided to the students. This is followed by running the standard SM&T method for 15 min while describing the main pitfalls that could stem from the gradual increase of the extractant pH (e.g., the lower extracting capability of the acetic acid and the underestimation of the real hazard of contaminated soils by trace elements) The SM&T single extraction test was executed by adding 2.0 g of a calcareous agricultural soil (Palma de Mallorca, Spain) to 80 mL of 0.43 mol/L CH3COOH subjected to controlled mechanical agitation (500 rpm) throughout. A simple Cocosoft script is given below as an educational example:
The role of every single instruction in the automatic analytical method above for continuous monitoring of the pH is to be explained, including setting up of the communications, data acquisition, and data plotting. An illustrative profile of pH monitoring is shown in Fig. 3A.
Upon critical evaluation of the pH profile, students should be asked to search possibilities and write viable scripts for automatic addition of HCl aliquots to the extraction vessel. They are distributed in small groups to trigger collaborative work in line with EHEA recommendations. The lecturer should indicate a number of instructions available of the CAVRO XP3000 pump while providing tips and tricks of what could be done with the equipment assembled.
The prevailing answer expected is to add fixed-minute volumes of HCl at predefined times aided by the syringe pump. The amount to be added, however, varies from soil to soil, and although this protocol is recommended in the standard CEN/TS 14429:2005, as students have access to smart automatic equipment, no further attention was given to this method. Emphasis was placed on the word ‘smart’, and on the possibility to further improve the pH monitoring system if we succeed in programming a viable automatic method with feedback from the sample.
After further discussion in a team collaborative environment, the possibility of design of a pH comparison system should come up. This was the case in a last year undergraduate course of advanced analytical chemistry laboratory in which this tutorial example was introduced as an innovative learning tool. The basis of the comparison algorithm is as follows: ‘when the extract pH is higher than the nominal pH of the extractant, a given volume of acid should be added (in our case 200 μL (see script below)’. The fluidic system was initialized manually, and four instructions were added to the previous monitoring script:
Before the loop:
And in the loop, before the plotting:
The time-based pH profile of the SM&T test was projected at real time in the class (see Fig. 3B) and synchronized with the visualization of the Cocosoft script. This test served in triggering students’ learning outcomes and engagement as a great expectation was observed after the first addition of acid in as much as most students were eagerly waiting for the pH to exceed the nominal extractant pH for the automatic actuation of the syringe pump.
Several teams, however, soon realized that the main drawback of the smart comparative procedure is that once the carbonate pool of the solid material is almost entirely dissolved in the acetic acid milieu, occurring within a few-minute timeframe, the difference of 100th of a pH unit still triggered the addition of an aliquot of 200 μL of 3 mol/L HCl with the consequent sharp decrease in the extractant pH, which is not to be buffered. The lecturer should make students aware that the dissolution of further mineralogical phases that have slower leaching kinetics will be carried out at lower pH values, which will in turn overestimate the pools of bioaccessible trace elements in risk assessment studies of metal contaminated soils. As a result, the students in the laboratory course should identify the need of a different algorithm to tackle this issue. Our experience indicates that at this point the educator should give some further tips and tricks prior to introducing the so-called proportional algorithm in which the volume of the acid aliquot added to the extraction medium is proportional to the difference between the nominal and current pH values rather than adding a steady volume regardless of the absolute pH value, as is the case with the comparator algorithm. The procedure is the same as in the previous example, but the smart conditional line should be replaced from:
CAVRO_XP3000.dispense_ uL (200)
CAVRO_XP3000.dispense_ uL (1000*(Eutech_PC2700.get_pH()-Initial_pH))
This demonstrates the versatility of the Cocosoft software package in method development and optimization of analytical procedures. With the proportional algorithm, two simple demonstrations could be introduced to illustrate the fact that a difference of one pH unit will lead to the addition of a 1000 μL acid aliquot to the extraction vessel, whereas a difference of 100th of a pH unit will trigger the automatic addition of a mere 10 μL aliquot. The new monitoring profile (proportional algorithm) is compared in Fig. 3C with that of the previous procedure (comparator algorithm).
As a further team collaborative work, students might be asked to enumerate potential improvements to the algorithm. We do expect that they envisage the feasibility of the first derivative for monitoring of the rate of pH change. Optimization of the parameters of the proportional algorithm, including the time between consecutive measurements and the constant of proportionality between the acid volume and the difference between the nominal and actual pH values, needs also to be perceived and illustrated via practical examples in the laboratory.