A Source of Systematic Errors in the Determination of Critical Micelle Concentration and Micellization Enthalpy by Graphical Methods in Isothermal Titration Calorimetry

Isothermal titration calorimetry is frequently employed to determine the critical micelle concentration and the micellization enthalpy of surfactants in terms of geometrical characteristics of the titration curves. Previously we have shown theoretically that even for an infinitesimal injection, the heat per titrant mol depends on the stock solution concentration. In this work, we explore experimentally the influence of the stock solution concentration on the geometrical characteristics of the titration curve and its effect in determining the critical micelle concentration and the micellization enthalpy of surfactants. The systematic study of this phenomenology involves a great number of measurements at different temperatures with several repetitions carried out using a robotic calorimeter. As surfactant hexadecyltrimethylamonium bromide was used. The magnitude and shape of the heat titration depend on the stock solution concentration. As a consequence, the inflexion-point, break-point, and step-height decrease until a limiting value. A qualitative analysis suggests that the limiting value depends only on substance. This work shows that graphical methods could not be suitable for the calculation of the critical micelle concentration and micellization enthalpy because the magnitude and shape of the titration curve depend on the stock solution concentration. Micellar properties should be calculated by the application of theoretical models as in the ligand-binding studies.

Virtual lab on analytical chemistry for pharmacy students of distance learning

Extramural education along with certain advantages has some disadvantages, including the lack of the possibility of acquiring experimental work skills. Pharmacy students of correspondence courses should study a number of chemical disciplines, including analytical chemistry. Virtual labs could solve the problem of lack of laboratory training, at least partially. Virtual laboratory works on chemistry, described in the literature, are built on the basis of multimedia technologies (audio, video, animations, presentations and other visual effects). The purpose of this work is the creation of a computer program that would not only visually simulate the process of acid-base titration, but also create a task for a student, i.e. – definition of titrant volume and calculation the concentration of an analyte in a virtual sample. Virtual laboratory work was developed in the LabVIEW12 package (National Instruments, USA). A file of virtual laboratory work along with all educational materials is received via e-mail by all students. When you open the laboratory work file, the front panel of the virtual instrument shows the setup for titration, a series of indicator windows for displaying information, and a virtual monitor screen for displaying the titration curve. Then the program guides the student through the following steps: registration of personal data and definition embodiment, visualization of titration and formation of the titration curve, mapping the first derivative of the titration curve, increasing the scale of the graph and determining the volume of titrant at the equivalence point. After finishing the task, the student makes a print-screen, inserts it into his report with the calculations and sends it to the teacher via e-mail for verification in a timely manner. Thus, the described laboratory work allows the student to observe the formation of the titration curve and its subsequent transformation, as well as to master the processing of titration results. Development of virtual laboratory works similar to other methods for quantitative analysis will form a complete virtual laboratory practice, which can increase the level of training of pharmacy students and eventually move to distance learning.

Base Neutralizing Capacity of Agricultural Soils in a Quaternary Landscape of North-East Germany and Its Relationship to Best Management Practices in Lime Requirement Determination

Despite being a natural soil-forming process, soil acidification is a major agronomic challenge under humid climate conditions, as soil acidity influences several yield-relevant soil properties. It can be counterbalanced by the regular application of agricultural lime to maintain or re-establish soil fertility and to optimize plant growth and yield. To avoid underdose as well as overdose, lime rates need to be calculated carefully. The lime rate should be determined by the optimum soil pH (target pH) and the response of the soil to lime, which is described by the base neutralizing capacity (BNC). Several methods exist to determine the lime requirement (LR) to raise the soil pH to its optimum. They range from extremely time-consuming equilibration methods, which mimic the natural processes in the soil, to quick tests, which rely on some approximations and are designed to provide farmers with timely and cost-efficient data. Due to the higher analytical efforts, only limited information is available on the real BNC of particular soils. In the present paper, we report the BNC of 420 topsoil samples from Central Europe (north-east Germany), developed on sediments from the last ice age 10,000 years ago under Holocene conditions. These soils are predominantly sandy and low in humus, but they exhibit a huge spatial variability in soil properties on a small scale. The BNC was determined by adding various concentrations of Ca(OH)2 and fitting an exponential model to derive a titration curve for each sample. The coefficients of the BNC titration curve were well correlated with soil properties affecting soil acidity and pH buffer capacity, i.e., pH, soil texture and soil organic matter (SOM). From the BNC model, the LRs (LRBNC) were derived and compared with LRVDLUFA based on the standard protocol in Germany as established by the Association of German Agricultural Analytic and Research Institutes (VDLUFA). The LRBNC and LRVDLUFA correlated well but the LRVDLUFA were generally by approximately one order of magnitude higher. This is partly due to the VDLUFA concept to recommend a maintenance or conservation liming, even though the pH value is in the optimum range, to keep it there until the next lime application during the following rotation. Furthermore, the VDLUFA method was primarily developed from field experiments where natural soil acidification and management practices depressed the effect of lime treatment. The BNC method, on the other hand, is solely based on laboratory analysis with standardized soil samples. This indicates the demand for further research to develop a sound scientific algorithm that complements LRBNC with realistic values of annual Ca2+ removal and acidification by natural processes and N fertilization.

DETERMINATION OF THE SURFACE FUNCTIONALITY OF NANOCARBON ALLOTROPES BY BOEHM TITRATION

Critical point (CP) in a titration curve to quantify carboxylic, lactonic, and phenolic groups on multi-walled carbon nanotube (CNT) and Vulcan carbon (VC). Titration technique used was Boehm type and tipping point measurement from titration curve was done by second derivative method. The calculation functional groups on nanocarbons (NCs) presents lowest uncertainty previously published. An order of [Formula: see text]meq/g is the precision of the values obtained, one order of magnitude below reported. Both NCs display a high quantity of carboxylic groups created after being treated with regard to the other functional groups generated, mainly for functionalized Vulcan carbon VC-F.

Considerations on the Derivative of the Titration Curve of a Weak Diacid

We show that the derivative of the titration curve of a diacid can be used to assess the first equivalent as well as the second equivalent point, but also the value for which pH=pKa. This is controlled by the difference between both pKa's of the diacid. A bifurcation is observed for the values of volume of titrant for which the first derivative cancels.<br>

Considerations on the Derivative of the Titration Curve of a Weak Diacid

We show that the derivative of the titration curve of a diacid can be used to assess the first equivalent as well as the second equivalent point, but also the value for which pH=pKa. This is controlled by the difference between both pKa's of the diacid. A bifurcation is observed for the values of volume of titrant for which the first derivative cancels.<br>