Search results for "ionic strength"
showing 10 items of 198 documents
Silicate adsorption by goethite at elevated temperatures
2009
Abstract Batch adsorption experiments with relatively low silica concentrations between 10 µM and 100 µM were conducted at three different ionic strength (0.01 − 0.1 M), and four different temperatures between 10 °C and 75 °C, yielding in a total of 550 experimental data points. The residual concentration of monosilicic acid is controlled by an adsorption equilibrium which is dependent on pH. The % Si adsorbed vs. − log[H + ] curves reveal an upward bend with a maximum at about a pH of 9. With acidification below pH 9 the residual Si concentration in the suspensions steadily increases, as well as in the increasingly alkaline pH range. The slope of the latter is becoming steeper with increa…
Toward Mechanistic Design of Surrogate Buffers for Dissolution Testing of pH-Dependent Drug Delivery Systems
2020
The in vivo dissolution of enteric-coated (EC) products is often overestimated by compendial in vitro dissolution experiments. It is of great interest to mimic the in vivo conditions as closely as possible in vitro in order to predict the in vivo behavior of EC dosage forms. The reason behind this is the overly high buffering capacity of the common compendial buffers compared to the intestinal bicarbonate buffer. However, a bicarbonate-based buffer is technically difficult to handle due to the need for continuous sparging of the media with CO2 to maintain the desired buffer pH. Therefore, bicarbonate buffers are not commonly used in routine practice and a non-volatile alternative is of inte…
Functionalized halloysite nanotubes for enhanced removal of lead(II) ions from aqueous solutions
2018
In this study, environmental friendly halloysite nanotubes and their amino derivatives were used as adsorbent materials for lead(II) ions. The adsorption ability of both nanomaterials towards Pb2+ ions has been studied in NaClaq, at I = 0.1 mol L−1, in the pH range 3–6. Moreover, the effect of ionic strength on the adsorption process was evaluated at the pH of maximum efficiency of the adsorbent materials. Kinetic and equilibrium experiments were carried out by using the Differential Pulse Anodic Stripping Voltammetry (DP-ASV) technique to check the metal ion concentration in solution after contact with the two adsorbents. Different isotherm and kinetic equations were used to fit the experi…
Equilibrium studies in natural waters: Speciation of phenolic compounds in synthetic seawater at different salinities
1995
Interactions between some phenolic compounds and macro-constituents of synthetic seawater (Na{sup +}, K{sup +}, Ca{sup 2+}, Mg{sup 2+}, Cl{sup {minus}}, and SO{sub 4}{sup 2{minus}}), at 20, 35, and 45 {per_thousand} salinity, have been investigated potentiometrically by using the [H]-glass electrode. The formation constants of phenol, o- and p-cresol, o-a dn p-nitrophenol complexes with sodium, potassium, calcium, and magnesium ions have been determined in the ionic strength range 0 {le} I {le} 1 mol/L. A comparison between the apparent protonation constants of phenols determined in synthetic seawater, and those simulated by a suitable complex formation model, is discussed. The possibility …
HPLC-SEC: a new approach to characterise complex wastewater effluents
2016
ABSTRACTThis work investigates the use of HPLC-SEC to characterise dissolved organic matter (DOM) of complex wastewater effluents. A silica-based column, sodium acetate eluent and multiple detections were employed: UV-254 absorbance for humictype, and tryptophan-like (Ex/Em = 270/355) and tyrosine-like (Ex/Em = 270/310) fluorescence for protein type compounds. Effects of eluent pH, eluent ionic strength and injection volume on separation efficiency were tested. Humic-type and protein-type fractions were clearly differentiated and eluted within and out of calibration range. Eluent ionic strength had the greatest influence on global resolution; the lowest eluent concentration of 0.01 M produc…
A program for the simultaneous potentiometric determination of protonation constants and electrode calibration parameters
2005
A program for the potentiometric determination of the protonation constants of mononuclear polyprotic substances is described. A maximum of twelve parameters can be determined simultaneously, including up to six protonation constants, four electrode calibration parameters, the protolysis constant of the solvent and the titrant concentration. Optimization is carried out by using the non-simplified Newton-Raphson method, which is potentiated by the Marquardt algorithm and a distance speeding-up coefficient. A direct search method is also used to improve the initial set of values. Variances are calculated very accurately, since the real Hessian function is used. Statistical weights and ionic s…
Analysis of furan in coffee of different provenance by head-space solid phase microextraction gas chromatography-mass spectrometry: effect of brewing…
2009
A simple, sensitive and accurate method for the analysis of furan in roasted coffee has been used based on headspace-solid-phase micro-extraction (HS-SPME) coupled to gas chromatography-mass spectrometry (GC-MS). The extraction was performed using 75-microm carboxen/polydimethylsiloxane fiber. Ionic strength, extraction time and temperature, and desorption time were assessed as the most important parameters affecting the HS-SPME procedure and d(4)-furan was used as the internal standard. The linearity range was in the range 0.0075-0.486 ng g(-1); the LOD and LOQ calculated using the signal-to-noise ratio approach were 0.002 and 0.006 ng g(-1), respectively. The inter- and intra-day precisio…
Hydrolysis of methyltin(IV) trichloride in aqueous NaCl and NaNO3 solutions at different ionic strengths and temperatures
1999
The hydrolysis of methyltin(IV) trichloride (CH 3 SnCl 3 ) has been studied in aqueous NaCl and NaNO 3 solutions (0 < l/mol dm -3 ≤1), at different temperatures (15 ≤ T/°C ≤ 45) by potentiometric measurements (H + -glass electrode). By considering the generic hydrolytic reaction pCH 3 Sn 3+ + qH 2 O = (CH 3 Sn) p (OH) q 3p-q +qH + (logβ pq ), we have the formation of five species and logβ 12 = -3.36, logβ 13 = -8.99, logβ 14 = -20.27 and logβ 25 = -7.61. The first hydrolysis step is measurable only at very low pH values and was not determined: a rough estimate of the hydrolysis constant is logβ 11 = -1.5 (± 0.5). The dependence on ionic strength of logβ pq is quite different in NaNO 3 and N…
Behaviour of Nanomaterials in the Environment: A Study of Interaction between Humic Acids and Fullerene C60
2010
Interaction between buckminsterfullerene and humic substances of different origin was investigated using fluorescence spectrometry as function of concentration of humic substances, pH and ionic strength. Binding constants of humic substances and buckminsterfullerene were calculated. It can be suggested that the complexation was driven by hydrophobic interaction depending on properties of the interacting compounds. Hydrophobic interaction model was indicated by linear Stern–Volmer plot, and high Kd values characterized interaction between buckminsterfullerene and humic acids. The results of this study support the development of understanding of fate of nanomaterials in environment as well as…
Hydrolysis and chemical speciation of (C2H5)2Sn2+, (C2H5)3Sn+ and (C3H7)3Sn+ in aqueous media simulating the major composition of natural waters
2001
The hydrolysis of (C 2 H 5 ) 2 Sn 2+ , (C 2 H 5 ) 3 Sn + and (n-C 3 H 7 ) 3 Sn + has been studied, by potentiometric measurements ([H + ]-glass electrode), in NaNO 3 , NaCl, NaCl/Na 2 SO 4 mixtures and in a synthetic seawater (SSWE), as an ionic medium simulating the major composition of natural seawater, at different ionic strengths (0 ≤ I ≤ 5 mol dm -3 ) and salinities (15 ≤ S ≤ 45), and at t = 25 °C. Five hydrolytic species for (C 2 H 5 ) 2 Sn 2+ , three for (C 2 H 5 ) 3 Sn + and two for (C 3 H 7 ) 3 Sn + are found. Interactions with the anion components of SSWE, considered as single-salt seawater, are determined by means of a complex formation model. A predictive equation for the calcul…