Alkali Titration Research Articles

Graphene-supported Cu2O-CoO heterojunctions for high performance supercapacitors

Transition metal oxides are excellent electrode materials for supercapacitors because of their high theoretical capacitance, good stability, environmental friendliness, and tunable structure. In this study, Cu2O-CoO heterojunction materials loaded on graphene oxide (Cu2O-CoO/GO) are prepared via the precipitation, hydrothermal, and heat treatment processes. Through the adsorption of transition metal ions by the oxygen-containing functional groups of GO, followed by alkali titration as well as hydrothermal reaction of Cu2+ and Co2+ ions, the hydroxides of metals are adsorbed on GO. Meanwhile, the reduced GO completes the self-assembly, and the GO with good spatial structure supports the formation of Cu2O-CoO heterojunction structure. The Cu2O-CoO/GO electrode material combines the high conductivity of GO with the excellent internal conductivity of Cu2O-CoO, and its abundant active sites and specific surface area, thereby ensuring good electrochemical performance. Thus, it enables reversible ion adsorption and desorption, exhibits the pseudocapacitance of copper oxide and cobalt oxide, and has higher capacitance performance than Cu2O/GO and CoO/GO electrodes. The specific capacitance of Cu2O-CoO/GO at 1 A g−1 is 723 F g−1. Cu2O-CoO/GO/ASC exhibits an energy density of 44.1 Wh kg−1 at 794 W kg−1 and capacitance retention rate of 89.3 % after 10,000 cycles. This indicates that the material has significant potential for a wide range of supercapacitor applications.

Comparison of the different monosodium urate crystals in the preparation process and pro-inflammation

ObjectivesThe deposition of monosodium urate (MSU) crystals within synovial joints and tissues is the initiating factor for gout arthritis. Thus, MSU crystals are a vital tool for studying gout’s molecular mechanism in animal and cellular models. This study mainly compared the excellence and worseness of MSU crystals prepared by different processes and the degree of inflammation induced by MSU crystals.MethodsMSU crystals were prepared using neutralization, alkali titration, and acid titration methods. The crystals’ shape, length, quality, and uniformity were observed by polarized light microscopy and calculated by the software Image J. The foot pad and air pouch models were used to assess the different degrees of inflammation induced by the MSU crystals prepared by the three different methods at different time points. Paw swelling was evaluated by caliper. In air pouch lavage fluid, inflammatory cell recruitment was measured by hemocytometer, and the level of IL-1β, TNF-α, and IL-18 by ELISA. Inflammatory cell infiltration was assayed by immunohistochemistry of air pouch synovial slices.ResultsFor the preparation of MSU crystals with the same uric acid, the quantity acquired by the alkalization method was highest, followed by neutralization, with the acid titration method being the lowest. The crystals prepared by neutralization were the longest. The swelling index of the foot pad induced by MSU crystals prepared by acid titration was significantly lower than that of the other methods at 24 h. The inflammatory cell recruitment and level of IL-1β, TNF-α, and IL-18 in air pouch lavage fluid were lowest in animals with crystals prepared by acid titration. IL-1β secretion induced by MSU crystals prepared by acid titration was significantly lower than that of the other two groups, but there was no significant difference in IL-18 secretion between the three groups in THP-1 macrophages and BMDMs.ConclusionsAll three methods can successfully prepare MSU crystals, but the levels of inflammation induced by the crystals prepared by the three methods were not identical. The degree of inflammation induced by MSU crystals prepared by neutralization and alkalization is greater than by acid titration, but the quantity of MSU crystals obtained by the alkalization method is higher and less time-consuming. Apparently, the window of inflammation triggered by acid titration preparation is shorter compared to other forms of crystal preparation. Overall, MSU crystals prepared by the alkaline method should be recommended for studying the molecular mechanisms of gout in animal and cellular models.

Kinetics of native and added carbon mineralization on incubating at different soil and moisture conditions in Typic Ustochrepts and Typic Halustalf

The carbon dynamics in soils is of great importance due to its links to the global carbon cycle. The prediction of the behavior of native soil organic carbon (SOC) and organic amendments via incubation studies and mathematical modeling may bridge the knowledge gap in understanding complex soil ecosystems. Three alkaline Typic Ustochrepts and one Typic Halustalf with sandy, loamy sand, and clay loam texture, varying in percent SOC of 0.2; S1, 0.42; S2, 0.67; S3 and 0.82; S4 soils, were amended with wheat straw (WS), WS + P, sesbania green manure (GM), and poultry manure (PM) on 0.5% C rate at field capacity (FC) and ponding (P) moisture levels and incubated at 35 °C for 1, 15, 30 and 45 d. Carbon mineralization was determined via the alkali titration method after 1, 5, 7 14, 21, and 28 d. The SOC and inorganic carbon contents were determined from dried up (50 °C) soil samples after 1, 15, 30, and 45 d of incubation. Carbon from residue mineralization was determined by subtracting the amount of CO2-C evolved from control soils. The kinetic models; monocomponent first order, two-component first order, and modified Gompertz equations were fitted to the carbon mineralization data from native and added carbon. The SOC decomposition was dependent upon soil properties, and moisture, however, added C was relatively independent. The carbon from PM was immobilized in S4. All the models fitted to the data predicted carbon mineralization in a similar range with few exceptions. The residues lead to the OC build-up in fine-textured soils having relatively high OC and cation exchange capacities. Whereas, fast degradation of applied OC in coarse-textured soils leads to faster mineralization and lower build-up from residues. The decline in CaCO3 after incubation was higher at FC than in the P moisture regime.

Coordination Behaviors of Cu2+ With Azo Dye Acid Black 10B

This study mainly explored the coordination relationship between Cu2+ and functional groups of acid black 10B, including sulfonic group, azo group, amino group, hydroxyl group and nitro group. By means of UV-vis spectrum scanning, FTIR and XPS techniques, the complex reaction process of cooper ion with dye molecule and their precipitate formed during alkali titration were analyzed. The results showed that copper ion had coordination with all the above groups, and its coordination with azo group and sulfonic group can be able to form a closed ring structure similar to chelate. The spectra characteristics of precipitate revealed that copper ions could play bridging role between amino and hydroxyl and its coordination process would be affected by the pH value of the solution.

Green synthesis of Tungsten-doped CeO2 catalyst for selective catalytic reduction of NOx with NH3 using starch bio-template

Herein, a novel tungsten-doped CeO2 catalyst was fabricated via the spread self-combustion (SSC) method using sweet potato starch as bio-template. The doping of tungsten refines the pore diameter of CeO2 and restrains the growth of cubic CeO2 crystal formed in it. However, the annealing at 700 °C promotes the growth of cubic CeO2 crystal and the formation of Ce4W9O33 crystal instead of the amorphous tungsten species compared with the catalyst calcined at 550 °C. It is also found that the doping of tungsten increases the concentration of the adsorbed oxygen on the surface of CeO2-550 and enhances the reducibility of the surface oxygen of stoichiometric ceria and the acid sites. The as-prepared Ce4W2Oz-550 catalyst exhibits higher catalytic performance than Ce4W2Oz-CP-550 synthesized via the urea hydrothermal co-precipitation method due to the better dispersion of cerium species and the larger molar ratio of Oα/(Oα + Oβ). However, the enhancement of annealing temperature from 550 °C to 700 °C decreases the acidic and the redox properties of Ce4W2Oz, and thereby depresses the promotional effect of tungsten-doped on the NH3-SCR activity of CeO2-550. The suitable starch dosage (liquid/solid ratio), ion concentration, alkali titration and annealing temperature are 1 g (10 mL/g), 1 mol/L, ammonia water (pH ≈ 7.0) and 400–550 °C for the as-prepared Ce4W2Oz catalyst, respectively.

The quality of ice cream samples made from buffalo milk

The aim of this study was to evaluate physical, chemical and organoleptic properties the ice creams made from buffalo milk, cow milk, and their mix during storage for 21 days. Before the cow milk ice-cream samples were made, cow milk was standardized to fat ratio of 7.10 % with fresh cow milk cream. The highest acidity values were determined in ice cream samples made from buffalo milk using the alkali titration method . The pH of ice cream samples was found at between 6.44 and 6.60. The average pH of ice cream samples was found as 6.53. The overrun values of ice cream samples ranged from 34.10 % to 43.23 % and the highest overrun values were determined in samples made with buffalo and cow milk mix. The first melting time of ice cream samples was between 1109s and 1510s. Generally, the first melting time of ice cream samples made from buffalo milk was longer than that of only cow milk ice cream samples. The highest L color value was determined at ice cream samples made from only buffalo milk. Generally, buffalo milk ice cream samples were preferred by the panelist with the highest scores when compared with ice cream samples made from cow and mix of cow and buffalo milk. As a result, it can be surely said that the use of buffalo milk in dairy products such as ice cream enriches the sensory, chemical and physical properties of the product.

Change of the main components of a chemical composition of frozen strawberries under previous treatment in solutions with structure-retaining properties

The objective of the research lies in the study of the main indices of the chemical composition of strawberries during freezing and storage under its previous treatment in solutions with structure-retaining properties. The main indices of the chemical composition of strawberries during freezing and a six-month storage under previous treatment in the solutions with structure- retaining properties were studied, namely in 1 %, 2 % and 3 % solutions of potato starch and 1 % and 2 % sugar-pectin solution.The main components of the chemical composition were identified. The content of soluble solids was determined using refractometer, sugar content was measured by means of ferricyanide method, organic acids were identified by alkali titration, ascorbic acid content was determined by iodometric method.It has been found that during freezing and storage of frozen strawberries there are losses of the main components of chemical composition, the amount of which considerably depends on the type and concentration of the solution for treatment.Application of the previous treatment of strawberries before freezing helps to retain the main components of chemical composition of strawberries during freezing and storage. Losses of soluble solids of previously treated strawberries during freezing were by 0,9 – 1,2 % lower compared to control. Sugar content went down to1,8 – 3,3 % as the result of freezing. Previous treatment of berries before freezing promotes their preservation by 0,1-0,7 %. Organic acids content in strawberries remained practically unchanged. Ascorbic acid content during freezing of strawberries suffered essential losses that made up 43,8 – 50,3 % under the lowest in the sample that were previously treated in 2 % sugar-pectin solution and by 13 % lower than in control. During storage the losses of ascorbic acid in strawberries without previous treatment were by 4,4 – 7,6 % higher

Enhanced Coagulation-Flocculation Performance of Iron-Based Coagulants: Effects of PO4(3-) and SiO3(2-) Modifiers.

PO4 3- and SiO3 2- are often used as modifier to improve stability and aggregating ability of the iron-base coagulants, however, there are few reports about their detailed comparison between the coagulation performance and mechanisms. In this study, three coagulants—polyferric phosphoric sulfate (PFPS), polysilicon ferric sulfate (PFSS), and polyferric sulfate (PFS) were synthesized; their structure and morphology were characterized by Fourier transformed infrared (FT-IR) spectroscopy, X-ray diffraction (XRD) and Scanning electron microscope (SEM). Alkali titration and Ferron species analysis were employed to investigate the hydrolysis performance and species distribution. Jar test was conducted to measure their coagulation behaviors at different dosage, pH, and temperatures in which the flocs properties were measured. The results showed that a number of new compounds were formed due to the presence of PO4 3- and SiO3 2-. Moreover, PFPS and PFSS had similar level in Fea as well as Feb. Among them, PFPS produced more multi-core iron atoms polymer and content of Feb, and the formed flocs were larger and denser. It exhibited superior coagulation performance in terms of turbidity reduction, UV254 removal and residual ferric concentration. Jar test and floc breakage/regrowth experiments indicated other than charge neutrality, the dominated mechanism involved in PFSS was the adsorption between polysilicic acid and solution particle, while PFPS was sweeping, entrapment/adsorption resulting from larger polymer colloid of Fe-P chemistry bond.

Measurement and modeling of acid dissociation constants of tri-peptides containing Glu, Gly, and His using potentiometry and generalized multiplicative analysis of variance

We report pK(a) values with measurement uncertainties for all labile protons of the 27 tri-peptides prepared from the amino acids glutamic acid (E), glycine (G) and histidine (H). Each tri-peptide (GGG, GGE, GGH, …, HHH) was subjected to alkali titration and pK(a) values were calculated from triplicate potentiometric titrations data using HyperQuad 2008 software. A generalized multiplicative analysis of variance (GEMANOVA) of pK(a) values for the most acidic proton gave the optimum model having two terms, an interaction between the end amino acids plus an isolated main effect of the central amino acid.

A new and efficient colorimetric high-throughput screening method for triacylglycerol lipase directed evolution

A new and efficient method was developed for the first time using the targeted substrate, triacylglycerol, for the rapid screening of a large library of mutants for lipase directed evolution. The high-throughput method applies calcium chloride to convert the fatty acid produced from hydrolysis reaction to hydrochloric acid which is easy to release H+ detected by pH indicator. This new method exhibits significant advantages compared with the conventional method of applying a substituent of the actual targeted substrate, p-nitrophenyl palmitate as a high-throughput screening substrate. The results determined by the new method correlate well with those obtained from alkali titration and HPLC analysis. By applying the new method, a significantly enhanced mutant enzyme was obtained with lipase activity increased more than five-fold compared to wild type after only two cycles of directed evolution. In addition, the esterification activity of the mutant was also improved and the regioselectivity of the lipase was not changed.

Structure elucidation of a sodium salified anthraquinone from the seeds of Cassia obtusifolia by NMR technique assisted with acid–alkali titration

A new sodium salt of anthraquinone named sodium emodin-1-O-β-gentiobioside, together with nine known compounds, viz. rubrofusarin-6-O-β-D-gentiobioside, chrysophanol-1-O-β-D-glucopyranosyl-(1-3)-β-D-glucopyranosyl-(1-6)-β-D-glucopyranoside, obtusifolin-2-O-β-D-glucopyranoside, aurantio-obtusin-6-O-β-D-glucopyranoside, physcion-8-O-β-D-glucopyranoside, 1-hydroxyl-2-acetyl-3,8-dimethoxy-6-O-β-D-apiofuranosyl-(1-2)-β-D-glucosylnaphthalene, toralactone-9-O-β-D-gentiobioside, aurantio-obtusin, rubrofusarin-6-O-β-D-apiofuranosyl-(1-6)-O-β-D-glucopyranoside, was isolated from the seeds of Cassia obtusifolia and its structure was elucidated by (1)H and (13)C NMR technique assisted with acid-alkali titration. The change of chemical shifts of sodium emodin-1-O-β-gentiobioside before and after acid-alkali titration was also characterized.

Macro Analysis Would Like to Drop Half a Drop of Solution

By strong alkali (acid) titration strong acid (alkali), alkali (acid) titration mono-weak acid (alkali), precipitation titration and coordination titration to drop by half a drop, and settling the titration error specific terms, to illustrate the macro titration analysis would like to have drop half a drop of a solution. When strong alkali titrate strong acid or monobasic weak acid, there is no need to drop half a drop. As strong acid titrate strong alkali, there is must to drop half a drop as methyl orange for indicator. But phenolphthalein as indicator, there is doesn't need to drop half a drop. Strong alkali titrate monobasic weak acid, there is no need to drop half a drop. Eriochrome Black T for indicator titrate magnesium ion with EDTA, there is must to drop half a drop. Titration of zinc by EDTA, don't need to drop half a drop. Titration chloride ion with Moore method can not drop half a drop.

Physicochemical study of the nanodiamond surface

Suspensions of five types of ultrafine diamond of a detonation synthesis in distilled water and 0.9 mol/l NaCl solution were found to have pHs in the range of 3–6, which results from the intrinsic acidity of their surfaces. The possibility of quantitatively determining the protogenic groups of a surface is found using the pH-potentiometric method. The data on the kinetics of the varying pH of suspensions in water and 0.9 mol/l NaCl solution and the data on the curves of the alkali titration of the suspensions of nanodiamond in the indicated systems make it possible to infer the presence of one or two types of acid groups on the surface of a diamond. The constants of dissociation (pK 1) of these groups on the assumption of their single-base nature are estimated. From 1 × 10−3−5 × 10−2 mol/l chloride solutions of H[AuCl4] and RhCl3 on the surface of diamond, gold(III) and rhodium(III) are found to be adsorbed; the adsorption of methylene blue from its 2.5 × 10−4−5 × 10−4 mol/l solutions reaches more than 90% (upon adsorption from 5 ml of the initial solution by a 0.05-g nanodiamond).

Complex-radical copolymerization of N-vinyl pyrrolidone with isostructural analogs of maleic anhydride

Radical copolymerizations of N-vinyl-2-pyrrolidone (VP) with isostructural analogs of maleic anhydride (MA), such as citraconic anhydride (CA) and N-substituted maleimides [maleimide (MI), N-ethylmaleimide (EMI) and N-phenylmaleimide (PhMI)] were studied. Compositions of copolymers synthesized in a wide range of monomer feed ratios were determined by alkali titration (for anhydride copolymers), FTIR and 1H NMR spectroscopy using 1495 and 630 cm-1 (for VP-MI), 1289 and 1225 cm-1 (for VP-EMI) and 1050 and 3067 cm-1 (for VP-PhMI) analytical bands and integral areas of CH2 (pyrrolidone ring) and CH (MI), CH3 (EMI) and CH= (benzene ring in PhMI) groups, respectively. Electron-donor VP monomer was found to have substantially different reactivities in the radical copolymerization with MA, CA and N-substituted (H, C2H5 and phenyl) malemides as electron-acceptor comonomers. Effects of H-bonding and N→O=C coordination on the monomer reactivity ratios were evaluated. Tendency to alternation of the monomer pairs increases in the order of VP–MA > VP–CA > VP-MI > VP-PhMI > VP-EMI. Structure-thermal property-relationship for the synthesized copolymers was also studied.

Coupling pH-stat and DO-stat titration to monitor degradation of organic substrates

A combined titration methodology at constant pH and dissolved oxygen concentration to monitor the degradation of organics as carbon dioxide evolution rate (CER) and oxygen uptake rate (OUR) is presented. Equations necessary to assess CER from alkali titration rates and the instrument used to test the technique are described. Experiments were performed on samples of activated sludge from a domestic wastewater treatment plant using glucose and ethanol as substrates. OUR and CER were calculated for both substrates, as well as the respiratory quotient RQ = CER/OUR and biomass to substrate yields coefficients. RQ data for ethanol compared better with corresponding stoichiometric values than those for glucose.

Dissolution of Sparingly Soluble Inorganic Compound in Aqueous Suspension Containing Ion Exchange Resin (Part 9)

Dissolution process of CaMoO4 in aqueous suspension containing H-R, a strongly acidic ion exchange resin with hydrogen form, was studied mainly by chemical analysis of the liquid phase. Several analytical methods such as titration using NaOH solution or H-R suspension, calorimetry, thermal analysis and XRD were carried out in order to confirm the dissolution mechanism. CaMoO4 does not dissolve according to the equation, CaMoO4+2(H-R)→Ca-R2+H2MoO4 owing to the aggregation to polyanion by the acidification of MoO42-. By considering the experimental results, it is concluded that CaMoO4 dissolves to yield an acid having a composition to be H4Mo8O26. The dissolution process of CaMoO4 and alkali titration for the resultant acid are summarized as follows: (1) The dissolution process of CaMoO4 as a function of H-R (g) is expressed by the following two-stages reactions, 8CaMoO4+12(H-R)→Ca2Mo8O26(aq)+6(Ca-R2)+6H2O Ca2Mo8O26(aq)+4(H-R)→H4Mo8O26(aq)+2(Ca-R2). With increasing the amount of H-R, the concentrations of Ca and Mo increase at the initial stage which is followed by the decrease in Ca2+ concentration at the final stage. The formation of the intermediate, Ca2Mo8O26(aq), is proved by the experimental fact of ca. 4 of Mo/Ca molar ratio. (2) The neutralization between the final acid product, H4Mo8O26(aq) and NaOH solution proceeds according to the following two-stage reactions, H4Mo8O26(aq)+4NaOH(aq)→Na4Mo8O26(aq)+4H2O, Na4Mo8O26(aq)+12NaOH(aq)→8Na2MoO4(aq)+6H2O.

Effect of a light-induced pH gradient on purple-to-blue and purple-to-red transitions of bacteriorhodopsin

Bacteriorhodopsin-containing vesicles that were able to alkalize the extravesicular medium by greater than 1.5 pH units under illumination, i.e., inside-out vesicles, were reconstituted by reverse-phase evaporation with Halobacterium halobium polar lipids or exogenous phospholipids. Acid titration of a dark-adapted sample was accompanied by a color change from purple to blue (pKa = 2.5-4.5 in 0.15 M K2SO4), and alkali titration resulted in the formation of a red species absorbing maximally at 480 nm (pKa = 7 to greater than 9), the pKa values and the extents of these color changes being dependent on the nature of lipid. When a vesicle suspension at neutral or weakly acidic pH was irradiated by continuous light so that a large pH gradient was generated across the membrane, either a purple-to-blue or a purple-to-red transition took place. The light-induced purple-to-red transition was significant in an unbuffered vesicle suspension and correlated with the pH change in the extravesicular medium. The result suggests that the purple-to-red transition is driven from the extravesicular side, i.e., from the C-terminal membrane surface. In the presence of buffer molecules outside, the dominant color change induced in the light was the purple-to-blue transition, which seemed to be due to a large decrease in the intravesicular pH. But an apparently inconsistent result was obtained when the extravesicular medium was acidified by a HCl pulse, which was accompanied by a rapid color change to blue. We arrived at the following explanation: The two bR isomers, one containing all-trans-retinal and the other 13-cis-retinal, respond differently to pH changes in the extravesicular and the intravesicular medium. In this relation, full light adaptation was not achieved when the light-induced purple-to-blue transition was significant; i.e., only the 13-cis isomer is likely to respond to a pH change at the N-terminal membrane surface.

Effect of Surface Structure of Carbon fiber on Bending Strength of C/C Composite

The carbon fibers, the surface of which was hydrogenated and oxidized, were sub-mitted to be examined concerning the effect of the surface structure of carbon fiber on the strength of C/C composite. After the sizing agent on carbon fiber surface was first removed by boiling with ethyl methyl ketone, the surface was hydrogenated with H2 at 1000°C and further the surface hydrogenated was oxidized with aqueous H2O2 solution at the boiling temperature for various hours respectively. The amount of surface oxides, in this case, was measured by means of alkali titration, on the assumption that the surface oxides belong to the acidic groups such as hydroxyls, carboxyls and so on. The tensile strength of carbon fibers barely changed through such chemical surface modifications. It was found that the bending strength of C/C composites, in which the carbon fibers had been used as a reinforcing filler, linearly increased with an increase in the amount of surface acidity, that is, the surface acidic sites played a major role in the increase of the bending strength of C/C composite.Such effects of surface oxides could be ascribed to the strong adhesion between the fiber surface and the binder in the first stage and the following formation of C-C bond between the fiber surface and the binder the pyrolysis of surface oxides, during the molding of composite by means of hot-pressing.

Constantes de stabilité de complexes organo-minéraux. Interactions des ions plombeux avec les composés organiques hydrosolubles des eaux gravitaires de podzol

The complexation of lead(II) ions by water-soluble soil organic matter as ligand (recovered by percolating water through a A2 horizon of a podzolic soil) has been studied by potentiometric methods at 25°C in 0.1 M NaClO 4 medium. The total acidity of these solutions was resolved into strong, weak and very weak acidity. During the alkali titration, the dissociation of humic samples was characterized by an α coefficient that may be determined according to a procedure derived from Irving-Rossotti's method. The so-called Henderson-Hasselbalch equation was used to obtain pK A values for each of the two weak acidities ( pK A = 4.80 (COOH) and pK A = 8.85 (OH)). For the determination of stability constants of metal-humate complexes (HA) in the acidic range, three methods have been investigated: • —an approach using only pH measurements. • —direct estimation of the free metal ion concentrations from ISE determinations. • —application of Marinsky's method accounting for complications arising from the electric field at the surface of the polyelectrolyte which determines the effective concentration [A]. In these experiments typical values used for total lead(II) ion concentrations and total ligand concentrations are respectively in the range 4.10 −5 M to 4.10 −4 M and 0,33 to 0,57 meq·1 −1. All results agree with the existence of mono and bis-complexes PbA and PbA 2 with stability constants (protometric determinations): log K 1 = 4.2 and log K 2 = 3.7. The values obtained by direct estimation of free lead(II) concentration are slightly lower. An increase in stability constants was observed with increase in pH and also with decrease in total metal ion concentration (at constant pH). Ligand conformational properties are expected to be of great importance in the complexation phenomena.

New approach for the determination of stability constants of metal ions-soil organic matter

The binding of metal ions to organic materials in soil and water is an area of considerable interest. A better understanding of the equilibria that determine the behaviour of the chemical elements in the superficial environment is clearly essential for effective geochemical research. Naturally occurring organic compounds are an integral part of soils and aquatic systems. These substances play a major role in the retention and deactivation of anthropogenic organic and mineral chemicals and are involved in the geotransport of metals. Humic substances make up the bulk of organic matter and they probably control the abundance and distribution of trace metals in the environment by complexation or chelation processes through functional groups including carboxylic, phenolic, alcoholic and enolic OH, carbonyl and NH 2 sites at which binding may occur [1]. The determination of stability constants of metal-humic and fulvic acid complexes permits the prediction of the chemical speciation of many metals and the elucidation of their dispersion cycles. The present work deals with the interactions of lead(II), cadmium(II) and copper(II) ions with water soluble soil organic matter. Cu(II) like Zn(II), Mn(II)…, is a metal which is essential to plants while Cd(II) and Pb(II) are without known function in plants but are of considerable interest in view of their toxicity and accumulation in the environment. A variety of methods have been applied for determining stability constants of metalhumate complexes, as reviewed by Stevenson [1]. Considerable progress has been made, but agreement has not yet been reached as to how the data can best be analyzed and interpreted. One of the most popular methods is base titration, which uses the competition of metal ions and protons for complexing sites on the ligand. It is such a modified approach that has been developed in this work. Humic substance samples (in this context, this term will be used to designate a system of organic water soluble compounds including polymers) were recovered by percolating rain-water through a A2 horizon of a podzolic soil and purified by filtration, ultracentrifugation, and treatment by passage through ion exchange resins [2]. Stock solutions of metallic perchlorates were prepared from commercial products and standardized against EDTA. Samples containing constant quantities of humic substances and perchloric acid were titrated by sodium hydroxide in the absence and in the presence of metal ion. Protometric measurements were performed at 25 °C in 0.1 NaClO 4 medium under nitrogen atmosphere. An automatic titrator ‘Tacussel TT Processor’ fitted with extension glass and calomel electrodes was used for recording direct and differential titration curves; pH readings were combined with determinations of free metal ion concentrations by means of ion selective electrodes (ISE). Titration and Protonation Constants of Humic Samples For humic substances which contain COOH and OH groups of various acid strengths we shall define the ligand concentration in terms of titratable acidity. The total acidity content was resolved into a strong, weak and weak acidity by applying linear titration plots derived by McCallum and Midgley [3] in combination with Gran's plots as previously used by Takamatsu and Yoshida [4]. In order to obtain pK a values for each of the two weak acidities, the so-called extended Henderson-Hasselbalch equation was used pH = pK a + n log α/(1 − α) During the alkali titration, the dissociation of humic samples was characterized by a coefficient that may be determined from titration curves of perchloric acid alone and in the presence of humic substances, according to a procedure derived from Irving-Rossotti's method [5]. Figure 1 shows typical Henderson-Hasselbalch plots leading for instance to results such as: • strong acidity: 0.32 meq 1 −1 • weak acidity groups (COOH): 0.33 meq 1 −1; pK a = 4.54; n = 2.15. • very weak acidity groups (OH): 0.34 meq 1 −1; pK a = 9.33; n = 2.59. ▪ Stability Constants of MetalHumate Complexes (HA) Three different methods have been investigated in the acidic range: • an approach closely related to that of Gregor . [6] but the experimental values of [HA] and [A −] are determined using an original derivation of equations resulting from the application of Irving and Rossotti's concepts [5]. • direct estimation of the free metal ion concentrations from ISE measurements. • application of Marinsky's method [7] accounting for complications arising from the electric field at the surface of the polyelectrolyte which determines the effective concentration [A −]. Reasonable agreement is observed. The existence of mono-complexes MA is expected for the three metal ions, while bis-complexes MA 2 are detected with Pb(II) and Cu(II) ions. Stability of the complexes followed the order Cu > Pb 》Cd. An increase in stability constant was observed with increase in pH and also with decrease in total metal ion concentration (at constant pH), as exemplified by Fig. 2.