Masking of manganese in relation to stepwise complexometric determination of calcium, magnesium and manganese

A critical study has been made of the masking of manganese with potassium cyanide in alkaline medium for complexometric titration of calcium and magnesium. It has been found that there is incomplete oxidation to manganese(III) unless the solution is aged for a sufficient period or air is bubbled through the solution at 35 ± 5° for 10 min. The manganese(III) complex can be reduced with ascorbic acid for titration with EDTA. Procedures are given for stepwise titration of magnesium, manganese and calcium in silicate materials. Mixed indicators are used, to improve the end-points.     

A simultaneous EDTA-metric determination of calcium and magnesium with antipyrylazo III and thymolphthalexon

The simultaneous determination of calcium and magnesium in natural materials (limestone and dolomite) can be made by EDTA-metric titration at pH 13.0–13.3 and 10.2–10.5 indicated by antipyrylazo III (diantipyrylaxo; 3,6-diantipyrylazo-4,5-dihydroxy-2, 7-naphthalenedisulfonic acid) and thymolphthalexon (3′,3″-bis[bis(carboxymethyl)aminomethyl]-thymolphthalein), respectively. Sodium cyanide and triethanolamine are used as masking reagents, hydroxylamine hydrochloride is the bleaching (reducing) reagent to remove the antipyrylazo III color for the subsequent magnesium titration. Antipyrylazo III is also a convenient indicator for EDTA-metric determination of copper(II) in aqueous ammonia.     

Determination of calcium in biological material The use of calcein as an indicator in the edta titration

The EDTA titration of calcium using the change in fluorescence of Calcein indicator under ultraviolet illumination at the end-point was very satisfactory for essentially pure calcium chloride solutions. In order to use this method for biological samples, it was found best to separate the calcium by an oxalate precipitation at a pH of 4.7, convert the calcium oxalate to calcium carbonate, and dissolve the calcium carbonate in hydrochloric acid.     

Synthesis of Two N,N′-bis(N,N-dialkylamide) Derivatives of Diethylenetriaminepentaacetic Acid and the Stabilities of Their Complexes with Gd3+, Ca2+, Cu2+, and Zn2+

Two bis(N,N-dialkylamide) derivatives of DTPA [(carboxymethyl)iminobis (ethylenenitrilo) tetraacetic acid], DTPA-BDMA = the bis(N,N-dimethylamide) and DTPA-BDEA = the bis(N,N-diethylamide) were synthesized. Their protonation constants were determined by potentiometric titration in 0.10 M Me₄NNO₃ and by NMR pH titration at 25.0 ± 0.1 °C. Stability and selectivity constants were measured to evaluate the possibility of using the corresponding gadolinium(III) complexes for magnetic resonance imaging contrast agents. The stability constants of gadolinium(III), copper(II), zinc(II), and calcium(II) complexes with DTPA-BDMA and DTPA-BDEA were investigated quantitatively by potentiometry. The stability constant for gadolinium(III) complexes is larger than those for Ca(II), Zn(II), and Cu(II) complexes. The selectivity constants and modified selectivity constants of the amides for Gd³⁺ over endogenously available metal ions were calculated. Effectiveness of these two ligands in binding divalent and trivalent metal ions in biological media is assessed by comparing pM values at physiological pH 7.4. Spin-lattice relaxivity values R₁ for Gd(III) complexes were also determined. The observed relaxivity values were found to decrease with increasing pH in the acid range below pH 4 and relaxivity values became invariant with respect to pH changes over the range of 4–10. ¹⁷O NMR shifts showed that the [Dy(DTPA-BDMA)] and [Dy(DTPA-BDEA)] complexes had one inner-sphere water molecule. Water proton spin-lattice relaxation rates for the [Gd(DTPA-BDMA)] and [Gd(DTPA-BDEA)] complexes were also consistent with one inner-sphere gadolinium(III) coordination position.     

Potentiometric determination of EDTA and NTA in detergents

A method is described for the potentiometric titration with iron(III) of EDTA and NTA in detergents, a platinum electrode being used as indicator. EDTA and NTA were extracted at pH 9 and 50–60 °C in the presence of magnesium(II). Interference from polyphosphates was minimized by hydrolysis to orthophosphate followed by remotion as magnesium ammonium phosphate. Complexometric titration was carried out at pH 4.7 in acetate medium, in the presence of ferroin. A preliminary cleaning of the platinum electrode with thiosemicarbazide in hydrochloric acid was found to improve significantly the potential measurements. A mean recovery of 93% for EDTA and 89% for NTA was observed.     

Gibbs energies of formation of hydroxides of lanthanides and yttrium

The pH values of the formation of hydrates in solutions of yttrium(III), cerium(III), samarium(III), europium(III), erbium(III), and ytterbium(III) were determined by conductometric titration. The solubility products and Gibbs energies of formation for hydroxides for the elements listed were calculated. The average Gibbs energy of dissolution for lanthanide hydroxides was found to be approximately 149.83 ± 0.90 kJ/mol. The Gibbs energies of formation for hydroxides of the remaining lanthanides were assessed on this basis.     

Determination and differentiation of nitrilotriacetic acid and ethylenediaminetetra-acetic acid

Ethylenediaminetetra-acetic acid (EDTA) and nitrilotri-acetic acid (NTA) can be differentiated and determined by titration with metal ions to visual metallochromic dye end-points. EDTA can be determined without interference from NTA, either by titrating with copper(II) at pH 5 using PAN indicator, or by titrating with iron(III) at pH 6 and 70 degrees using Tiron indicator. The total chelating power (EDTA + NTA) can be determined either by titrating with lead(II) at pH 4.4 using dithizone indicator, or by titrating with iron(III) at pH 3.5 using Tiron indicator ; NTA is determined by difference. The lowest concentration at which NTA can be determined in EDTA by titration to the iron(III)-Tiron end-point is about 1 wt.%. The apparent stability constants of the iron(III)-Tiron complexes under the conditions of the titration at pH 3.5 and pH 6 have been determined using the method of continuous variations.     

光度络合滴定(Ⅰ)——微量铁(Ⅲ)的滴定

本文提出在pH=0.8和波长540nm,以二甲酚橙作指示剂,用铋盐作回滴剂光度滴定3-100微克铁。本法选择性很高,大量铝、钛(Ⅳ)、铬(Ⅲ)、铜、铅、锌、镉、锰、镧、铈(Ⅲ)、钨(Ⅶ)、钼(Ⅵ)、钒(Ⅴ)、砷(Ⅲ)、镁、钙、银以及适量的汞、钍、锑(Ⅲ)、镍、氟离子、氯离子和磷酸根等不干扰,应用适当的隐蔽剂,400倍于铁的铝以及适量的钍、锆和锡也不干扰。应用本法,不必分离便可滴定石英石、石英砂、铝合金、纯铝以及水样中的铁。     

配位滴定法测定锌含量的合适酸度条件

调节待测体系的pH值是配位滴定中一种重要的选择性滴定手段。采用CTE1.0程序计算了不同酸度条件下的终点误差,同时用配位滴定法测定了模拟样品和市售药品中的锌含量。计算结果和实测结果均表明,在不含钙离子的待测体系中,测定锌离子含量的合适酸度条件为pH=6;在钙、锌混合离子溶液中,测定锌含量的合适酸度条件为pH=5。平行测定实验的相对标准误差为1.63%,加标回收率为99.1%-101.8%。     

Natural pyrite as an electrochemical sensor for potentiometric titrations with EDTA, mercury(II) and silver(I)

The results obtained in potentiometric titrations of copper(II), mercury(II) and iron(III) with standard EDTA solutions are presented. The titration of copper(II) at pH values in the range from 8.11 to 10.99 (ammonia buffer) and the titration of mercury(II) and iron(III) at pH values from 3.59 to 5.65 (acetate buffer) were performed. The titration end-point (TEP) was detected with an indicator electrode made from natural crystalline pyrite as an electrochemical sensor. The results obtained in potentiometric titration with the pyrite electrode were compared with those obtained using a platinum electrode (Fe³⁺), a Cu ion selective electrode (Cu²⁺) and a Hg electrode (Hg²⁺). Accurate and reproducible results with good agreement were obtained, but higher potential changes at the TEP were obtained using the pyrite electrode. In the course of the titration the potential was established within less than 1 min, whereas at the TEP it was within about 2–3 min. The potential changes at the TEP were in the range from 60 to 200 mV per 0.1 ml EDTA, according to the stability constant of the complex formed. The highest potential changes, ranging from 160 to 200 mV, were obtained in the titration of iron(III) at pH 3.59. Reverse titration was also performed and accurate and reproducible results were obtained. Moreover, titration of halogenide and thiocyanate with standard mercury(II) solutions, as well as cyanide with silver(I) solution, were performed and accurate and reproducible results were again obtained. Received: 20 February 1998 / Accepted: 19 November 1999     

Sequential potentiometric complexometric redox determination of iron(III) and cobalt(II) with application to alloys

A simple potentiometric method is presented for successive determination of iron(III) and cobalt(II) by complexometric titration of the iron(III) with EDTA at pH 2 and 40 degrees , followed by redox titration of the cobalt(II) complex with 1,10-phenanthroline or 2,2'-bipyridyl at pH 4-5 and 40 degrees , with gold(III). There is no interference in either determination from common metal ions other than copper(II), which severely affects the cobalt determination but can be removed by electrolysis. The method has been successfully applied to determination of iron and cobalt in Kovar and Alnico magnet alloys.     

ESR study of aqueous micellar solutions of perfluoropolyether surfactants with the use of fluorinated spin probes

The acid-base properties of synthetically prepared and well-characterized hydroxyapatite (HAP) in contact with KNO3 solutions were investigated at 25 degrees C, through potentiometric titrations, zeta-potential measurements, and surface complex modeling. Aliquots of suspension were withdrawn every 0.5 pH unit during the titration procedure and analyzed for calcium and phosphate. It was found that, even for rapid titration experiments, a remarkable amount of H+ ions (H+dissol.) is consumed in the bulk solution in reacting with species coming from the dissolution of HAP. These H+ ions must be taken into account in the H+ mass balance, in order for true value for the point of zero charge (pzc=6.5+/-0.2) and consequently true value for the surface charge (sigma0) to be obtained. Besides the conventional potentiometric titration technique, it was found that pzc may be determined much more easily as the intersection point of the suspension titration curve and the blank one modified to include the amount of H+dissol. obtained at one ionic strength. Finally, a surface complexation model was proposed for the development of surface charge. Experimental data were satisfactorily fitted by using the value of 4.2 F m-2 for the capacitance.     

Isothermal titration calorimetric studies of the pH induced conformational changes of bovine serum albumin

Bovine serum albumin (BSA) is a soft globular protein that undergoes conformational changes through several identified transition steps in the pH range 2–13.5. The ability to change conformation makes BSA capable of complexing different ligands from fatty acids to cations or drugs and carries them in the bloodstream. Microcalorimetric titration of BSA with NaOH solution was performed to measure the enthalpy of conformational changes. Two exothermic enthalpy changes were found in the course of the titration between pH 3 and 9.5, which can be identified with the E–F, and the F–N transitions. The enthalpy change at pH 3.5 (transition from the E to the F form of BSA, folding of intra-domain helices in domain I) is independent of the protein concentration. The second transition (F–N, folding of domain III) was observed at pH 4.8 for the 0.1% BSA solution, but it shifted to higher pH values as the protein concentration increased to 0.2% and 0.3%. The tightening of the protein structure with increasing pH was verified measuring intrinsic fluorescence of tryptophan residues. At even higher pH value, pH 10.5, fluorescence measurements revealed protein expansion. The BSA conformational changes were also measured by dynamic light scattering. The hydrodynamic diameter was smaller at the i.e.p. of BSA (5–7 nm at pH ~5) and larger at the two ends of the pH range (17.5 nm at pH 2 and 8.3 nm at pH 10).     

EDTA络合滴定法快速测定矿石中钙、镁

试样用盐酸、硝酸、氢氟酸、高氯酸分解,在pH值为6～9时,经六次甲基四胺-铜试剂分离铁、铝、镍、钴、铅、锌、铜、镉、锰等干扰元素后,在pH=10的氨水和氯化铵缓冲溶液中,以酸性铬蓝K-萘酚绿B为指示剂,用EDTA络合滴定法测定钙镁合量;另在氢氧化钾溶液中,用钙试剂为指示剂,以EDTA络合滴定法测定钙量,从而计算镁的含量。当样品钙高镁低或者镁高钙低时,低含量的镁量或钙量(<5%)可用电感耦合等离子体原子发射光谱(ICP-AES)法准确测定,使结果更准确。实验中对三个标准样品中的钙和镁进行多次测定,结果与认定值相符,相对标准偏差在0.69%～1.3%(n=7),加标回收率在99%～102%。方法实用性强,已经成功应用于各类矿石中钙镁的检测。     

Sorption of Eu(III) on GMZ bentonite in the absence/presence of humic acid studied by batch and XAFS techniques

Bentonite has been extensively studied because of its strong sorption ability and low permeability. In this work, the Na-bentonite from Gaomiaozi County (China) has been characterized by XRD, FTIR and acid-base titration. The sorption of Eu(III) on Na-bentonite in the absence/presence of humic acid (HA) was studied at T = 25 ± 2 °C and in 0.01 mol/L NaClO₄ solution. The effects of pH, HA, contact time and initial Eu(III) concentrations were also investigated. The results indicate that the sorption of Eu(III) on Na-bentonite was dependent on pH values. The presence of HA had little effect on Eu(III) sorption at low pH values, but decreased Eu(III) sorption at high pH values. X-ray absorption fine structure spectroscopy (XAFS) was applied to characterize the local structural environment of the adsorbed Eu(III) on bare Na-bentonite and HA-bentonite hybrids. The results indicate that Eu(III) was bound to O atoms at a distance of about 2.39 Å at pH 4.15. The results are crucial for the evaluation of the sorption and migration of other trivalent lanthanides and actinides in bentonite as backfill materials.     

Amperometric titrations of cobalt(II) with hexacyanoferrate(III) in ammonium citrate and glycine media

An amperometric titration of cobalt(II) with hexacyanoferrate(III) in aqueous ammonium citrate or aqueous glycine solution at pH 9.8 or pH 8.0 respectively, is reported. Cobalt concentrations of 2-30 mg/l were successfully determined. In citrate solutions cerium(III) and iron(III) interfered, and in glycine solutions, copper(II) and vanadium(V).     

The titration of bismuth with complexone III at pH 2.0–2.8

A titration procedure for Bi is described; by adding an excess of complexone III (Versene) and back-titrating at pH 2.0–2.8 with standard thorium nitrate solution using alizarin-S as an indicator, it is possible to determine Bi with a precision of about 0.3%.By choice of the proper pH it is possible to eliminate most of the interfering elements, but at pH 2.0 Fe and Ni still interfere, and Cu only to a slight extent.Attention is drawn to the fact that it may be possible to determine Bi at pH 2.0 by direct titration with complexone III in the presence of much tartrate, using thiourea as an indicator. This method might be of value in the presence of Sn and Sb.     

Chelatometric titration of bismuth with hedta

A new chelatometric method for the determination of Bi in bismuth-base alloys, low-melting alloys and reagent-grade bismuth salts is proposed. It is based on the chelation of Bi(III) with HEDTA, which is added in substoichiometric amount in perchloric acid medium to eliminate the interfering side-reactions of olation and oxolation, followed by pH adjustment and titration at pH 2 with Semi-Xylenol Orange as indicator. It is more selective than the classical EDTA method, and the accuracy and precision are enhanced. The coefficient of variation was found to be ˜ 0.1%.     

Determination of formation constants of hydroxo and carbonate complexes of Pr(3+) in 2 M NaCl at 303 K

The hydrolysis of praseodymium III in 2 M sodium chloride at 303 K was studied. Two methods were used: pH titration followed by a computational refinement and solvent extraction in the presence of a competitive ligand. The hydrolysis constants obtained by pH titration were: logbeta(1,H)=-7.68+/-0.07, logbeta(1,2H)=-15.10+/-0.03, and beta(1,3H)=-23.80+/-0.04. The stability constants of praseodymium carbonate complexes were determined by pH titration as well and were: logbeta(1,CO(2-)(3))=5.94+/-0.08 and logbeta(1,2CO(2-)(3))=11.15+/-0.15. Praseodymium carbonate species were taken into consideration for calculating the first hydrolysis constants by the solvent extraction method and the value obtained was: logbeta(1,H)=-7.69+/-0.27. The values for logbeta(1,H) attained by both methods are the same. The species-distribution diagram was obtained from the stability constants of praseodymium carbonate complexes and hydrolysis products in the conditions of the present work.     

Recovery and separation of Ce<Superscript>3+</Superscript> and Y<Superscript>3+</Superscript> ions from aqueous solutions by ion flotation

Experimental dependences of the distribution coefficients of cerium(III) and yttrium(III) dodecyl sulfates on the pH value of the equilibrium aqueous phase in the course of ion flotation are reported. Conditions for separation of cerium(III) and yttrium(III) are discussed. A value of the dissociation constant found from experimental results of potentiometric titration of dodecylsulfuric acid with sodium hydroxide is presented.