滴定分析中常见的误差及其对分析结果的影响

滴定分析中不可避免地存在各种误差,绝大多数误差会对分析结果造成一定的影响。然而,不少同学对误差的判定感到难以理解,经常得到错误的结论。为了帮助学生理解误差产生的根本原因,正确分析误差对分析结果的影响,本文结合实例根据滴定分析中误差产生的原因对误差进行了分类(主要包括由终点指示方式、试剂及分析人员的操作引起的误差),并结合公式对于误差对分析结果造成的影响进行了分析。

Determination of Stability Constants of Cadmium-Glycine Complexes by a Unified Treatment for Potentiometric and Polarographic Data

The cadmium（Ⅱ）-glycine system was studied by the two experimental techniques, ion sensitive electrode （ISE） and differential pulse polarography （DPP）, and the experimental data obtained were used by a unified mathematical treatment to calculate the complex stability constants. The combination of the two techniques is of many advantages as ISE can be performed at low [LT]：[MT] ratios and significantly higher [MT], whereas DPP could be used well at large [LT]：[MT] ratios and much smaller [MT]. This makes it possible to study a metal-ligand system in a relatively broader range of experimental conditions that, in turn, provides more information about the metakligand system of interest. Applying the unified mathematical treatment to the cadmium-glycine system, two new complexes MHL and ML2（OH） as well as three complexes ML, ML2 and ML3, reported in literatures, could be modeled and all their stability constants have been refined.

电位法测定土壤pH值不确定度评定

土壤pH值是土壤分析中的一项重要指标,许多土壤分析项目需要根据土壤的pH值进行选择相应的分析方法,检测结果也都与土壤pH值有关联,是审核其他项目结果的重要依据。本文根据测量不确定度评定理论,结合电位法对土壤pH值测量不确定度进行全面分析。从测定的重复性、样品称重、浸提前处理、仪器示值误差、仪器校准5个方面归纳不确定度来源,对各不确定度分量详细评定,并由各不确定度分量合成得到土壤pH值测定的相对标准不确定度及扩展不确定度,得出土壤pH值的测量不确定度为8.24±0.03。在分析过程中,对比各个不确定度分量对土壤pH值测定不确定度的贡献大小,其中pH标准缓冲溶液校准引入的不确定度分量最大。

Potentiometric detection of polyions based on functionalized magnetic nanopaiticles

A novel potentiometric detection strategy based on functionalized magnetic nanoparticles has been developed for rapid and sensitive sensing of polyions.Highly dispersed magnetic nanoparticles coated with ion exchanger and plasticizer could promote an in situ cooperative ion-pairing interaction between the ion exchanger and the polyion analyte in sample solution by dramatically reducing the mass-transfer distance.With applying a magnetic field,the nanoparticles can be attached to the surface of ion exchanger free polymeric membrane.The observed potential signals are related to the polyion concentrations.The proposed polymeric membrane electrode exhibits a linear relationship between the greatest potential response slope（dE/dt） and the logarithm of protamine concentration in the range of 0.05-5μg/mL with a lower detection limit of 0.033μg/mL.

Cyclam Modified Carbon Paste Electrode as a Potentiometric Sensor For Determination of Cobalt(Ⅱ) Ions

A new modified carbon paste electrode based on cyclam as a modifier was prepared for the determination of Co(Ⅱ) ions. The proposed electrode shows a Nernstian slope 28.4 mV per decade over a wide concentration range 5.0×10 -6_1.0×10 -1 mol/L of Co 2+ ions with detection limit 2.5×10 -6 mol/L. The sensor exhibits good selectivities for Co 2+ over a wide variety of other cations. It can be used as an indicator electrode in potentiometric titration of cobalt(Ⅱ) ions as well as in direct determination of cobalt(Ⅱ) ions in wastewater of acidic cobalt electroplating bath. The electrode shows Nernestian behavior in a solution of 25% ethanol.

Studies on the Coordination of Pr(Ⅲ) with Adrenaline by Potentiometry and Absorption Spectroscopy

The stability constant for complex of Pr(Ⅲ) with adrenaline has been determined by potentiometric titration under biological conditions (37℃ and 0.15mol/L NaCl). The absorption spectra of the Pr(Ⅲ)-adrenaline system exhibit characteristic bands of Pr(Ⅲ) at lower pH values. However, the charge transfer band which is due to the coordination of Pr(Ⅲ) with adrenaline has been observed at higher pH values.

Potentiometric Determination of Chloride and Cyanide Ions in Nanoliter Samples with Capillary Indicating Electrodes

The samples of 25 nL of chloride and cyanide were determined with a kind of glass capillary indicating electrodes by potentiometry. Silver was deposited by the silvermirror reaction on the inner surface of the glass capillary with a volume of 25 nL. The AgCl film and the AgI film were formed by the treatment of the capillary with a FeCl 3 solution and further with a KI solution. The samples were collected automatically into the capillary by the capillary ascension. The procedure in the measurement was merely to place a reference electrode and a capillary indicating electrode vertically on a strip filter paper containing a 01 mol/L NaNO 3 solution. The various i.d. of the capillary electrodes were used to determine samples ranging from 25 nL to dozens of microliters. The ranges of the linear response to chloride and cyanide were 32×10 -4 —22×10 -1 mol/L and 10×10 -5 —10×10 -1 mol/L with an average slope of 552 and 52—62 mV/decade , respectively. The samples with various concentrations of chloride and cyanide were determined, and the relative standard deviations were less than 7%.

Potentiometric determination of diclofenac in pharmaceutical formulation by membrane electrode based on ion associate with base dye

The characteristics, performance and application of membrane electrode based on ion associate of diclofenac with base dye Safranine T are described. The electrode response to diclofenac has the sensitivity of 47 ± 1.0 mV decade(?1) over the range of 5 × 10(?5) to 5 × 10(?2) mol/L at pH 6–12, and the detection limit of 3.2 × 10(?5) mol/L. The electrode is easy assembled at a relatively low cost has fast response time (2–4 s) and can be used for a period up to 3.5 months without any considerable divergence in potential. The proposed sensor displayed good selectivity for diclofenac in the presence of different substances. It was used to determine diclofenac in pharmaceuticals by means of the standard addition method.

In situ modifed screen printed and carbon paste ion selective electrodes for potentiometric determination of naphazoline hydrochloride in its formulation

The construction and performance characteristics of new sensitive and selective in situ modified screen printed （ISPE） and carbon paste （ICPE） electrodes for determination of naphazoline hydrochloride （NPZ-HC1） have been developed. The electrodes under investigation show potentiometric response for NPZ-HC1 in the concentration range from 7.0 x 10-7 to 1.0 x 10-2 M at 25 ~C and the electrode response is independent of pH in the range of 3.1-7.9. These sensors have slope values of 59.7＋0.6 and 59.2＋0.2 mV decade-l with detection limit values of 5.6 ~ 10-7 and 5.9 x 10-7 M NPZ- HC1 using ISPE and ICPE, respectively. These electrodes show fast response time of 4-7 s and 5-8 s and exhibits lifetimes of 28 and 30 days for ISPE and ICPE, respectively. Selectivity for NPZ-HC1 with respect to a number of interfering materials was also investigated. It was found that there is no interference from the investigated inorganic cations, anions, sugars and other pharmaceutical excipients. The proposed sensors were applied for the determination of NPZ-HC1 in pharmaceutical formulation using the direct potentiometric method. It showed a mean average recovery of 100.2% and 102.6% for ISPE and ICPE, respectively. The obtained results using the proposed sensors were in good agreement with those obtained using the official method. The proposed sensors show significantly high selectivity, response time, accuracy, precision, limit of detection （LOD）and limit of quantification （LOQ） compared with other proposed methods.

Determination of Trace Molybdenum(VI) by Oscillographic Potentiometric Catalyzing Kinetic Method of Simplex Optimization

Under the optimum experimental conditions (i. e., pH=1.6, c KI=10.6 mmol/L, =5.4 mmol/L, =1.30 mmol/L) derived from simplex operations, trace molybdenum(VI) was determined through the agency of its catalytic effect on the reaction of H2O2 with I? in acid medium. The detection limit and linear calibration range of molybdenum were obtained as 0.5×10?6 mol/L and 0.5×10?6～120×10?6mol/L respectively. This method was applied to the examination of the industrial waste water, and the recovery ratios were found to be in the range of 101% and 107%. Key words oscillographic potentiometry - catalyzing kinetic analysis - simplex optimization - molybdenum Supported by the National Natural Science Foundation of China (Grant No. 20175013) and the Higher Education Development Foundation of Shanghai in China (Grant No. 01A17)

Potentiometric Determination of Stability Constants of Sulphathiazole and Glycine-Metal Complexes

Binary and ternary complexes of (Fe(III), Pb(II), Co(II), Al(III), La(III), Sr(II), Cr(III), Ti(II) and Zr(II)) with sulphathiazole (as primary ligand) and amino acid glycine (as secondary ligand) have been studied potentiometrically at 25°C ± 0.1°C and I = 0.1 M NaClO4 in 25% (v/v) pure ethanol-water medium. Although there are many methods available to study the stability of metal-ligand complexes, pH-metry is most frequently used. In extension of our study on solution equilibria, we used Calvin-Bjerrum method for the calculation of stability constants. Stoichiometries and stability constants of binary systems containing the above metal ions in a 1:1 and 1:2 and/or 1:3 ratios were also determined to compare the effect of the secondary ligand on (1:1) Metal:Sulphathiazole system. The protonation constants of the complexes were evaluated for the system M:Sulphathiazole:Glycine = 1:1:1. The order of stability of the binary and ternary complexes was examined. It was found that glycine adds preferably [M-Sulfathiazole] rather than to the aqueous complexes of metal ions. In all cases 1:1:1 complex was formed.

Carbon Nanotubes as Nanosensor for Differential Electrolytic Micropotentiometry

Carbon nanotubes as nanosensor were synthesized on the surface of the silver electrode using floating catalyst chemical vapor deposition reactor. Acetylene gas was used as a carbon source, ferrocene as a source of the iron nanocatalyst and hydrogen as a carrier and an activate agent. Several runs were performed to find the optimum conditions that produce sensitive Ag-CNTs electrodes. The electrodes obtained at each run were characterized by SEM. The optimum conditions that produce sensitive Ag-CNTs were found to be at a reaction time of 15 minutes, reaction temperature of 700°C, hydrogen flow rate of 25 ml/min and an acetylene flow rate of 75 ml/min. These optimum conditions were confirmed from the normal behavior of the resulting Ag-CNTs electrodes in the titration of 10 μL of chloride using dc differential electrolytic potentiometry. When conditions that differ from the optimum ones were applied in the preparation of the Ag-CNTs electrodes, abnormal titration curves were obtained. The superiority of the Ag-CNTs electrodes was demonstrated by the successful applications of these electrodes as an indicating system in the micro titrations of different volumes of cyanide solution with silver nitrate reach. By applying this technique a volume of 1.2 μL sample of cyanide was successfully titrated. The normal behavior of the Ag-CNts electrodes was compared to that of the normal silver electrodes which exhibit an abnormal behavior.

A New Potentiometric Sensor for Determination of Pethidine hydrochloride in Ampoules and Urine

A simple, precise, rapid and low-cost potentiometric method for pethidine determination in pharmaceuticals and urine is proposed. A chemically modified carbon paste electrode (CMCPE) for pethidine hydrochloride (PDCl) based on pethidine-phosphotungstate (PD-PT) as ion-pair complex was prepared and fully character-ized in terms of composition, usable pH range, response time and temperature. The pethidine electrode showed Nernstain responses in the concentration range 2.1 × 10–6-1.0 × 10–2 M with a detection limit of 7.3 × 10-7 and usable within the pH range 3.5-6.6. This sensor exhibited a fast response time (about 5-8 s), good stability. The value (dE/dt) of the electrode was found to be 0.00071 V/?C, which indicates fairly high ther-mal stability. Selectivity coefficients determined by matched potential method (MPM) and separate solution method (SSM) showed high selectivity for PDCl with respect to a large number of inorganic cations, organic cations, sugars and some common drug excipients. The sensor could be used successfully in the estimation of PDCl in ampoules and in spiked urine samples.

Imprinted Polymer Inclusion Membrane Based Potentiometric Sensor for Determination and Quantification of Diethyl Chlorophosphate in Natural Waters

Biomimetic potentiometric sensor for the determination of diethyl chlorophosphate was developed using im- printed polymer inclusion membrane strategy. Semi-covalent imprinted and non-imprinted polymer particles were synthesized and found that non-imprinted polymer inclusion membrane was unstable in contrast to im- printed polymer inclusion membrane in determination and quantification of diethyl chlorophosphate. Im- printed polymer inclusion membrane based sensor found to be pH dependant with a 5 min equilibrium response time at pH = 10.5 and linearly responds to diethyl chlorophosphate in the concentration range of 1 × 10–9 to 1 × 10–4 and 1 × 10–4 to 1 × 10–1 mol●L–9 with a detection limit of 1 × 10–9 mol●L–1 (0.17 ppb). It was found that diethyl chlorophosphate response was selective against various selected interferents like pinacolyl methylphosphonate, dimethyl methyl phosphonate, methylphosphonic acid, Phorate and 2, 4-D. The devel- oped sensor was found to be stable for 3 months and can be reusable more than 30 times without loosing sensitivity. The developed sensor was successfully applied for the determination of diethyl chlorophosphate in natural waters.

Analytical expressions of steady-state concentrations of species in potentiometric and amperometric biosensor

A mathematical model of potentiometric and amperometric enzyme electrodes is discussed. The model is based on the system of non-linear steady-state coupled reaction diffusion equations for Michaelis-Menten formalism that describe the concentrations of substrate and product within the enzymatic layer. Analytical expressions for the concentration of substrate, product and corresponding flux response have been derived for all values of parameters using Homotopy analysis method. The obtained solution allow a full characterization of the response curves for only two kinetic parameters (The Michaelis constant and the ratio of overall reaction and the diffusion rates). A simple relation between the concentration of substrate and products for all values of parameter is also reported. All the analytical results are compared with simulation results (Scilab/Matlab program). The simulated results are agreed with the appropriate theories. The obtained theoretical results are valid for the whole solution domain.

Potentiometric, spectrophotometric, conductimetric and thermodynamic studies on some transition metal complexes derived from 3-methyl-1-phenyl- and 1, 3-diphenyl-4-arylazo-5-pyrazolones

A new 3methyl1phenyl4arylazo5pyrazolone and 1, 3diphenyl4arylazo5pyrazolone have been synthesized and characterized by elemental analysis, IR, mass and 1H NMR spectra. The acid dissociation constants (pKa values) of the investigated ligands were determined potentiometrically and spectrophotometrically. The stability constants of the transition metal ions (VO2+, Cr3+, Mn2+, Co2+, Ni2+, Cu2+ and Zn2+) with the investigated ligands were determined potentiometrically at different ionic strengths (0.167, 0.1, 0.05 and 0.025 M) NaCl at 25oC and different temperature (25, 30, 35, and 45oC). The values of stability constants were found to decrease with increasing ionic strengths and temperature. The stoichiometries were studied using spectrophotometric and conductimetric methods, the results indicate the existence of 1:1 and 1:2 (M:L) metal:ligand species. The relationships between the stability constants of the complexes, ionization constants of the ligands have been discussed and correlated. The thermodynamic parameters (?G, ?H and ?S) and the thermodynamic stability constants for all of the investigated complexes were determined potentiometrically.

Differential Electrolytic Potentiometric, a Detector for FIA Determination of Procainamide in Pharmaceutical Preparations

Differential electrolytic potentiometry （DEP） was coupled with Flow injection analysis （FIA） technique for the determination of Procainamide in pharmaceutical preparations. Platinum electrodes were used as an indicating system to follow the oxidation of Procainamide with cerium（IV）, and permanganate in an acidic medium. The oxidation reactions of Procainamide with Ce（IV） and/or permanganate are fast enough to permit its determination by flow injection in sulfuric acid media. The univariate method was employed to optimize the variables such as the current density, the flow rate, the oxidant concentration and the concentration of sulfuric acid. The proposed method was linear in the range 20-100 μg.mL^-1 , the DL and R2 values were 12 μg.mL^-1 and 0.995 respectively. The procedure was applied successfully to the determination of Procainamide in commercial tablets. The results of this study were favorably compared statistically with those obtained with official methods.

Application of H-Point Standard Addition Method and Multivariate Calibration Methods to the Simultaneous Kinetic-Potentiometric Determination of Cerium(IV) and Dichoromate

A kinetic-potentiometric method for simultaneous determination of Cerium(IV) and dichoromate (Cr2O72-) by H-point standard addition method (HPSAM), partial least squares (PLS) and principal component regression (PCR) using fluoride ion-selective electrode (FISE) is described. In this work, the difference between the rate of the oxidation reaction of Fe(II) to Fe(III) in the presence of Ce4+ and Cr2O72- is based of the method. The rate of consume fluoride ion for making complex is detected with a FISE. The results show that simultaneous determination of Ce4+ and Cr2O72- can be done in their concentration ranges of 1.0-30.0 and 0.1-20.0 μg/mL, respectively. The total relative standard error for applying the PLS and PCR methods on 8 synthetic samples was 2.97 and 3.19, respectively in the concentration ranges of 1.0-30.0 μg/mL of Ce4+ and 0.1-20.0 μg/mL of Cr2O72-. In order for the selectivity of the method to be assessed, we evaluated the effects of certain foreign ions upon the reaction rate and assessed the selectivity of the method. The proposed methods (HPSAM, PLS and PCR) were evaluated using a set of synthetic sample mixtures and then applied for simultaneous determination of Ce4+ and Cr2O72- in different water samples.

等电位间隔-电位滴定法

对两点电位滴定法作出改进,提出了等电位间隔-电位滴定法。由数学推导证明,当P1的数值(P1为V1与Ve的比值以百分率表示;V1为在第1测量点所耗滴定剂的体积;Ve为滴定终点时所耗滴定剂的体积)达到95%,且在两点间的电位差值(ΔE)大于30mV时,滴定的相对误差(T)的计算值可达到小于0.1%。据此,在滴定过程中,记录两测量点之间的ΔE为30mV时的V及E值,当有两组数据的ΔE达到30mV并算得此时的T值≤0.1%时,即可停止滴定,并根据所给公式计算Ve及滴定结果。将此方法应用于包括中和反应、沉淀反应及氧化还原反应等多种滴定体系的电位滴定,所得结果与常规电位滴定法的测定结果一致。