Artificial Neural Networks Applied to the Quantitative Structure-Activity Relationship Study of Para-substituted Phenols

The artificial neural network (ANN) model with back-propagation of error is used to study the quantitative structure-activity relationship of para-substituted phenol derivatives between the biological activity and the physicochemical property parameters. Network parameters are optimized, and an empirical rule for dynamically adjusting the network's learning rate is proposed to improve the network's performance. The results showthat the three-layer ANN model gives satisfactory performance, with f(x)=1/(1+exp(-x)) as the network node's input-output transformation function and the number of hidden nodes 10. The network gives the mean square error (rose) of 0.036 when predicting the biological activity of 26 para-substituted phenol derivatives. This result compares favourably with that obtained by the conventional methods.     

强弱酸混合体系被分步直接准确滴定的判据

以一元强酸和一元弱酸组成的混合体系为对象,并以指示剂确定终点,推导能否实现混酸分步直接准确滴定的判据。研究发现:只有待滴定酸的浓度≥0.544 mol·L-1且弱酸的p Ka处于5.6–8.3之间时,才有可能在滴定误差不超过0.1%的要求下实现同浓度混合液中一元强酸和弱酸的分步直接准确滴定,这两个条件会随着滴定准确度要求的变化而变化。     

Rapid determination of the equivalence volume in potentiometric acid-base titrations to a preset pH-II Standardizing a solution of a strong base, graphic location of equivalence volume, determination of stability constants of acids and titration of a mixture of two weak acids

A newly proposed method of titrating weak acids with strong bases is applied to standardize a solution of a strong base, to graphic determination of equivalence volume of acetic acid with an error of 0.2%, to calculate the stability constants of hydroxylammonium ion, boric acid and hydrogen ascorbate ion and to analyse a mixture of acetic acid and ammonium ion with an error of 0.2-0.7%.     

Graphic determination of equivalence volumes in potentiometric titrations of mixtures of weak acids-II Titration of a dibasic acid and determination of its two acid species H(2)A and HA(-)

The method given in part I is used to determine the mixed stability constants of a dibasic acid. Values of log K(H)(HA(2)) = 2.878 and log K(H)(HA(2)) = 4.011 for tartaric acid and 4.077 and 5.335 for succinic acid at mum = 0.1 and T = 25 degrees have been obtained. The method is also applied to titrations of these acids, the error obtained being about 0.5 %, and to titrate a mixture of tartaric acid and potassium hydrogen tartrate with an error of 0.7-1.6%     

偏最小二乘近红外光谱法测定瘦肉脂肪酸组成的研究

利用偏最小二乘将瘦肉的近红外光谱数据分别与其棕榈酸、棕榈油酸、硬脂酸、油酸、亚油酸含量建立校正模型,并用交互校验和外部检验来考查模型的可靠性.各脂肪酸模型的校正相关系数分别为0.9998、0.9844、0.9963、0.9754、0.9969,均方估计残差(RMSEC)分别为0.0231、0.0485、0.111、0.373、0.311,交互校验均方残差(RMSECV)分别为0.509、0.115、0.225、0.848、0.649.应用所建立的各脂肪酸近红外模型对瘦肉脂肪酸组成进行预测,并对各脂肪酸的预测值与气相色谱法测定值进行配对t-检验,结果表明两者差异均不显著(p>0.05).     

A coulometric method for the assay of some easily oxidized organic substances with iodine as an oxidizing agent

A coulometric method for the assay of easily oxidized organic substances, using iodine as an oxidizing agent, has been developed. The principle of the method is to oxidize the substance with an excess of iodine in a water-acetic acid medium and then titrate the iodide formed, with anodically generated silver ion. The titration is followed by dead-stop indication using two platinum electrodes. The method has been applied to the assay of dihydric phenols with hydroxyl groups in the ortho or para positions and to the determination of ascorbic acid in various pharmaceutical preparations. The absolute relative error varied between 0.2 and 2.8% for the 8 phenols investigated and the relative standard deviation between 0.2 and 1%. For standard solutions of ascorbic acid the absolute relative error was 0.9% and for the 6 pharmaceutical preparations investigated the relative standard deviation lay in the range 0.3-0.9%. The method is easy to apply and has a number of advantages over current iodimetric methods.     

Empirical procedure for the calculation of ionization constants of organic compounds in water from their molecular volume

An empirical procedure was used to calculate 278 ionization constants of 271 organic compounds, including phenols, NH acids, benzoic acid derivatives, and mono- and dihydric carboxylic acids, in water. The examined compounds were divided into 11 structural groups. The ionization constants for compounds belonging to a single group were calculated from constant empirical coefficients, molecular volumes, and formulas with an average error of less than 3.5%, the maximal error not exceeding 9.9%.     

Potentiometric determination of the total acidity and concentration of citric acid in wines

The method of potentiometric titration with a copper electrode is used for the determination of the total acidity and concentration of citric acid (CA) in identifying the adulteration of wines. The procedure is suitable for the determination of citric acid in wines in the range from 0.1 to 3.5 g/L in the presence of 30-fold amounts of tartaric, acetic, malic, succinic acids and a 10-fold amount of oxalic acid after the separation of organic carboxylic acids on an AV-17-8 anion exchanger. The procedure was developed and certified for the potentiometric determination of the mass fraction of citric acid in table wines and wine materials with an error not exceeding 20%. The criteria (mass fraction of citric acid, the percentage of citric acid in the total acidity, and the shape of the curves of potentiometric titration) were proposed for revealing the adulteration of the acid composition of wines.     

Processing and error analysis of signals in flow-injection analysis

The processing and error analysis of signals in flow-injection systems were systematically studied by simulation and experimental measurements. The content includes an error analysis for peak-height and peak-area signal, a least-squares filtering procedure applied to the flow-injection curve and a peak recognition to remove interferences from air bubbles. Simulation results were obtained by statistical processing of peak-height and peak-area values from Gaussian curves to which noise had been added. The experimental measurements were done by an automatic flow-injection device to obtain detailed information for each individual point of a peak. 2-(2-Arsenophenylazo)-7-(2,6-dichlorophenylazo-4-sulphonic acid)-1,8-dihydroxynaphthalene-3,6-disulphonic acid (DCSA) was used for measuring physical dispersion alone, and Fe(II)-o-phenanthroline for the measurement of both physical dispersion and chemical reaction. The results from computer simulation and experiments agreed well.     

Empirical method for consideration of solvent effect on the dissociation constants of carboxylic acids

An empirical method was used to calculate 363 dissociation constants of 33 benzoic and acetic acid derivatives in 11 solvents. The relative error in the calculations did not exceed 9.5%, and the average error was no higher than 3%. The calculated values were compared with those obtained by quantum-chemical and other methods for determination of acid dissociation constants in different solvents.     

Kinetic determination of formaldehyde and hexamethylenetetramine with a cyanide-selective electrode

An automatic potentiometric reaction-rate method is described for the determination of formaldehyde and hexamethylenetetramine. The formaldehyde reacts with cyanide, and the reaction rate is followed with a cyanide-selective electrode. The time required for the reaction to consume a fixed amount of cyanide, and therefore for the potential to increase by a preselected amount (8.0 mV), is measured automatically and related directly to the formaldehyde concentration. The average error for the determination of 60–300 μg of formaldehyde in a sample volume of 1.00 ml was about 1.3%. Amounts of hexamethyl-enetetramine in the range 50–250 μg in a sample volume of 0–050 ml were determined after acid hydrolysis to formaldehyde with an average error of about 1.6%. Measurement times were in the range 18–80 s for both determinations. The method has been applied to the determination of hexamethylenetetramine in pharmaceutical preparations.     

Rapid determination of the equivalence volume in potentiometric acid-base titrations to a preset pH-I Theory and applications

A new approach to shorten the time needed for an acid-base titration has been made. The method developed is based on the equation for acid-base titrations derived by Ingman and Still. The equation is transformed into such a form that only one titration point is needed to calculate the equivalence volume when the titration is carried out to a preset pH which can be chosen according to the experimental conditions. The method is used for titration of acetic acid, log K(H)(HA) = 4.65, hydroxylammonium ion, log K(H)(HA) approximately 6.2, and boric acid, log K(H)(HA) approximately 9.1, with an error of 0.1-0.5%. In titration of hydrogen ascorbate ion, log K(H)(HA) approximately 11.3, the error obtained was about 0.3-2%.     

Routine determination of hydroxyphenytoin in urine by high-performance liquid chromatography using an automatic column-switching technique

A high-performance liquid chromatographic method for the determination of the main phenytoin metabolite, hydroxyphenytoin, in the urine of epileptic patients is described. The use of an automated column-switching technique greatly simplifies the pretreatment steps. Thereby, both time and chemicals are saved. The possibility of error arising during the several pretreatment steps is considerably reduced. Following acid hydrolysis of the hydroxyphenytoin glucuronic acid conjugate the sample is diluted with water and after centrifugation is injected onto the pre-column. After washing for a short time with water, the substances which were absorbed on the head of the pre-column were backflushed with water--acetonitrile as eluent onto the analytical column. Separation is achieved by gradient elution using an ODS reversed-phase column with a particle size of 5 microns.     

Étude de l'erreur acide de l'électrode de verre en milieu aqueux: Tome III. Solutions mixtes d'acide et de sel alcalin

The acid error of the glass electrode is studied in mixed solutions of acid and alkaline salt with common anion. Measurements have been made by direct comparison with the hydrogen electrode. The experimental results confirm the view that the acid error arises from anion penetration into the glass. An empirical equation is proposed in which this error is expressed in terms of cation concentration in mixed solution.     

石墨炉原子吸收测定土壤中的Cd Pb

摘要：为了提升石墨炉原子吸收测定土壤样品中Cd和Pb的准确度和测试效率,对样品前处理和仪器测试等条件进行优化选择,确定了低温电热板消解样品时,硝酸、氢氟酸、高氯酸的三酸比例和复溶方式以及采用海光GGX-920型石墨炉原子吸收测定Cd和Pb的升温程序、样品和基体改进剂加入量等条件。应用选定方法条件分别对土壤标准物质GBW07403、GBW07449水系沉积物 GBW07309进行6次测定,测定值Cd的相对标准偏差RSD：2.49%~4.67%,相对误差RE：-2.31% ~ 2.22%,测定值Pb的相对标准偏差RSD：1.98%~ 2.54%,相对误差RE：-1.88% ~ 2.11%。此方法精密度和准确度均达到标准要求,是一种准确而快速的检测方法,适合大批量的环境样品检测。     

二阶导数预处理法在中红外光谱定量分析中的应用研究

介绍了运用最小二乘法建立傅立叶变换中红外光谱定量分析模型的原理和方法。以苯甲酸和邻苯二甲酸氢钾为实验材料获取红外吸收光谱,采用MAT-LAB工程语言编程,分别以吸收光谱和二阶导数光谱,进行中红外光谱定量分析建模波数信息选择,优选出2~4个波数点的光谱信息,建立了中红外光谱定量分析模型。用此模型预测混合物的量,预测值与实际值的相对误差小于5.0%。     

Analysis of a mixture of a known and an unknown weak acid by titration to a preset pH

The preset-pH titration method has been used to indicate the presence of a second acid when the titration curve (pH vs. volume of added titrant) seems to indicate only one acid. By use of the method even small amounts of propionic acid can be detected in an acetic acid solution despite the small value of Δ log K^H_HA = 0·18. Binary mixtures of acids may bs analysed when one acid is known, and log k^H_HA for the unknown acid may be found. Acetic acid, as the known acid, has been determined together with hydrochloric, mandelic, hydroxyacetic or boric acid or ammonium ion, with an error of about 1%. The method can be used in some cases for titration of ternary mixtures of one known and two unknown acids. Only the sum of the unknown acids can then be determined together with the known acid.     

Étude de l'erreur acide de l'électrode de verre en milieu aqueux: Tome II. Solutions d'acides purs

The acid error of glass electrode is studied in pure acid solutions of different natures by direct comparison with hydrogen electrode. This deviation from the hydrogen function is depending on the nature of both the acid involved and the glass electrode used. The potential of this electrode can be expressed by an empirical formula where two parameters appear: one of them is inversely proportional to molar weight of acid, while the other one is also dependent on the glass electrode used.     

Usage of Artificial Neural Network (Back Propagation) in Optimising Salicylic Acid Determination with Ferric(III) Nitrate

Abstract

The use of artificial neural networks (ANN) in optimizing salicylic acid (SA) determination is presented in this paper. A simple and rapid spectrophotometric method for salicylic acid (SA) determination was carried out based on the complexation of salicylic acid–ferric(III) nitrate, SAFe(III). The SA forms a stable purple complex with ferric(III) nitrate at pH 2.45. The useful dynamic linear range is 0.01–0.35 g/L. It has a maximum absorption at 524 nm and the stability is more than 50 hours. The results were used for artificial neural networks (ANNs) training to optimize data. For training and validation purposes, a back‐propagation (BP) artificial neural network (ANN) was used. The results showed that ANN technique was very effective and useful in broadening the limited dynamic linear response range mentioned to an extensive calibration response (0.01–0.70 g/L). It was found that a network with 22 hidden neurons was highly accurate in predicting the determination of SA. This network scores a summation of squared error (SSE) skill and low average predicted error of 0.0078 and 0.00427 g/L, respectively.