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Purity of potassium hydrogen phthalate, determination with precision coulometric and volumetric titration - A comparison
Abstract
The mass fraction of potassium hydrogen phthalate (KHP) from a specific batch was certified as an acidimetric standard. Two different analytical methods on a metrological level were used to carry out certification analysis: precision constant current coulometric and volumetric titration with NaOH. It could be shown that with a commercial automatic titration system in combination with a reliable software for the end-point detection it is possible to produce equivalent results with the same accuracy in comparison to a definite method handled by a fundamental apparatus for traceable precision coulometry. Prerequisite for titrations are that a high number of single measurement are applied which are calibrated with a high precision certified reference material.
... Initially, coulometry has been implemented by some National Metrology Institutes (NMIs) for the quantification of pure substances, such as potassium hydrogen phthalate (KHP), 10 as well as other acidimetric standards. [11][12][13] Coulometric titration involves the measurement of physical quantities such as time, mass, potential difference, and electrical resistance. As in any experimental technique, a thorough understanding of its physical and chemical principles is necessary to minimize the impact of variables that could affect the quality of results. ...
... As in any experimental technique, a thorough understanding of its physical and chemical principles is necessary to minimize the impact of variables that could affect the quality of results. 11,14 The previously mentioned quantities must be accurately measured, recorded, and analyzed during the coulometric experiment to obtain reliable substance content results. This process is complex because it requires several instruments to obtain the necessary data. ...
... It also uses the TC(NLR) model 15 for the IT, Equation 11: (11) and for the FT, Equation 12: ...
Coulometry is a well-established technique for certified reference materials (CRM). However, commercial coulometers are currently expensive and utilize outdated software. The National Metrology Institute of Colombia (INMC) has developed a cheaper and modular coulometer with powerful software capable of controlling the instrument, and acquiring pH, the potential difference between generator electrodes, and the electrical current at any stage of the coulometric titration. The results obtained from this new coulometer align well with the novel method for calculating the amount of substance content of potassium hydrogen phthalate (KHP). This method involves the experimental data fitting with the theoretical titration curve TC(NLR), also developed by the INMC. TC(NLR) replaces the traditional empirical W-function (eWf), which has no chemical basis and does not account for CO2 as an impurity. The KHP content of the CRM KHP 84L from NIST was measured using the developed coulometer, resulting in 4.9069 ± 0.0090 mol kg-1 with a bias of 0.21% for the eWf, and a more accurate result of 4.8983 ± 0.0047 mol kg-1 with a bias of 0.041% for the TC(NLR). The challenges during the development of the coulometer, particularly in terms of instrumentation, as well as their impact on uncertainty, are also discussed.
... The coulometric technique shows some important advantages over the classical titration methods such as high sensitivity, the usage of aa very low amount of reagent required, and/or unstable compounds as titration agents, no primary standard is needed and the main reactant (electrons) can be precisely controlled. Nevertheless, some drawbacks that can be overcome are also present such as a more complex instrumentation, the non-commercial availability of specific experimental devices, specific training for the laboratory specialist, and normally, a higher budget hardware [6][7][8]. ...
... An increase in the stirring frequency and/or the extent of titration increased this absolute excess error. This interference was also observed in manual titrations of acids with phenolphthalein, where the pink color disappeared several seconds after completion of the titration, which has been addressed in [8,40,41]. The elimination and study of this particular interference at the detection levels employed was beyond the scope of this paper. ...
In this work, a coulometer was developed from a digitally controlled galvanostat. A simple colorimeter based on a RGB LED was used as a light emitter coupled to light detectors, while light dependent resistance (LDR) and photodiodes have been developed as endpoint detectors. Both hardware and software have been adapted from the original galvanostat design. Regarding the hardware, new electrical signal conditioners (filters and voltage dividers) were included to optimize the working system. The software was developed based on an open source Arduino UNO microcontroller. The different variables that control the titration process are managed by an add-in module for Excel data acquisition software that is freely available. A study of the possible variables that influence the titration process has been carried out. The system was tested with two classical coulometric titrations such as iodometry (thiosulfate, ascorbic acid) and acid/base (potassium acid phthalate as standard). The developed system is versatile as different endpoint color indicators can be employed (starch and phenolphthalein for the investigated reactions). Different experimental arrangements have been studied: the nature of the electrodes (Pt, Ag), type of cells (two separate compartments or a single compartment), and light detectors (LDR, photodiode). The influence of several experimental parameters (both electrical, light, and integration time) was studied and chosen to obtain the best performance of the complete system. Reproducibility results below 1% can be obtained under controlled conditions. In the case of acid/base titrations, the presence of atmospheric carbon dioxide was detected, whose interference was mainly affected by the stirring rate and the titration time.
... The composition of the remaining material (in this case phthalate as its mono potassium salt) is assumed. If the assay is expressed on an amount-of-substance content basis, it is also assumed that a stoichiometric relationship exists between the detected species (replaceable H + for KHP) and the remaining portion of the molecule [132]. ...
The “purity” of an organic compound typically refers, in practice, to an assignment of the mass fraction content of the primary organic component present in the material. The “purity” value of an organic primary calibrator material is the ultimate source of metrological traceability of any quantitative measurement of the content of that compound in a given matrix. The primary calibrator may consist of a Certified Reference Material (CRM) whose purity has been assigned by the CRM producer or a laboratory may choose to value-assign a material to the extent necessary for their intended application by using appropriately valid methods. This report provides an overview of the approach, performance and applicability of the principal methods used to determine organic purity including mass balance, quantitative NMR, thermal methods and direct-assay techniques. A statistical section reviews best practice for combination of data, value assignment as the upper limit values corresponding to 100 % purity are approached and how to report and propagate the standard uncertainty associated with the assigned values.
... One of the most important reference materials in chemistry is potassium hydrogen phthalate (KHP) because it is a primary standard for the typical acid−base titrations. 11,12 The coulometric cell is represented as follows (generator electrodes) ...
In metrology, the certification of potassium hydrogen phthalate (KHP) as a reference material is made using the potential primary method of coulometric titration. Usually, this titration is performed in three steps at constant current, where two endpoint (EP) times are determined from the nonlinear regression that fits the empirical W-function (eWf) to the experimental data. As an alternative, we propose the implementation of the theoretical coulometric titration curve (TC). The TC allowed us to compute the KHP amount of substance, the influence of CO2 in the system, the acid dissociation constants for carbonic and phthalic acids, assuming that those species are the only acids present during titration, and the EP times. The amount of substance of KHP estimated with the EP time and obtained from the TC was compared with the results of the eWf, and no statistical difference was found, while the amount of substance, when estimated directly as a parameter of the nonlinear regression of the TC, was lower. Therefore, the traditional method finds the total acidity of the dissolution, and our method finds the KHP purity. In addition, the acid dissociation constants for H2CO3 and phthalic acid estimated in this work agreed with the data reported in the literature. Finally, the description of the coulometric system using the theoretical TC has a solid and well-known chemical support that is not present in the eWf; this fact is essential for the uncertainty budget and the scientific support for coulometry as a potential primary method.
... 7,13,18 A variety of substances have been analyzed by coulometric titrations, including porphyrins, 24 mercury complexes, 25 thiols, 26 cations and anions, 2,27,29 complexes of nickel, 21 biodiesels, 12 and reference standards. 5,6 Coulometric Karl− Fischer analysis was applied to characterize microemulsions 19 and porphyrin assemblies. 9 Laboratory experiments of an academic interest present coulometric analysis of common household products. ...
Feasibility of performing multiple coulometric titrations in a single course of electrolysis is presented. In these titrations, three pairs of cathode and anode compartments were connected with a network of electrodes and salt bridges. Passage of current through the cell caused concurrent electrolysis in cathode and anode compartments. Electrogenerated reagents produced in these compartments were used as titrants for quantifying the analyte samples. Endpoints of the titrations were determined from the visual color change of an indicator. The charge passing through the cell was monitored and Faraday's laws of electrolysis were applied to assess the quantitative relation between the charge and analyte concentration. Experimentally determined coulombs required to titrate aqueous potassium hydrogen phthalate, MnO 4⁻ , OH ⁻ , and S 2 O 3²⁻ were 0.100, 0.466, 0.103, and 0.0934 C, respectively. These results matched with estimated values of 0.0965, 0.482, 0.0965, and 0.0965 C, respectively. Agreement between the coulombs determined from experimental results and reaction stoichiometry suggests a feasible application of concurrent coulometric titrations. Efficacy of the method was tested for determining the active ingredients in household vinegar and vitamin C dietary supplement tablets. Quantities of acetic acid and ascorbic acid in these products were 5.1% and 980 mg, respectively, agreeing with the quantities determined from volumetric titrations (5.1% and 990 mg) and manufacturer's label (5.0% and 1000 mg).
... Although the titration techniques are considered good repeatability methods and are adequate for homogeneity studies, this technique introduced slightly more uncertainty to the CRM developed. However, comparable uncertainties to this present work were obtained by Recknagel et al. [39] when they used the volumetric titration to estimate the homogeneity and the characterization for a candidate CRM of KHP in a higher batch of material than this work. The metrology institutes adopt the Mutual Recognition Arrangement (MRA) [40]. ...
The use of pure materials in chemical analysis is currently in high demand due to the need to provide reliable measurement results. In the Electrochemistry Laboratory of the National Institute of Metrology from Brazil (Inmetro) is located the primary system of coulometry, which is considered the standard of the measurement for the amount of substance. The main function of this primary system is to certify primary reference materials to ensure reliability and metrological traceability for titration analyses. Some important certified reference materials (CRMs), homogeneous, stable, and characterized with an accurate property value have been developed. This work aims to present the stages for certifying the primary reference materials in a metrological approach: potassium hydrogen phthalate for acid-base titration, potassium chloride for precipitation titration, and potassium dichromate for redox titration. In addition, the key comparisons that were necessary to demonstrate the national traceability structure's equivalence for these CRMs' international recognition will be shown.
In this study, the coulometric method with titratable impurity analysis and the mass balance method were successfully applied in the quantification of the certified reference material of potassium hydrogen phthalate (KHP) with accurate metrological traceability of chemical purity value (99.983% with an expanded uncertainty of 0.024%, k = 2). In contrast to the general coulometric titration method, the coulometric method with titratable impurity analysis enables the conversion of acidimetric purity to chemical purity: The acidimetric purity was determined by coulometric titration method, and then the impurities that may be titrated as principal components were found as far as possible using various methods and the result of deducting these impurities from the acidimetric purity can be considered as chemical purity. The mass balance method also accounted for all possible types of impurities as much as possible to improve the accuracy of the determined result. The accuracy and reliability of the purity results were subsequently verified by a two-step quantitative nuclear magnetic resonance (qNMR) method. This KHP certified reference material was the first hydrophilic internal standard of qNMR (applied in polar solvents) with an expanded uncertainty lower than 0.03%, which will become a major reference standard in the organic chemistry traceable calibration chain, especially when evaluating hydrophilic organic compounds to obtain purity values with very low uncertainty.
A new quantitative analysis method is proposed, aiming at the difficulty in accurately determining the trace content of free acid in lithium-ion battery electrolytes in the past 30 years. The presented method overcomes the three restrictive factors upon the lithium-ion battery electrolytes, such as poor thermal stability, formation of hydrofluoric acid with water and difficulty in accurate determination of trace free acid. Free acid in lithium-ion battery electrolytes is directly titrated with ethoxide ion, which is generated through the electrolyzation of 0.50 mol·L-1 LiCl ethanol solution. The content of free acid is obtained according to the Faraday’s law, and the whole determination process can be completed in 5 minutes. The relative standard deviation is below 2.0% as the content of free acid is 2.0 μg and above. The detection limit is 1.0 μg and the recovery rate is 99.5%‒102.5%. In this method, the free acid determination is not affected by temperature change and the existence of small amount of water. Thus, it provides a simple, fast and accurate analytical method with lower detection limit for the free acid in the lithium-ion battery industry, and comprehensively improves the quality of lithium-ion battery electrolytes and the performance of lithium-ion battery products.
This paper describes the Coulometry system implemented at National Metrology Institute (NMI) of Colombia for purity certification of potassium hydrogen phthalate (KHP). The system aims to become a primary measurement system. This study includes an evaluation of precision in terms of relative standard deviation (%RSD), and its associated uncertainty by repeatability and end-point detection such as type-A uncertainty; And by electric resistance, time, potential difference and sample mass such as type-B uncertainty sources. The implementation of this system will allow the certification in purity of different reference materials.
Gravimetric and coulometry are recognized as primary method in actual measurement, so their accuracy can be appraised by their precision. The precision of measurements of ZrO2+HfO2 by mass balance of difference and gravimetric by precipitation mandelic acid was compared. It shows that the precision of mass balance of difference (0.022%) is better. There is no significant difference between the two averages. So the accuracy of mass balance of difference is higher as its precision is better.
The two-abrupt-change coulometric titration was proposed for the analysis of the degree of deacetylation of chitosan. The potential was indicated by an antimony electrode which could sensitively response to the variation of activity of hydrogen ion in the turbid and viscous chitosan solution. Through this new approach, the deficiency of combination pH glass electrode in that medium is successfully overcome. The degree of deacetylation of chitosan was obtained by the difference of electrolysis time between two inflection points in the electrolysis process. The measurement error, which is caused by the residual acid or alkali in the chitosan, could be eliminated. The degree of deacetylation of four chitosan samples were investigated by this new method, which found to be consist with those determined by proton nuclear magnetic resonance (1H NMR). The standard deviations were found to be lower than 0.60 % and the whole experimental process can be completed within 15 min. With merit of simplicity, convenience, quickness, accuracy, the proposed new method would possess extensive practicability in scientific research and industrial production.
A guide for instructors and laboratory assistants to prepare some common aqueous reagents used in an undergraduate laboratory is presented. Dilute reagents consisting of H+(aq), I3–(aq), Ce4+(aq), and Ag+(aq) were prepared by electrolytic oxidation of respective precursors. Electrolysis was promptly stopped when potassium hydrogen phthalate (KHP) was completely neutralized by OH–(aq) resulting from an electrolytic reduction of water in the cathode compartment. The neutralization of KHP was monitored by a visual color change of an indicator. The amount of reagent produced in the anode compartment was directly determined from the mass of KHP added to the cathode compartment. The concentrations of reagents determined from the mass of KHP and from an independent volumetric titration were in agreement. The electrolysis cell, powered by an 18 V dc power supply, eliminated the requirement of a coulometer. Setup of an electrolysis cell and experimental details for preparing reagents are presented.
A reliable, sensitive, and rapid coulometric titration (CT) method was developed for the purity determination of luteolin reference material candidates. The results obtained from the proposed CT method were compared with those obtained from differential scanning calorimetry (DSC) and mass balance method. The performance of the CT method was comparable, and the quantitative results of the proposed methods were in good agreement with those of DSC and mass balance methods. CT has the advantages of minimal sample requirement, rapid measurement, high accuracy, good reproducibility, and no corresponding reference standard requirement. This method can be used for luteolin purity assays and related production quality control.
This paper describes the history and scheme of reference materials and standard solutions for titrimetry in Japan. Titrimetry
is one of the most fundamental and precise methods for determination of a constituent, based on the effective purity of reference
materials in stoichiometric analysis. It has wide-ranging applications based on titrimetric reactions such as neutralization,
redox, chelatometric, and precipitation titration processes, which are used in various analytical fields all over the world.
Japanese Industrial Standards (JIS) have played an important role in establishing a stable supply of reference materials for
titrimetry since the 1950s. There are several reports of preparations and their determination, including coulometric studies,
in order to establish highly reliable reference materials. This paper briefly reviews the schemes and studies of standardization
through the provision of reference materials and standard solutions based on JIS, and several applications in other standards.
Apparatus and techniques are described for the titration of acids and bases by constant-current coulometry. The precision and accuracy of the method are indicated by titrations of benzoic acid, potassium acid phthalate, adipic acid, hydrochloric acid, and sodium carbonate. Standard deviations of ±0.003 percent have been obtained which indicate that the reliability of the method is equal to or exceeds that of classical analytical procedures.
The latest evaluation of atomic weight determinations and other cognate data has warranted 16 changes for the standard atomic weights of the elements, A,(E), from those published previously in the 2001 Table of Atomic Weights. The revised standard atomic weights are as follows: A(r)(Al) = 26.9815386(8), A(r)(Bi) = 208.980 40(1), A(r)(Cs) 132.905 4519(2), A(r)(CO) = 58.933 195(5), A(r)(Au) = 196.966 569(4), A(r)(La) 138.905 47(7), A(r)(Mn) = 54.938 045(5), A(r)(Nd) = 144.242(3), A(r)(P) = 30.973 762(2), A(r)(Pt) = 195.084(9), A(r)(Sm) = 150.36(2), A(r)(Sc) = 44.955 912(6), A(r)(Na) = 22.989 769 28(2), A(r)(Ta) = 180.947 88(2), A(r)(Tb) = 158.925 35(2), A(r)(Th) = 232.038 06(2). A recommendation is made that delta C-13 values of all carbon-bearing materials be measured and expressed relative to Vienna Pee Dee Belemnite (VPDB) on a scale normalized by assigning consensus values of -46.6 parts per thousand to L-SVEC lithium carbonate and +1.95 parts per thousand to NBS 19 calcium carbonate.
The purity of the potassium hydrogen phthalate (NBS SRM 84d) was determined by precise coulometric titration. The purity obtained was 99.995±0.002% (standard deviation: 0.003%), and was in excellent agreement with the purities of the same SRM determined already in three laboratories in U.S.A. These results support the recommendation on the use of the Faraday constant as an international standard for titrimetric analysis.
Automated constant-current coulometric acidimetry, based on Faraday's Laws, is uncertain to less than 1 part in 20 000 (relative standard deviation) and requires no chemical standardization. It is applicable to strong and weak acids and bases, with bases back-titrated after addition of excess strong acid. Initial setup and sample introduction are the sole manual steps. Assays of HCl, benzoic acid, Na2CO3, and tris(hydroxymethyl)aminomethane are presented. In the endpoint determination procedure, a generalized titration equation yields the theoretical charge remaining to the endpoint before each charge addition. The ratio of the experimental to the theoretical charge for the preceding aliquot corrects for experimental deviations.
Performance characteristics of coulometric titration equipment of a commercially available kind were studied by carrying out precise coulometric assays on potassium hydrogen phthalate and constant boiling hydrochloric acid. The cells investigated were of the shield-tube type in which separation of anode and cathode is accomplished by restricted flow of electrolyte. Tests were made on the conventional cell arrangement and on a modified version involving counterflow separation. From results obtained it is concluded that such cells and the associated coulometric instrument are capable of high measurement accuracy.
In the 1990s the CCQM (Comité consultatif pour la quantité de la matière) defined the term 'primary method'. It also identified some methods that are considered to be primary. One of these methods is titrimetry. In this paper, the principle of titrimetry and two practical examples performed at the highest metrological level in the frame of key comparison CCQM-K8 are discussed. It is evident that titrimetry is a very powerful method leading to highly accurate results when performed correctly, and does not depend on whether the classical metrological approach or a new experimental design using modern laboratory technology is applied.
The accuracy and uncertainty of coulometric titration of Japanese certified reference materials (CRMs) for oxidation–reduction titration were examined in this study. The results for potassium dichromate, sodium oxalate, and potassium iodate are presented. Potassium dichromate was directly determined by coulometric titration, and sodium oxalate and potassium iodate were determined by volumetric analysis using potassium dichromate assigned by coulometry.The uncertainty of the method was investigated by examining the dependency on the sample size and on the electrolysis current. Changes in the titration parameters did not result in any significant effects on the titration results. The coulometric system used primarily consists of a constant current source, timer, switching circuit, indicator unit, and voltmeter. They were controlled using the coulometry software by a PC/AT compatible computer. A highly automated coulometric system achieves repeatabilities of less than one part in 30,000 (k = 2) and uncertainties of less than one part in 15,000 (k = 2). In addition, using volumetric method, SI units traceable sodium oxalate and potassium iodate (purity standards for redox reaction) CRMs were developed.Reference materials for volumetric analysis are the most basic substances used in analytical chemistry. These materials are analyzed by several analytical methods and are produced globally; however, their purities have not been compared at the international level. Therefore, the relationship between the purity and reliability of these materials has not yet been established. In this paper, we determine the relationship between these parameters by titrating each material obtained from different laboratories.
This paper gives the 1998 self-consistent set of values of the basic constants and conversion factors of physics and chemistry recommended by the Committee on Data for Science and Technology (CODATA) for international use. Further, it describes in detail the adjustment of the values of the subset of constants on which the complete 1998 set of recommended values is based. The 1998 set replaces its immediate predecessor recommended by CODATA in 1986. The new adjustment, which takes into account all of the data available through 31 December 1998, is a significant advance over its 1986 counterpart. The standard uncertainties (i.e., estimated standard deviations) of the new recommended values are in most cases about 1/5 to 1/12 and in some cases 1/160 times the standard uncertainties of the corresponding 1986 values. Moreover, in almost all cases the absolute values of the differences between the 1998 values and the corresponding 1986 values are less than twice the standard uncertainties of the 1986 values. The new set of recommended values is available on the World Wide Web at physics.nist.gov/constants.
The homogeneity of samples intended for metrological intercomparison studies must be granted without ambiguity. This holds equally true for aqueous solutions for which the determination of between-bottle variations requires measurement techniques with extremely high precision. Therefore measurement series were designed for automated analysis techniques such as titrimetry, optical emission spectrometry and ion chromatography which offer very high precision of results. Between-bottle relative standard deviations (RSD) of at best 0.008% were obtained with titrimetry. With optical emission spectrometry and ion chromatography, between-bottle RSDs of 0.02% and 0.05% were obtained. The contributions from these measurements were included in a conservative approach to the uncertainty budgets of the gravimetrical reference values for the analytes in the samples.
Problems with understanding, explaining and communication of the present definition of primary method of measurement are described
and amendments put forward for discussion. The conclusion is drawn that in many cases more attention should be given to the
measurement result and its uncertainty statement, rather than to a method. Some cases are discussed where methods might have
a fundamental characteristic that other methods do not have, a condition for the epitheton ’primary’.
In 1995, international agreement was reached on a definition for a primary method of measurement. We provide some practical
advice about the use of primary methods of measurement in a question and answer format.
The accuracy and uncertainty of the coulometric measurement results of reference materials for acidimetric titration were
examined in this study. The results for amidosulfuric acid and potassium hydrogen phthalate are presented. The uncertainty
was investigated by examining the dependency on the sample size and on the electrolysis current. Changes in the titration
parameters did not result in any significant effects on the titration results. Acidimetric standards with the certified value
linked to the SI were developed. In addition, the intercomparison of acidimetric standards was carried out by gravimetric
titration, and the relationship between our coulometric results was determined. Furthermore, due to recent internationalization,
not only the traceability to the SI but also the relationship and consistency of their analytical data have gained increasing
importance. Our results were validated using certified reference materials (CRMs) obtained from different National Metrology
Institutes (NMIs), and their relationships are presented.
The possible approaches to realising a link to the SI system and the status of primary direct methods in the traceability
chain of chemical measurements are discussed. Some results obtained with the new coulometric standard system are presented.
Automated constant-current coulometry, based on Faraday's achieves uncertainties (relative standard deviation) of less than 1 part in 20 000 without chemical standardization. It is applicable to acid-base, redox, and precipitation titrations of high-purity compounds and solutions. Automation of the technique permits unsupervised operation and reduces operator-dependent errors. Initial setup and sample introduction are the sole manual steps. Each assay consists of a main titration at high, constant current, bracketed by the initial and final endpoint routines, each at a lower current. The coulometric assay is analogous to a conventional titration in which two different concentrations of the titrant are used to attain optimum accuracy. The initial endpoint determination corresponds to the blank determination in a classical titration includes a statistical analysis of the random and systematic uncertainties associated with the analysis. Individual steps in the procedure are performed by a hierarchical series of subroutines to reduce program complexity. Results are presented for K2Cr2O7, benzoic acid, and solutions of strong acids.
A cell has been designed for the high-precision coulometric titration, with externally generated titrant, of materials which otherwise undergo undesirable reactions at the working electrodes. With this cell potassium dichromate has been titrated, via its hydrolysis reaction, with hydroxyl ion generated at the cathode, cathodic reduction of the chromium(VI) being circumvented. In this cell 99.9% of the titrant required is generated in one chamber and transferred to another for reaction; the titration is then completed with titrant generated at a second, drip-type electrode working at much lower current. By means of commercially available Leeds and Northrup coulometric titration electrical equipment, titration of NBS 136b Potassium Dichromate gave a purity of 99.976%, standard deviation 0.005%, and of NBS 84d Potassium Acid Phthalate (done as a check) 99.991%, standard deviation 0.005%, both values being in excellent agreement with other work.
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