电解镍液中添加剂的定量核磁共振

ORIGINAL PAPERQuantitative nuclear magnetic resonance for additives determination in an electrolytic nickel bathMiren Ostra &Carlos Ubide &Maider VidalReceived:14October 2010/Revised:18November 2010/Accepted:29November 2010/Published online:17December 2010#Springer-Verlag 2010Abstract The use of proton nuclear magnetic resonance (1H-NMR)for the quantitation of additives in a commercial electrolytic nickel bath (Supreme Plus Brilliant,Atotech formulation)is reported.A simple and quick method is described that needs only the separation of nickel ions by precipitation with NaOH.The four additives in the bath (A-5(2X),leveler;Supreme Plus Brightener (SPB);SA-1,leveler;NPA,wetting agent;all of them are commercial names from Atotech)can be quantified,whereas no other analytical methods have been found in the literature for SA-1and NPA.Two calibration methods have been tried:integration of NMR signals with the use of a proper internal standard and partial least squares regression applied to the characteristic NMR peaks.The multivariate method was preferred because of accuracy and precision.Multivariate limits of detection of about 4mL L −1A-5(2X),0.4mL L −1SPB,0.2mL L −1SA-1and 0.6mL L −1NPA were found.The dynamic ranges are suitable to follow the concentration of additives in the bath along electrodeposition.1H-NMR spectra provide evidence for SPB and SA-1consumption (A-5(2X)and NPA keep unchanged along the process)and the growth of some products from SA-1degradation can be followed.The method can,probably,be extended to other electrolytic nickel baths.Keywords qNMR .Ni electroplating .Additives determination .Process analysisIntroductionNuclear magnetic resonance (NMR)is a versatile technique since it can offer a high number of data of several molecules in a single spectrum.No other spectroscopic method contains equally detailed structural and dynamic information about chemical systems under investigation [1].The availability of high field instruments in conjunction with improvements in probe design and electronic perfor-mance have considerable increased sensitivity,resolution,precision and applicability of quantitative NMR (qNMR)determinations from the beginning of the nineties [2].qNMR has gained growing interest ever since and it has been successfully applied in numerous fields.In electrolytic baths,additives are of prime relevance,but the additive control has frequently been accomplished in an empirical way,due to the lack of rapid and accurate analytical methods for every additive [3].In most cases,the experience of the operator decides what agents and in which concentration should be added,sometimes based on distillation and colorimetric,spectrophotometric or chro-matographic monitoring [4].V oltammetry has also been used [5].However,the automatic addition,based on the time of use of the bath (ampere-hour),is frequently preferred [4,6].In general,quantitation usually requires a specific method and a standard as reference.The use of qNMR also requires the choice of an adequate signal for every analyte,and the integration respect to an internal standard is frequently needed.The choice of the signals for both the analyte and the standard is critical and a wrong decision can strongly affect results,especially in the case of crowded spectra with impurity signals overlapping resonance.One of the main limitations of qNMR is probably the need ofM.Ostra :C.Ubide (*):M.VidalDepartamento de Química Aplicada,Facultad de Química,Universidad del País Vasco,Apdo.1072,San Sebastián 20080,Spain e-mail:carlos.ubide@ehu.esAnal Bioanal Chem (2011)399:1907–1915DOI 10.1007/s00216-010-4573-zhuman intervention during processing operations that closely influence integral values.Several software com-mands are available in order to reduce at minimum the subjective decisions of the operator,but they have demonstrated to perform worse than a manual processing made by a skilled operator[7].The increasing apparition of powerful computers and software has led to the increase of chemometric applica-tions for NMR signals,as far as a large amount of data is produced and peaks can be strongly overlapped[1].Thus, several references applying chemometric tools to NMR spectra of oils[8],tobaccos[9],alcohol mixtures[1]or biological samples[10]can be found,and a paper comparing the quantitation results obtained by partial least squares(PLS)regression and by integration of NMR peak areas is available[7].PLS is the multivariate quantitation algorithm most frequently used;it improves accuracy and allows the determination of components with overlapped signals,but when the number of calibration samples is short,the algorithm classical least squares(CLS)may outperform PLS,specially when non-linearities are absent.No previous papers dealing with NMR in nickel electro-plating baths have been found in the literature,but the organic composition of additives makes NMR a suitable technique for quantitation.The problem arises because plating baths usually contain a big amount of inorganic salts and,in this case,a large amount of nickel ions that are paramagnetic(they have unpaired electrons)and,so,they relax so quickly that the typical signals of other compounds in the NMR spectrum are precluded.In the present paper,a commercial nickel bath is used and a method for nickel elimination is reported.Reproducible NMR spectra can be obtained and the four additives present in the bath(A-5 (2X),Supreme Plus Brightener(SPB),SA-1and NPA, commercial names)can be determined through their NMR signals.Univariate calibration procedures as well as PLS are applied and compared.There are some precedents on the use of UV-chemometric procedures applied to additives in electrolytic zinc and nickel baths[11–14];because of that UV–vis spectrophotometry is applied as a reference technique for those additives showing UV–vis absorption; namely,A-5(2X)and SPB.ExperimentalReagentsA volume of1.8L of a commercial nickel bath(Supreme Plus Brilliant,Atotech®formulation)was used with the following composition:NiSO4·6H2O(250g L−1), NiCl2·6H2O(50gL−1)and H3BO3(45gL−1)as non-additive solution;SA-1(2.6ml L−1),A-5(2X)(20ml L−1),NPA(2ml L−1)and SPB(1ml L−1)as additives.The chemical composition of the additives is unknown.The final pH was4.0and it was maintained constant along the process with the addition of either NiCO3or H2SO4as required.Non-additive chemicals were of analytical reagent grade(Panreac or Fluka)and used without further purifi-cation.Additives were obtained from Atotech and used as received.Doubly distilled water was used throughout.An amount of0.00150g of3-(trimethylsilyl)-2,2,3,3-tetradeu-teroprionic acid sodium salt(TSP)dissolved in5mL D2O was used as a reference forδ=0.00ppm.Succinic acid (20gL−1)as an internal standard was also prepared.HCl and NaOH at several concentrations were also used.ApparatusThe following instrumentation was used:an electrodeposi-tion vessel with a water jacket for the nickel bath;a Crison 501pH meter;a Haake water bath thermostat controlled by an external probe dipped into the nickel bath and a rectifier (±20A/30V)from HQ Power(Nedis BV,model no.PS 3020)(Fig.1).A Bruker Avance-500spectrometer was used to record500mHz1H-NMR spectra at a temperature of30°C.An amount of128scans of64K data points was acquired every time with a spectral width of8012Hz (16ppm),acquisition time of2.2s.,recycle delay of9.0s, flip angle of90ºand a constant gain of11,585.The solventFig.1Manifold for process analysis of nickel electroplating.a, Vessel for electrodepostion;b,anodes;c,cathode;d,temperature probe;e,magnetic stirrer;f,current source;g,water bath thermostat; h,pH meter1908M.Ostra et al.signal suppression was achieved using the watergate pulse sequence [15].The data were acquired at a rate of 24min per sample.Micropipettes brand or Eppendorf were used throughout.Sample preparation and NMR spectra acquisition A volume of 19.25μL of succinic acid solution (internal standard)was added to 2.5mL of the nickel bath solution to be analysed in a conical test tube.Then,3.5mL of NaOH 10M were added in order to remove Ni(II)from the solution through the formation of nickel hydroxide precip-itate.The process was assisted by a glass stick and a combination of heating and stirring in an ultrasonic bath during 5min at 65°C.After some cooling,the solution was centrifuged during 5min at 4,000rpm.An amount of 1.0mL of concentrated HCl was added to a volume of 2.5mL of the supernatant solution and the pH was adjusted approximately to 4.0(a pH range between 3.95and 4.05was considered acceptable)with a diluted solution of HCl.The solution was taken to 10mL in a volumetric flask with HCl 10−4M (pH=4.00).A volume of 500μL of this final solution was placed in a 5mm NMR tube and 50μL of the D 2O-TSP solution was added.D 2O served as the field frequency lock.The final concentrations were: 4.1×10−3g L −1TSP and 9%D 2O.The whole process entails a dilution of 10.6-fold from the original bath concentration.A sketch of the process is depicted in Scheme 1.CalibrationBath solutions (no current through)were prepared main-taining the non-additive bath components at their standard concentrations.The concentration level of additives was fixed among the following:5.0,10,15,20and 25mL/L for A-5(2X);0.10,0.25,0.50,0.75,1.00and 1.25mL/L for SPB;0.5,1.3,2.5and 3.0mL/L for SA-1;and 1.0,1.5,2.0and 2.5mL/L for NPA.Initially,the whole set of samples shaped a tetrahedron that,afterwards,was enlarged to cover some poorly represented concentrations.A total amount of 82samples was used,including 15replicates.The calibration set was formed by 51samples and the validation set by 31samples.The distribution of samples in the two sets was random,except the tetrahedron extremes that were included in the calibration set.Replicates were always kept in the same cross-validation segment.To avoid overesti-mation,several additive concentrations could be varied at the same time.Nickel electrodepositionNickel electrodeposits were obtained galvanostatically in a glass cylindrical vessel (10.5cm inner diameter,21cm height)of approximately 2L of capacity (Fig.1)with a lid that minimized heat and solvent losses.The electrodeposi-tion was carried out on both sides of 16.5×3cm commer-cial steel sheets at a temperature of 65°C with magnetic stirring under 4Adm −2current density for 15min.Prior to each electrodeposition,the steel sample was cleaned with soap and water,then with calcium carbonate,and finally etched with hydrochloric acid/Beizentfetter®solution for 30s and rinsed with water.Two 20×3A-cm Ni pieces were used as anodes.An amount of 53steel sheets were nickel plated along the bath life.After that,the bath was considered to have run out.Additive determination in an electrolytic nickel bathV olumes of 2.5mL were regularly extracted from the bath during electrodeposition for NMR analysis (see “Sample preparation and NMR spectra acquisition ”),until the nickel bath was considered to have run out.A total of 20aliquots were measured along the whole process (one aliquot every three steel sheets approximately).Software and data processingPreliminary data processing was carried out with Bruker software,TOPSPIN 1.3.The free induction decay signals were Fourier transformed (1.0-Hz line broadening),the spectra were phased and the baseline corrected through data analysis with the MestRe-C 4.8.6.0software package2.5 mL nickel bath + 19.25 µL inner standard (succinic acid)3.5 mL NaOH + Heat (5min, 65ºC) + Stirring (sonication)Ni precipitate (discard) solutiontake 2.5 mL1mL. conc. HCl + diluted HCl10.00 mL (pH 4.0)take 500 µLNMR measurement+ 50 µL D 2O·TSPScheme 1NMR measurementqNMR for additives determination in an electrolytic nickel bath 1909(Santiago de Compostela University,Spain).The resulting spectra were aligned by right or left shifting,as necessary,using the TSP signal as a reference and the i coshift algorithm (Matlab®7.4.0environment).The Unscrambler®v.9.7(Camo A/S,Trondheim,Norway,2007)software package was used for PLS application.To test the prediction capability of the developed models,the statistic relative error (RE)was used:RE ¼100ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiP m i ¼1^y i Ày i ðÞ2P m i ¼1y 2iv u u u u u twhere ^y i is the estimated analyte concentration in the sample i and y i is the analyte concentration in the same sample.RE can be applied either to the calibration (RE cal )or the prediction (RE val )sets.The calibration and prediction (validation)sets were always defined before any data processing and remained unchanged along the whole work.The cross-validation procedure was used to assess the robustness of the constructed models.Choosing the optimum number of factors (LV s )to be used with the PLS model was made through a previously established method [16].That is,the lowest number of LV for which the cross-validation variance value does not differ signifi-cantly from the minimum,according to an F test with probability P =0.25,was chosen.Results and discussion NMR spectraAll the additives in a nickel bath show NMR signal (Fig.2).A-5(2X)(peak 2)and NPA (11)show signals where no other bath component absorbs.SPB and SA-1show theindependent signals (1)and (3),respectively,but they bothalso show the signals (4)and (6),that correspond to the same species which is present in both additive preparations,so peaks (4)and (6)in Fig.2have not quantitative usefulness in this case.Signal (5)is for water,signal (8)is for the internal standard,succinic acid,and signal (12)is for the displacement reference,TSP (δ=0ppm).New peaks coming from the degradation products of additives appear as current passes through;they correspond to signals (7),(9)and (10).Linear calibrationUnivariate (linear)calibrations with NMR signals make use of peak areas,but in the present case the sample is subject to a previous precipitation and separation of nickel;this makes necessary the use of an internal standard to account for any possible uncontrolled loss of sample.The choice of an internal standard for quantitation was carefully accom-plished,taking into account that several assumptions must be fulfilled.Thus,it was necessary to find a compound with a clean and simple signal in the NMR spectra at pH 4and not overlapping with any additive signal.It should be water-soluble and not excessively expensive because the internal standard must be added at the beginning of the procedure and bath aliquots were diluted 10.6-fold in the NMR tubes after the whole pre-treatment,resulting in a high internal standard consumption.The high price of TSP was the main reason for discarding it as internal standard,but it was still used as a displacement reference.Succinic acid demonstrated to be a suitable and reliable internal standard.The individual spectra were manually integrated for each analyte.Thus,in each spectrum,the area of succinic acid was taken to 1.000and the area of each additive peak was calculated accordingly.The quantitation by integration of signals needs to fix the integration limits with good precision to avoid large errors due to chemical-112345678910δ (ppm)Fig.2Typical NMR spectrum for a nickel bath sample after some current has passedthrough.New peaks are signals appearing along the electrode-postion process.1,SPB;2,A-5(2X);3,SA-1;4,SA-1+SPB;5,water;6,SA-1+SPB;7,9and 10,new peaks appearing along the electrodeposition process;8,succinic acid (internal standard);11,NPA;12,TSP (δ=0)1910M.Ostra et al.shifts that can be due,for instance,to small pH variations as well as intermolecular interactions.Manual integration overcomes the most of errors.The calibration models for the four additives are shown in Table 1when succinic acid was used as internal standard.The calibration line for A-5(2X)provides a good regression (low errors),but those for the rest of additives do not;that is,the internal standard is efficient only for A-5(2X),showing that the complex procedure involved is not the only relevant source of error in the determination of SPB,SA-1and NPA.Crowded NMR spectra and low signals are probably responsible for the poor results obtained.The relative calibration and prediction errors and some other features of the applied univariate calibration method applied for the A-5(2X)determination are given in Table 2.The linear calibration models for the other additives were not considered further owing to the poor results they provided.The limit of detection for A-5(2X)was obtained according to the 3s criterion recommended by IUPAC [17].Independent studies of accuracy and precision for A-5(2X)determination are given in Table 3.Measurements were taken along 1week,1month after the calibration model was built.The results obtained agree with expect-ations derived from Table 2.Mean systematic errors were lower than 8%and the precision was always better than 7%,except in one case (5.0mL/L;Table 3).So,the linear calibration model with internal standard can be proposed as a good quantitation method for the determination of A-5(2X)in electrolytic nickel baths.PLS regressionSimilarly to linear calibration,peak misalignments can deteriorate the chemometric modelling [18,19];so,spectra were always aligned prior to the multivariate modelling.The alignment procedure was made with the i coshift (interval correlation shifting )programme,based on Corre-lation SHIFTing of Spectral Intervals.The programme demonstrated to be highly efficient in solving signal alignment problems in metabonomic NMR data analysis and it works faster than similar methods found in the literature [20].Unlike linear calibration,when multivariate methods are used,no need of normalization with internal standard is usually required because small errors in the amount ofsample taken can be modelled with some extra latent variable.First of all,the algorithm CLS was tried,but PLS always provided better results,probably because CLS is more prone to signal variations,interferences from the matrix,etc.and it was not considered further.Table 2summarizes the results obtained for the four additives when PLS was applied,including some characteristics of the calibration models,the relative prediction errors and some estimations of the limits of detection.The NMR variables used to build the data matrices depend on each analyte.Only those variables with observable signal for any specific analyte were used,in order to reduce the amount of no-correlated variance in data.Agreeing with some other precedents [21],the models based on selected regions of the NMR spectra provided better predicting ability and needed a lower number of latent variables.The signals for SPB,SA-1and NPA are very weak after the sample has been highly diluted with consequences in precision.The RE values were similar for the calibration and validation sets (Table 2)as would be expected,and most of errors keep under 10%,which represent much lower errors for SPB,SA-1and NPA than those found by using relative areas as analytical signal.However,the additive A-5(2X)can be equally predicted by areas integration,probably due to the large NMR signal.The limit of detection in multivariate calibration was calculated through two different methods:Found vs .added concentration plots [22]and the recently proposed multivariate residuals procedure [23];in any case,similar values for LOD were found regardless the calibration method used.Data for independent studies of accuracy and precision are given in Table 3.The determination of additives was made in the presence of variable amounts of the rest of additives,trying to reproduce possible stages along the bath live.In general,both systematic and random errors have values according to expected;they all keep under 10%in most of cases.That precision is enough to follow the working conditions of the nickel bath.As a whole,lower errors are obtained for the additive A-5(2X)regardless areas integration or PLS calibration is used.Evaluation of modelsThe univariate method does not need a high number of standards for calibration and it does not require anyAnalyte Peak number (Fig.2)a ±s mb ±s br s y/x A-5(2X)2a+2b+2c 0.510±0.0140.1±0.20.9800.6SPB 10.21±0.03−0.01±0.030.7810.03SA-130.18±0.030.06±0.070.7320.08NPA110.157±0.03−0.09±0.060.6620.05Table 1Calibration data for the general equation A analyte /A TSP =a ×C (mL L −1)+bs m standard error for slope,s b standard error for intercept,s y/x standard error for regression lineqNMR for additives determination in an electrolytic nickel bath 1911Analyte Added/mL L −1Found/mL L −1Error (%)RSD (%)(SPB,SA-1and NPA)A-5(2X)a5.0 5.48.014(1.25,2.6and 2.0)5.0 4.8−4.06.6(0.1,2.6and 2.0)15.015.9 6.0 6.6(0.75,2.6and 2.0)25.025.3 1.2 2.8(0.1,2.6and 2.0)25.025.6 2.4 3.0(1.25,2.6and 2.0)(SPB,SA-1and NPA)A-5(2X)b5.06.020 3.8(1.25,2.6and 2.0)5.0 5.510 2.1(0.1,2.6and 2.0)15.013.6−9.3 6.8(0.75,2.6and 2.0)25.021.4−14 1.7(0.1,2.6and 2.0)25.023.7−5.28.3(1.25,2.6and 2.0)(A-5(2X),SA-1and NPA)SPB b0.750.73−2.711(15.0,2.6and 2.0)1.25 1.09−13 2.5(5.0,2.6and 2.0)1.25 1.14−8.88.2(25.0,2.6and 2.0)(A-5(2X),SPB and NPA)SA-1b1.3 1.515 1.6(20.0,1.0and 1.5)3.02.8−6.7 1.7(20.0,1.0and 1.5)3.0 3.0− 3.0(20.0,1.0and 2.5)(A-5(2X),SPB and SA-1)NPA b1.5 1.6 6.78.1(20.0,1.0and 1.3)2.5 2.3−8.09.9(20.0,1.0and 0.1)2.52.1−1610(20.0,1.0and 1.3)Table 3Accuracy andprecision in the determination of additives in synthetic nickel baths by NMR,using both linear and PLS calibration models (Tables 1and 2)(seven replicates)The last column shows the con-centration of the rest of addi-tives.Concentrations are in millilitres of additive per litre batha Linear regresión bPLS regresiónTable 2Calibration models,relative prediction errors and some figures of merit,found for the determination of A-5(2X),SPB,SA-1and NPA AnalyteSetNo.of samplesNMR signal/ppmAnalytical signalRegressionRE (%)LOD/mL L −1IUPAC aFound vs added bMR cA-5(2X)Cal d 597.5–8.2Analyte area/Internal standard area Linear 6.8 3.5Val e 31 6.5A-5(2X)Cal d 597.5–8.2Spectral profile,mean centred dataPLS 2LV 8.254Val e 319.1SPB Cal d 408.85–8.95PLS 1LV 9.30.30.4Val e 2114SA-1Cal d 36 3.55–3.8+5.25–5.55+5.8–6.1+8.85–8.95PLS 3LV7.70.60.2Val e 229.6NPA Cal d 400.84–0.90PLS 2LV 120.80.6Val e259.5The value of LOD is given in millilitres of additive per litre batha IUPAC criterion for linear calibration was applied [17]b Found vs added plots were used [22]c The multivariate residual method was used [23]d Calibration samples eValidation samples1912M.Ostra et al.chemometric knowledge to build the calibration models.The need of an internal standard can be expected because electroplating baths are complex matrices with a number of compounds at very high concentrations and the experimen-tal procedure requires nickel separation by precipitation.Oddly enough,only the A-5(2X)determination with integrated areas clearly improves by the use of an internal standard,probably because some other relevant sources of error are also present for the rest of additives.On the other hand,PLS requires a larger amount of calibration samples;this can become an arduous task,as long as nickel elimination is strictly needed before NMR measurements;however,no internal standard is needed.The algorithm PLS can be recommended for SPB,SA-1and NPA determi-nations,taking advantage that no internal standard is needed,though it could probably be used as a way to correct experimental and spectral reproducibility in a number of cases.On the contrary,because the univariatemethod tends to give slightly better results than the PLS method for A-5(2X)(Table 2),the calibration technique chosen will depend,in this case,on the particular situation;in this work both methods were used and compared.Additives determination along the electrodeposition process The concentration of the four additives in the bath can be monitored over time by taking out periodical aliquots and measuring the NMR spectrum after proper pre-treatment.The additives concentration was estimated by using the calibration methods given in Table 2.Concentrations were corrected to the initial volume to take into account the new volume after every extraction (at the end of the bath life the volume reduction amounts 2.8%).The results obtained are shown in Fig.3.The additives A-5(2X)and NPA do not change their concentration along the process,but SPB and SA-1do it.Because of its relatively high LOD (Table 2),Additive Found/mL L −1Added/mL L −1Total found/mL L −1Recovered Recovery (%)SA-12.630.48 2.980.35731.10 1.15 2.43 1.331160.84 1.48 2.39 1.55105NPA1.870.482.260.39811.780.96 2.88 1.101151.331.192.881.55130Table 4Concentrations of SA-1and NPA additives found in nickel electroplating baths with PLS calibration applied to NMR data.Concentrations are in mL additive/L bath0102030m L A -5(2X )/ L b a t hCurrent /A·h·L -1Current /A·h·L -1Current /A·h·L -1Current /A·h·L -1m L S P B /L b a t h(c)0123m L S A -1/L b a t h(d)012m L N P A /L b a t h1530153015307.515Fig.3Concentrations (infact volume fractions)of a A-5(2X),b SPB,c SA-1and d NPA found along the electrodeposi-tion process.Asterisks ,NMR-PLS;empty circles ,NMR-relative areas;filled circles ,UV-Visible (reference)qNMR for additives determination in an electrolytic nickel bath 1913the SPB concentration cannot be completely followed along the bath life;on the other hand,relatively high imprecision is obtained (Tables 2and 3),but the concentration data obtained are valuable enough if the main aim is to keep the additive concentration at its original value by successive additions as needed.The evolution of A-5(2X)and SPB can be compared with the concentration profiles followed by UV-visible spectrometry (Fig.3a,b ),that is used here as a reference technique.It can be concluded that similar results are obtained by both techniques though NMR data for SPB are more imprecise.However,UV –vis data show better precision and lower detection limit.The limit of detection (LOD)value of the NMR method precludes its use to follow the SPB evolution during the last third of the process.The additives SA-1and NPA do not show any UV –vis absorption;so that,the technique cannot be used as a reference.Because of that,recovery studies of both additives in spiked samples were made.Several bath aliquots were spiked with known concentrations of the additives and the recovery concentration was then calculated.Table 4summarizes the results obtained.Recovery results stand between 73and 130%,which can be considered according to expected taking into account the random errors found in the estimation of both additives (Fig.3c,d ).These results can be considered acceptable when data are obtained with bath control purposes.Additive degradation productsSeveral peaks arise in the NMR spectrum (Fig.2)as the nickel bath is being used.Figure 4shows the growing peaks evolution obtained along the bath life.The new species,therefore,build up as reaction products from the additives degradation.Because the additives composition is unknown,so is the structure of their degradation products,but the way in which the profile of the new peaks evolves obeys a pseudo-first-order rate law,according to the equation:relative area t ¼relative area 11Àe Àk obs t ÀÁð1ÞWhere the subscript t refers to any time,the subscript ∞means that the reaction has gone to completion and k obs is the experimental pseudo-first-order rate constant.Thisbehaviour is not singular in electrodeposition coating processes and some precedents can be found for both zinc [13,21]and nickel [14]baths.From Eq.1,the following equation can be deduced:ln relative area t Àrelative area 1ðÞ¼ln relative area 1ðÞÀk obs tð2ÞBy plotting Eq.2,a straight line is obtained from which k obs can be evaluated.The values of k obs deduced from the intensity increase of the three peaks in Fig.4do not differ significantly from each other [24]in any case (p =0.05)and they might come from the same additive (they may,even,correspond to the same species).A mean value for the experimental rate constant of products formation can be obtained:k obs ðproducts Þ¼0:049Æ0:006Ah =L ðÞÀ1ð3ÞThe evolution of additives in the nickel bath has been shown in Fig.3.The concentration of A-5(2X)and NPA do not significantly change along the process,so they cannot be a source of degradation products;on the contrary,the degradation products should come either from SA-1,from SPB or,perhaps,from both of them.It has already been shown that SPB decay obeys a first-order rate law [21]and using the data in Fig.3b (NMR data)the first order rate constant:k obs ðSPB Þ¼Àslope ¼0:099Æ0:011Ah =L ðÞÀ1ð4Þis deduced.The SA-1decay does not show a definite pattern,probably due to imprecision of measurements,but the SA-1data in Fig.3c can be tentatively adjusted to a first-order decay and then a value of:k obs ðSA À1Þ¼0:025Æ0:001Ah =L ðÞÀ1ð5Þis obtained.The value of k obs(products)(Eq.3)differs significantly from both k obs(SPB)(Eq.4)and k obs(products)(Eq.5),so nothing can be concluded from the origin of products.Perhaps either non-considered sources of uncertainty,insufficient number of measurements in the case of SPB,1…52.95 1...51.8/ppm1 (5)1.032.832.89 1.681.740.971.0/ppm /ppm Fig.41H-NMR spectra,for peaks a (7),b (9)and c (10)in Fig.2,along the electrode-position process.1,A·h/L;2,5A·h/L;3,13.3A·h/L;4,21.7A·h/L;5,28.9A·h/L1914M.Ostra et al.。