Determination of total antimony(III,V) by square-wave anodic stripping voltammetry with in situ plated bismuth-film electrode

A sensitive and reliable method is described for the determination of total Sb(III,?V) at traces levels by Osteryoung square-wave anodic stripping voltammery (OSWASV). This method is based on the co-deposition of Sb(III,?V) with Bi(III) onto an edge-plane pyrolytic graphite substrate at an accumulation step. OSWASV studies indicated that the co-deposited antimony was oxidised with anodic scans to give an enhanced anodic peak at about 450?mV vs. Ag/AgCl (sat. KCl). The anodic stripping peak current was directly proportional to the total concentration of antimony in the ranges of 0.01–0.10?µg?L^?1, 0.10–1.0?µg?L^?1 and 1.0–18.0?µg?L^?1 with correlation coefficient higher than 0.995 when 2.0?mol?L^?1 hydrochloric acid was used. The detection limits calculated as S/N?=?3 was 5.0?ng?L^?1 in 2.0?mol?L^?1 hydrochloric acid at 180?s deposition time. The relative standard deviation was 5% (n?=?6) at 0.10?µg?L^?1 level of antimony. The analytical results demonstrate that the proposed method is applicable to analyses of real water samples.     

Selenium-coated carbon electrode for anodic stripping voltammetric determination of copper(II)

We describe a new and promising type of selenium film electrode for anodic stripping voltammetry. This method is based on formation of copper selenide onto an in-situ formed selenium-film carbon electrode, this followed by Osteryoung square-wave anodic stripping voltammetry. Copper(II) is also in-situ electroplated in a test solution containing 0.01 mol L^-1 hydrochloric acid, 0.05 mol L^?1 potassium chloride and 500 µg L^?1 Se(IV) at a deposition potential of ?300 mV. The well-defined anodic peak current observed at about 200 mV is directly proportional to the Cu(II) concentration over the range from 1.0 to 100 µg L^?1 under the optimized conditions. The detection limit (three sigma level) is 0.2 µg L^?1 Cu(II) at 180 s deposition time. Relatively less interferences are shown from most of metal ions except for antimony(III). The method can be applied to analyses of river water and oyster tissue with good accuracy.     

Gold microelectrode ensembles: cheap,reusable and stable electrodes for the determination of arsenic (V) under aerobic conditions

The determination of total arsenic through As(V) anodic stripping voltammetry (ASV) is, in some cases, preferable over As(III) ASV. The As(V) ASV procedure has no chemical reduction step from As(V) into As(III), which results in decreased analysis time and no contamination from reducting reagents. A simple and reliable procedure of As(V) determination is proposed. Anodic stripping determination of trace As(V) at gold microelectrode ensembles in diluted HCl solution in the presence of dissolved oxygen is shown. The electrode is based on a carbon black (30%)–polyethylene composite. The sensor was prepared by gold electrodeposition on the surface of the composite electrode. The given sensor is cheap, reliable and stable, especially when electrochemical activation is employed. The experimental parameters for the electrochemical determination were optimized, namely 0.005?M HCl as the background electrolyte, the deposition potential ?2.2?V (versus Ag/AgCl in 1?M KCl) and 180?mV?s^?1 linear scan rate. Calibration curves were obtained and were linear in [As(V)] over the 1.5–45?µg?L^?1 range, with a LOD of 0.5?µg?L^?1. The effect of common interfering species is studied. The electrochemical behaviour of As(III) form is studied in the same experimental conditions. It was found that As(III) is deposited at lower potentials (starting at ?0.6?V) and the sensitivity of As(III) detection is higher, but dependant on the presence of dissolved oxygen. The speciation of inorganic forms of arsenic is discussed.     

Cathodic Stripping Voltammetric Determination of Tellurium(IV) with in situ Plated Bismuth-Film Electrode

Abstract

A very sensitive and reliable method is proposed for the determination of tellurium(IV) [Te(IV)] by Osteryoung square-wave cathodic stripping voltammetry. This method is based on the reduction of Te(IV) with bismuth(III) onto an edge-plane pyrolytic graphite electrode, followed by a cathodic potential scan. The reduced Te gave a well-defined catalytic hydrogen wave at ?1200 mV vs. Ag/AgCl. The peak height of the catalytic wave was directly proportional to the initial Te(IV) concentration in the concentration ranges of 0.01–0.10 and 0.1–1.0 µg L^?1 with 30 s deposition time. A 3σ detection limit of 1.0 ng L^?1 Te(IV) was obtained with the same deposition time. The relative standard deviation was 3% on replicate runs (n = 5) for the determination of 0.1 µg L^?1 Te(IV). Analytical results of natural water samples demonstrate that the proposed method is applicable to the determination of traces of Te(IV).     

Electroanalytical Determination of Arsenic(III) and Total Arsenic in 1 mol L−1 HCl Using a Carbonaceous Electrode Without a Reducing Agent

Abstract

Arsenic(V) [As(V)] was reduced to As(0) at pH 0.0 and As(III) at pH 4.5 on a carbon-paste electrode modified with hematite, which allowed their selective determination. Arsenic(V) suffered interference from copper (Cu) and bismuth (Bi). Arsenic(III) was almost free of them. Humic acid did not affect the signal of As(V) but increased the signal of As(III). Arsenic was preconcentrated at ?0.8 V for 100 s. The response was linear up to 70 µg L^?1 for As(V) and 50 µg L^?1 for As(III). The limits of detection were 2 µg L^?1 and 5 µg L^?1 respectively. This method was applied to drinking water and compost lixiviate.     

Simultaneous Determination of Antimony(III) and Molybdenum(VI) by Adsorptive Stripping Voltammetry Using Quercetin as Complexing Agent

A sensitive and selective voltammetric method for simultaneous determination of Sb(III) and Mo(VI) using Quercetin (Q) as complexing agent is described. Optimal conditions were found to be: pH 3.7, C_Q=6.0 µmol L^?1 and E_acc=?0.10 V. The LOD (3σ) for Sb(III) are 0.076 and 0.040 µg L^?1, whereas for Mo(VI) are 0.086 and 0.048 µg L^?1 with t_acc of 60 and 120 s, respectively. The method was validated using synthetic sea water (ASTM D665) and was applied to the determination of Sb(III) and Mo(VI) in natural waters with satisfactory results.     

Square wave anodic stripping voltammetric determination of lead(II) using a glassy carbon electrode modified with a lead ionophore and multiwalled carbon nanotubes

We report on a glassy carbon electrode (GCE) modified with a lead ionophore and multiwalled carbon nanotubes. It can be applied to square wave anodic stripping voltammetric determination of Pb(II) ion after preconcentration of Pb(II) at ?1.0?V (vs. SCE) for 300?s in pH?4.5 acetate buffer containing 400?μg?L^?1 of Bi(III). The ionophore-MWCNTs film on the GCE possesses strong and highly selective affinity for Pb(II) as confirmed by quartz crystal microbalance experiments. Under the optimum conditions, a linear response was observed for Pb(II) ion in the range from 0.3 to 50?μg?L^?1. The limit of detection (at S/N?=?3) is 0.1?μg?L^?1. The method was applied to the determination of Pb(II) in water samples with acceptable recovery.

Automated Determination of Cu(II), Pb(II), Cd(II) and Zn(II) in Environmental Samples by Square Wave Voltammetry Exploiting Sequential Injection Analysis and Screen Printed Electrodes

This paper describes the development of a methodology for quantification of Cu(II), Pb(II), Cd(II) and Zn(II) in waters and sediments by anodic stripping voltammetry (ASV) automated by Sequential Injection Analysis (SIA) using a graphite screen printed sensor modified with mercury. Determinations were made by standard addition automated by the SIA system. The limits of detection and quantification were, respectively, 1.3 and 4.3 µg L^?1 for Cu(II), 1.4 and 4.6 µg L^?1 for Pb(II), 0.6 and 1.8 µg L^?1 for Cd(II) and 4.2 and 14 µg L^?1 for Zn(II). These limits were obtained for a sample volume of 1000 µL, flow rate of 10 µL s^?1 (during the deposition step), and utilizing 3 flow reversals (volume of reversion=950 µL), totalizing a deposition time of 315 s. The potentiostat worked synchronically with the SIA system applying the conditioning potential of ?0.1 V vs. pseudo reference of Ag (100 s), deposition potential of ?1.0 V for Cu(II), Pb(II) and Cd(II) or ?1,3 V for Zn(II), square wave frequency of 100 Hz, potential step of 6 mV and pulse height of 40 mV. For quantification of Zn(II) in sediment extracts, deposition of Ga⁰ on the working electrode was necessary to avoid the formation of intermetallic between Zn⁰ and Cu⁰. The accuracy of the method was assessed by spike and recovery experiments in water samples which resulted recovery rates near 100 % of the spiked concentrations. Recoveries of concentrations in the certified sediment sample CRM‐701 undergoing the three steps sequential extraction procedure of BCR varied from 71.7 % for Zn(II) in the acetic acid extract to 112.4 % for Cu(II) in the oxidisable fraction, confirming that the standard addition approach corrected the matrix effects in the complex samples of sediment extracts.     

Speciation of Antimony(III) and Antimony(V) in Bottled Water by Hydride Generation-Inductively Coupled Plasma Optical Emission Spectrometry

Speciation of Sb(III) and Sb(V) was investigated using hydride generation with the selective formation of stibine from Sb(III). A continuous flow system using a homemade gas-liquid separator with inductively coupled plasma optical emission spectrometry was employed. The conditions and concentrations of NaBH₄, HCl, citric acid, and KI were optimized to obtain limits of detection of 0.05 for Sb(III) and 0.11 µg L^?1 for total Sb without preconcentration. An attractive sampling rate of 26 analyses h^?1 was obtained, suggesting application for routine analysis. The method was employed for the determination of Sb(III) and total Sb in bottled drinking water, and recovery values between 82.0 and 98.8% with relative standard deviation lower than 6.2% were observed, demonstrating appropriate accuracy and precision.     

Enhanced Specificity and Sensitivity for the Determination of Nickel(II) by Square-wave Adsorptive Cathodic Stripping Voltammetry at Disposable Graphene-modified Pencil Graphite Electrodes

Chemical sensors relying on graphene-based materials have been widely used for electrochemical determination of metal ions and have demonstrated excellent signal amplification. This study reports an electrochemically reduced graphene oxide (ERGO)/mercury film (HgF) nanocomposite-modified pencil graphite electrode (PGE) prepared through successive electrochemical reduction of graphene oxide (GO) sheets and an in situ plated HgF. The ERGO-PG-HgFE, in combination with dimethylglyoxime (DMG) and square-wave adsorptive cathodic stripping voltammetry (SW-AdCSV), was evaluated for the determination of Ni²⁺ in tap and natural river water samples. A single-step electrode pre-concentration approach was employed for the in situ Hg-film electroplating, metal-chelate complex formation, and non-electrolytic adsorption at –0.7 V. The current response due to nickel-dimethylglyoxime [Ni(II)-DMG₂] complex reduction was studied as a function of experimental paratmeters including the accumulation potential, accumulation time, rotation speed, frequency and amplitude, and carefully optimized for the determination of Ni²⁺ at low concentration levels (μg?L^?1) in pH 9.4 of 0.1 M NH₃–NH₄Cl buffer. The reduction peak currents were linear with the Ni²⁺ concentration between 2 and 16?μg?L^?1. The limits of detection and quantitation were 0.120?±?0.002?µg?L^?1 and 0.401?±?0.007?µg?L^?1 respectively, for the determination of Ni²⁺ at an accumulation time of 120?s. The ERGO-PG-HgFE further demonstrated a highly selective stripping response toward Ni²⁺ determination compared to Co²⁺. The electrode was found to be sufficiently sensitive to determine metal ions in water samples at 0.1?µg?L^?1, well below the World Health Organization standards.     

All-solid-state Cr(III)-selective potentiometric sensor based on Cr(III)-imprinted polymer nanomaterial/MWCNTs/carbon nanocomposite electrode

A novel potentiometric sensor, based on carbon paste electrode (CPE), modified with ion-imprinted polymer (IIP) and multi-walled carbon nanotubes (MWCNTs), is introduced for detection of chromium (III). The IIP nanomaterial was synthesised and characterised by using scanning electron microscopy and Fourier Transform Infrared. The modification of the CPE with the IIP (as a ionophore) resulted in an all-solid-state Cr(III)-selective sensor. However, the presence of appropriate amount of MWCNTs in the electrode composition was found to be necessary to observe Nernstian response. The optimised electrode composition was 76.7% graphite, 14.3% binder, 5% IIP, and 4% CNT. The proposed sensor exhibited Nernstian slope of 20.2 ± 0.2 mV decade^?1 in the working concentration range of 1.0 × 10^?6?1.0 × 10^?1 mol L^?1 (52 µg L^?1–5.2 g L^?1), with a detection limit of 5.9 × 10^?7 mol L^?1 (30.68 µg L^?1) and a fast response time of less than 40 s. It displayed a stable potential response in the pH range of 2–5. It exhibited also high selectivity over some interfering ions. The proposed sensor was successfully applied for the determination of Cr(III) in real samples (sea, river water and soil).     

Anodic Stripping Voltammetry Determination of Pb(II) and Cd(II) at a Bismuth/Poly(aniline) Film Electrode

Abstract

A novel voltammetric method—anodic—using a bismuth/poly(aniline) film electrode has been developed for simultaneous measurement of Pb(II) and Cd(II) at low µg L^?1 concentration levels by stripping voltammetry. The results confirmed that the bismuth/poly(aniline) film electrode offered high‐quality stripping performance compared with the bismuth film electrode. Well‐defined sharp stripping peaks were observed for Pb(II) and Cd(II), along with an extremely low baseline. The detection limits of Pb(II) and Cd(II) are 1.03 µg L^?1 and 1.48 µg L^?1, respectively. The bismuth/poly (aniline) electrode has been applied to the determination of Pb(II) in tap water samples with satisfactory results.     

Speciation of Cr(III) and Cr(VI) using solid phase extraction and determination of total As inductively coupled plasma atomic emission spectrometry

An inductively coupled plasma atomic emission spectrometric (ICP-AES) method was developed for speciation and simultaneous determination of Cr and As, since these two analytes are commonly determined in various water samples in order to assess their toxicity. The objective of this research was to study the speciation of Cr(III), Cr(VI) in the presence of As(III) and/or As(V) using solid phase extraction (SPE) and ICP-AES. For these measurements, four spectral lines were used for each analyte with the purpose of selecting the most appropriate for each element. Finally with the use for first time of a cation-exchange column filled with benzosulfonic acid and elution with HCl, the speciation in solutions which contained [Cr(III)?+?Cr(VI)?+?As(V)] and [Cr(III)?+?Cr(VI)?+?As(III)] was examined. It was demonstrated that the separation of the two chromium species is almost quantitative and the simultaneous determination of chromium species and total arsenic analytes is possible, with very good performance characteristics. The estimated limits of detection for Cr(III), Cr(VI), As(III) and/or As(V) were 0.9?µg?L^?1, 1.1 µg?L^?1, 4.7 µg?L^?1 and 4.5 µg?L^?1 respectively, the calculated relative standard deviations (RSDs) were 3.8%, 4.1%, 5.2% and 5.1% respectively, and finally the accuracy of the methods was estimated using a certified aqueous reference material and found to be 5.6% and 4.8% for Cr(III) and Cr(VI) respectively. The method was applied to the routine analysis of various water samples.     

Adsorptive Stripping Voltammetric Determination of Antimony by Using Alizarin Red S

Abstract

A sensitive and selective voltammetric method for determination of antimony(III) using Alizarin Red S (ARS) as complexing agent is described. The method is based on the monitoring the oxidation peak of antimony(III)-ARS complex at ?520 mV in ammonium-ammonia buffer (pH = 7.5). The peak current was measured by scanning the potential from ?700 mV versus Ag/AgClto more positive potentials without accumulation in the presence of 1 × 10^?6 mol L^?1 of ARS. The limit of detection (3 s) and limit of quantification (10 s) of the method were calculated from calibration curve as 1.45 µg L^? and 4.8 µg L^? respectively. The calibration plot for antimony(III) was linear in the range of 4.8–30 µg L^?. The interference of various ions was examined. Serious interference from Al(III), Fe(III), Cu(II), Pb(II), and Zn(II) was eliminated by addition of EDTA to the solution. The method was applied to drinking water samples. The recoveries were in the range 94% – 105%. The results obtained from the developed method were compared with those from the differential-pulse anodic-stripping method and no statistically significant difference was found.     

Determination of Sb(III) and Total Sb in Antileishmanial Drugs by Spectrophotometric Flow‐Injection Hydride Generation

Abstract

A spectrophotometric procedure based on hydride generation and flow analysis is proposed for determination of antimony (III) [Sb(III)] and total antimony (Sb) in pharmaceutical samples. Firstly, Sb(III) reacts with hydrogen species generated in the system, forming antimony hydride. The reaction leads to a decrease in the permanganate concentration and, hence, in the intensity of the color of this specie, which is spectrophotometrically measured at 528 nm. Total Sb is determined as Sb(III) after Sb(V) reduction using 0.02% (m/v) KI. Some parameters, such as the number of channels of the gas phase separator, injection volume, coil length, and KBH₄ concentration, are investigated. The system presents a frequency of ca. 100 h^?1 and precision <3.0% [expressed as relative standard deviation (RSD) of 30 measurements using a 3.0 mg L^?1 Sb(III) solution]. The analytical curve ranging from 0.5 mg L^?1 to 5.0 mg L^?1 (r>0.998; n=5) permits limit of detection (LOD) and limit of quantification (LOQ) of 83 and 250 µg L^?1. For total Sb, the accuracy is checked by atomic absorption spectrometry applying the t test and the results are in accordance at the 95% confidence level. Recovery tests are used to check the accuracy for Sb(III) determination, and the recoveries are between 95% and 105%.     

Application of ultrasonic-assisted cloud point extraction/flame atomic absorption spectrometry (UA-CPE/FAAS) for preconcentration and determination of low levels of antimony in some beverage samples

Due to be able to migrate or leach from food packaging materials into the foods and/or beverages, development of a new, sensitive and selective analytical methods for low levels of antimony as a food contaminant is of great importance in terms of food safety. In this context, an ultrasonic-assisted cloud point extraction method was developed for the preconcentration and determination of antimony as Sb(III) using 4-(2-thiazolylazo)resorcinol (TAR) and 2-(2-thiazolylazo)-p-cresol (TAC) as chelating agents and sodium dodecyl sulfate as signal enhancing agent at pH 6.0 and mediated by nonionic surfactant, t-octylphenoxypolyethoxyethanol by flame atomic absorption spectrometry. Using the optimized conditions, the calibration curves obtained from Sb(III) with TAR and TAC were linear in the concentration ranges of 0.5–180 and 1–180 μg L^?1 with detection limits of 0.13 and 0.28 μg L^?1, respectively. The precision (as relative standard deviations, RSDs) was lower than 3.9 % (25 and 100 μg L^?1, n: 6). The method accuracy was validated by the analysis of two standard reference materials. The results obtained were statistically in a good agreement with the certified values at 95 % confidence limit. The method has successfully been applied to the determination of Sb(III) and total Sb in selected beverages, milk and fruit-flavored milk products before and after pre-reduction of Sb(V) to Sb(III) with a mixture of KI/ascorbic acid in acidic media. The Sb(V) contents of samples were quantitatively calculated from analytical difference between total Sb and Sb(III) levels.     

The electrochemistry of uric acid at a gold electrode modified with L-cysteine, and its application to sensing uric urine

The electrochemistry of uric acid at a gold electrode modified with a self-assembled film of L-cysteine was studied by cyclic voltammetry and differential pulse voltammetry. Compared to the bare gold electrode, uric acid showed better electrochemical response in that the anodic peak current is stronger and the peak potential is negatively shifted by about 100 mV. The effects of experimental conditions on the oxidation of uric acid were tested and a calibration plot was established. The differential pulse response to uric acid is linear in the concentration range from 1.0?×?10^?6 to ~?1.0?×?10^?4 mol?L^?1 (r?=?0.9995) and from 1.0?×?10^?4 to ~?5.0?×?10^?4 mol?L^?1 (r?=?0.9990), the detection limit being 1.0?×?10^?7 mol?L^?1 (at S/N?=?3). The high sensitivity and good selectivity of the electrode was demonstrated by its practical application to the determination of uric acid in urine samples.

Speciation analysis of inorganic vanadium (V(IV)/V(V)) by graphite furnace atomic absorption spectrometry following ion-exchange separation

A sensitive and simple method of ion-exchange resin separation and graphite-furnace atomic absorption spectrometry (GFAAS) detection was proposed for the determination of inorganic vanadium species. Methylene Blue (MB) was used as a chelating agent of V(V) for ion-exchange separation. The complex of V(V) and MB could be trapped by ion-exchange resin at pH 3.0 and eluted by 1.0?mol?L^?1 NaOH. The vanadium species was determined subsequently by GFAAS. The concentration of V(IV) was calculated by subtracting the V(V) concentration from the total concentration of vanadium. Under the optimized experimental conditions, the detection limit of V(V) is 0.48?µg?L^?1 with RSD of 2.6% (n?=?5, c?=?2.0?µg?L^?1). In order to verify the accuracy of the method, a certified reference soil sample was analyzed, and the results obtained were in good agreement with the certified values. The range of recovery for V(IV) and V(V) was 97.8–99.3% and 101.7–103.6%, respectively. The proposed method was applied to the speciation analysis of vanadium in lake-water samples.     

Simultaneous Detection of Copper,Lead and Zinc on Tin Film/Gold Nanoparticles/Gold Microelectrode by Square Wave Stripping Voltammetry

In this work, three heavy metals (Cu(II), Pb(II) and Zn(II)) in wide potential window were simultaneously detected on tin film/gold nanoparticles/gold microelectrode (Sn/GNPs/gold microelectrode) by the method of square wave stripping voltammetry. The Sn/GNPs/gold microelectrode was fabricated by in situ plating of a Sn film on a gold nanoparticles (GNPs) modified gold microelectrode. The influence of hydrogen overflow on stripping of Zn(II) on the gold microelectrode was reduced by modification of GNPs, which made the stripping potential of target metals shift positively. The interference of sulfhydryl groups was reduced and the selectivity of the microelectrode was improved due to the addition of Sn in the detection solution. After accumulation at ?1.4 V for 300 s in acetate buffer solution (0.1 mol L^?1, pH 4.5), the Sn/GNPs/gold microelectrode revealed a good linear behavior in the examined concentration ranges from 5 to 500 µg L^?1 for Cu(II) and Pb(II), and from 10 to 500 µg L^?1 for Zn(II), with a limit of detection of 2 µg L^?1 for Cu(II), 3 µg L^?1 for Pb(II) and 5 µg L^?1 for Zn(II) (S/N=3). When compared with a Sb/GNPs/gold microelectrode and a Bi/GNPs/gold microelectrode, the Sn/GNPs/gold microelectrode showed the best stripping performance to Cu(II), Pb(II) and Zn(II). As a new type of environment‐friendly electrode, the Sn/GNPs/gold microelectrode has potential applications for detection of heavy metals.     

Unexpectedly high levels of antimony (III) in the pentavalent antimonial drug Glucantime: insights from a new voltammetric approach

Glucantime, a pentavalent antimonial drug, is commonly used for the treatment of leishmaniasis but the presence of residual trivalent antimony, Sb(III), is thought to be responsible for toxic side-effects observed in patients. Numerous analytical studies have focused on determining Sb(III) concentrations in Glucantime but without reaching a consensus: results span over 3 orders of magnitude. In this study, we present a detailed new analytical approach showing that: (1) Sb(III) levels are much higher than previously reported and represent more than 30 % of total Sb; (2) determination of Sb(III) concentrations in acidic conditions is hampered by fast oxidation rates. This latter point explains the large variations in previously reported results of Sb(III) concentrations in Glucantime. Measurements were made here at a vibrated gold microwire electrode by stripping voltammetry enabling measurement of Sb(III) in acidic, neutral or alkaline conditions. The developed methods are sensitive (e.g., detection limits of 19 pM for 120 s deposition at pH 4.5), stable (<6 %, N?=?100), precise (5 %, N?=?5) and robust (same electrode used for weeks) at all pH values. In diluted solutions of Glucantime, Sb(III) levels were strongly dependent both on pH and ionic strength. At pH?<?3, Sb(III) is oxidized with oxidation rates that increase as pH is decreased. At high pH, Sb(III) forms electro-inactive complexes. Highest Sb(III) levels were detected at pH ～3 and at low ionic strength. The presence of several Sb(III) and Sb(V) species was demonstrated by different reduction waves obtained by stripping scanned voltammetry. As an implication of these unexpectedly high Sb(III) concentrations, an alternative model can be proposed for the mode of action of pentavalent antimonials against leishmaniasis, in which antimony complexes may act as molecular carrier of Sb(III) and release it specifically in the acidic intracellular compartment where the Leishmania parasites reside.