troubleshooting, testing, & analysis

CONTROLCHEMICAL ANALYSIS OF

PLATING SOLUTIONS*

BY CHARLES ROSENSTEIN

TESSERA-ISRAEL, LTD., JERUSALEM, ISRAEL

AND STANLEY HIRSCH

LEEAM CONSULTANTS LTD., NEW ROCHELLE, N.Y.

Plating solutions must be routinely analyzed in order to maintain the recommendedbath formulation and to preempt the occurrence of problems related toimproper levels of bath constituents. Contaminant levels in the solutions mustalso be monitored. Manufacturers of plating systems establish optimum specificationsto ensure maximum solution efficiency and uniformity of deposits.The various factors that cause the concentrations of bath constituents to deviatefrom their optimum values are as follows:

1. drag-out;

2. solution evaporation;

3. chemical decomposition; and

4. unequal anode and cathode efficiencies.

A current efficiency problem is recognized by gradual but continuous changesin pH, metal content, or cyanide content (see Table I).The techniques employed for the quantitative analysis of plating solutions areclassified as volumetric (titrimetric), gravimetric, and instrumental. Volumetricand gravimetric methods are also known as “wet” methods. The analyst mustselect the method that is best suited and most cost effective for a particularapplication.The wet methods outlined here are simple, accurate, and rapid enough forpractically all plating process control. They require only the common analyticalequipment found in the laboratory, and the instructions are sufficiently detailedfor an average technician to follow without any difficulty. The determinationof small amounts of impurities and uncommon metals should be referred toa competent laboratory, as a high degree of skill and chemical knowledge arerequired for the determination of these constituents.Hull cell testing (see the section on plating cells elsewhere in this Guidebook)enables the operator to observe the quality of a deposit over a wide currentdensity range.

VOLUMETRIC METHODS

When titrants composed of standard solutions are added to a sample that containsa component whose concentration is to be quantitatively determined, themethod is referred to as a volumetric method. The component to be determinedmust react completely with the titrant in stoichiometric proportions. From thevolume of titrant required, the component’s concentration is calculated. Thesimplicity, quickness, and relatively low cost of volumetric methods make themthe most widely used for the analysis of plating and related solutions.Volumetric methods involve reactions of several types: oxidation-reduction,acid-base, complexation, and precipitation. Indicators are auxiliary reagents,which usually signify the endpoint of the analysis. The endpoint can be indicatedby a color change, formation of a turbid solution, or the solubilization ofa turbid solution.Some volumetric methods require little sample preparation, whereas othersmay require extensive preparation. Accuracy decreases for volumetric analyses ofcomponents found in low concentrations, as endpoints are not as easily observedas with the components found in high concentrations.Volumetric methods are limited in that several conditions must be satisfied.Indicators should be available to signal the endpoint of the titration. The component-titrant reaction should not be affected by interferences from other substancesfound in the solution.

GRAVIMETRIC METHODS

In gravimetric methods, the component being determined is separated from othercomponents of the sample by precipitation, volatilization, or electroanalyticalmeans. Precipitation methods are the most important gravimetric methods. Theprecipitate is usually a very slightly soluble compound of high purity that containsthe component. The weight of the precipitate is determined after it is filtered fromsolution, washed, and dried. Gravimetric methods are used to supplement theavailable volumetric methods.Limitations of gravimetric methods include the requirement that the precipitatedcomponent has an extremely low solubility. The precipitate must also be ofhigh purity and be easily filterable.Species that are analyzed gravimetrically include chloride, sulfate, carbonate,phosphate, gold, and silver.

INSTRUMENTAL METHODS

Instrumental methods differ from wet methods in that they measure a physicalproperty related to the composition of a substance, whereas wet methods relyon chemical reactions. The selection of an instrument for the analysis of platingsolutions is a difficult task. Analysts must decide if the cost is justified and if theanalytical instrument is capable of analyzing for the required substances with ahigh degree of accuracy and precision. Instruments coupled to computers canautomatically sample, analyze, and record results. Mathematical errors are minimizedand sample measurements are more reproducible than with wet methods.Instrumental methods are also extremely rapid when compared with wet methods.Unlike humans, instruments cannot judge. They cannot recognize impropersample preparation or interfering substances. Erroneous results are sometimesproduced by electronic and mechanical malfunctions.Analytical instruments frequently used in the analysis of plating solutions canbe categorized as spectroscopic, photometric, chromatographic, and electroanalytical.Spectroscopic methods (flame photometry, emission spectrometry, X-rayfluorescence, mass spectrometry, and inductively coupled plasma) are based onthe emission of light. Photometric methods (spectrophotometry, colorimetry,and atomic absorption) are based on the absorption of light. Chromatographicmethods (ion chromatography) involve the separation of substances for subsequentidentification. Electroanalytical methods (potentiometry, conductometry,polarography, amperometry, and electrogravimetry) involve an electric current inthe course of the analysis.The instrumental methods, comprehensively reviewed below, are most applicableto plating environments.

SPECTROSCOPIC METHODS

Spectroscopy is the analysis of a substance by the measurement of emitted light.When heat, electrical energy, or radiant energy is added to an atom, the atombecomes excited and emits light. Excitation can be caused by a flame, spark, X-rays,or an AC or DC arc. The electrons in the atom are activated from their groundstate to unstable energy shells of higher potential energy. Upon returning to theirground state, energy is released in the form of electromagnetic radiation.Because each element contains atoms with different arrangements of outermostelectrons, a distinct set of wavelengths is obtained. These wavelengths, from atomsof several elements, are separated by a monochromator such as a prism or a diffractiongrating. Detection of the wavelengths can be accomplished photographically(spectrograph) or via direct-reading photoelectric detectors (spectrophotometers).The measurement of intensity emitted at a particular wavelength is proportionalto the concentration of the element being analyzed.An advantage of spectroscopy is that the method is specific for the elementbeing analyzed. It permits quantitative analysis of trace elements without anypreliminary treatment and without prior knowledge as to the presence of the element.Most metals and some nonmetals may be analyzed. Spectroscopic analysisis also useful for repetitive analytical work.Disadvantages of spectroscopic analysis include the temperature dependenceof intensity measurements, as intensity is very sensitive to small fluctuationsintemperature. The accuracy and precision of spectrographic methods is not as highas some spectrophotometric methods or wet analyses. Spectrographic methodsare usually limited to maximum element concentrations of 3%. Additionally, sensitivityis much smaller for elements of high energy (e.g., zinc) than for elementsof low energy (e.g., sodium).Applications of spectroscopy include the analysis of major constituents andimpurities in plating solutions, and of alloy deposits for composition.

Flame Photometry

In flame photometry (FP), a sample in solution is atomized at constant air pressureand introduced in its entirety into a flame as a fine mist. The temperatureof the flame (1,800-3,100OK) is kept constant. The solvent is evaporated and thesolid is vaporized and then dissociated into ground state atoms. The valenceelectrons of the ground state atoms are excited by the energy of the flame tohigher energy levels and then fall back to the ground state. The intensities of theemitted spectrum lines are determined in the spectrograph or measured directlyby a spectrophotometer.The flame photometer is calibrated with standards of known composition andconcentration. The intensity of a given spectral line of an unknown can then becorrelated with the amount of an element present that emits the specific radiation.Physical interferences may occur from solute or solvent effects on the rate oftransport of the sample into the flame. Spectral interferences are caused by adjacentline emissions when the element being analyzed has nearly the same wavelengthas another element. Monochromators or the selection of other spectral linesminimize this interference. Ionization interferences may occur with the highertemperature flames. By adding a second ionizable element, the interferences dueto the ionization of the element being determined are minimized.An advantage of FP is that the temperature of the flame can be kept morenearly constant than with electric sources. A disadvantage of the method is thatthe sensitivity of the flame source is many times smaller than that of an electricarc or spark.FP is used for the analysis of aluminum, boron, cadmium, calcium, chromium,cobalt, copper, indium, iron, lead, lithium, magnesium, nickel, palladium, platinum,potassium, rhodium, ruthenium, silver, sodium, strontium, tin, and zinc.

Emission Spectrometry

In emission spectrometry (ES), a sample composed of a solid, cast metal or solutionis excited by an electric discharge such as an AC arc, a DC arc, or a spark.The sample is usually placed in the cavity of a lower graphite electrode, which ismade positive. The upper counterelectrode is another graphite electrode groundto a point. Graphite is the preferred electrode material because of its ability towithstand the high electric discharge temperatures. It is also a good electricalconductor and does not generate its own spectral lines.The arc is started by touching the two graphite electrodes and then separatingthem. The extremely high temperatures (4,000-6,000OK) produce emitted radiationhigher in energy and in the number of spectral lines than in flame photometry.Characteristic wavelengths from atoms of several elements are separated bya monochromator and are detected by spectrographs or spectrophotometers.Qualitative identification is performed by using available charts and tables toidentify the spectral lines that the emission spectrometer sorts out according totheir wavelength. The elements present in a sample can also be qualitatively determinedby comparing the spectrum of an unknown with that of pure samples ofthe elements. The density of the wavelengths is proportional to the concentrationof the element being determined. Calibrations are done against standard samples.ES is a useful method for the analysis of trace metallic contaminants in platingbaths. The “oxide” method is a common quantitative technique in ES. A sampleof the plating bath is evaporated to dryness and then heated in a muffle furnace.The resultant oxides are mixed with graphite and placed in a graphite electrode.Standards are similarly prepared and a DC arc is used to excite the sample andstandards.

X-ray Fluorescence

X-ray fluorescence (XRF) spectroscopy is based on the excitation of samples by anX-ray source of sufficiently high energy, resulting in the emission of fluorescentradiation. The concentration of the element being determined is proportional tothe intensity of its characteristic wavelength. A typical XRF spectrometer consistsof an X-ray source, a detector, and a data analyzer.Advantages of XRF include the nondestructive nature of the X-rays on thesample. XRF is useful in measuring the major constituents of plating baths suchas cadmium, chromium, cobalt, gold, nickel, silver, tin, and zinc. Disadvantagesof XRF include its lack of sensitivity as compared with ES.X-ray spectroscopy is also used to measure the thickness of a plated deposit.The X-ray detector is placed on the wavelength of the element being measured.The surface of the deposit is exposed to an X-ray source and the intensity of theelement wavelength is measured. A calibration curve is constructed for intensityagainst thickness for a particular deposit. Coating compositions can also be determinedby XRF.

Mass Spectrometry

In mass spectrometry (MS), gases or vapors derived from liquids or solids arebombarded by a beam of electrons in an ionization chamber, causing ionizationand a rupture of chemical bonds. Charged particles are formed, which may becomposed of elements, molecules, or fragments. Electric and magnetic fields thenseparate the ions according to their mass to charge ratios (m/e). The amount andtype of fragments produced in an ionization chamber, for a particular energy ofthe bombarding beam, are characteristic of the molecule; therefore, every chemicalcompound has a distinct mass spectrum. By establishing a mass spectrum ofseveral pure compounds, an observed pattern allows identification and analysisof complex mixtures.The mass spectrum of a compound contains the masses of the ion fragmentsand the relative abundances of these ions plus the parent ion. Dissociation fragmentswill always occur in the same relative abundance for a particular compound.MS is applicable to all substances that have a sufficiently high vapor pressure.This usually includes substances whose boiling point is below 450OC. MS permitsqualitative and quantitative analysis of liquids, solids, and gases.

Inductively Coupled Plasma

Inductively coupled plasma (ICP) involves the aspiration of a sample in a streamof argon gas, and then its ionization by an applied radio frequency field. The fieldis inductively coupled to the ionized gas by a coil surrounding a quartz torch thatsupports and encloses the plasma. The sample aerosol is heated in the plasma, themolecules become almost completely dissociated and then the atoms present inthe sample emit light at their characteristic frequencies. The light passes througha monochromator and onto a detector.The high temperature (7,000OK) of the argon plasma gas produces efficientatomic emission and permits low detection limits for many elements. As withatomic absorption (AA),ICP does not distinguish between oxidation states (e.g.,Cr3+ and Cr6+) of the same element—the total element present is determined.Advantages of ICP include complete ionization and no matrix interferences asin AA. ICP allows simultaneous analysis of many elements in a short time. It issensitive to part-per-billion levels.Disadvantages of ICP include its high cost and its intolerance to samples withgreater than 3% dissolved solids. Background corrections usually compensate forinterferences due to background radiation from other elements and the plasmagases. Physical interferences, due to viscosity or surface tension, can cause significanterrors. These errors are reduced by diluting the sample. Although chemicalinterferences are insignificant in the ICP method, they can be greatly minimizedby careful selection of the instrument’s operating conditions, by matrix matching,or by buffering the sample.ICP is applicable to the analysis of major components and trace contaminantsin plating solutions. It is also useful for waste-treatment analysis.

PHOTOMETRIC METHODS

Photometric methods are based on the absorption of ultraviolet (200-400 nm)or visible (400-1,000 nm) radiant energy by a species in solution. The amount ofenergy absorbed is proportional to the concentration of the absorbing species insolution. Absorption is determined spectrophotometrically or colorimetrically.The sensitivity and accuracy of photometric methods must be frequentlychecked by testing standard solutions in order to detect electrical, optical, ormechanical malfunctions in the analytical instrument.

Spectrophotometry and Colorimetry

Spectrophotometry involves analysis by the measurement of the light absorbed bya solution. The absorbance is proportional to the concentration of the analytein solution. Spectrophotometric methods are most often used for the analysisof metals with concentrations of up to 2%.Spectrophotometers consist of a light source (tungsten or hydrogen), a monochromator,a sample holder, and a detector. Ultraviolet or visible light of a definitewavelength is used as the light source. Detectors are photoelectric cells thatmeasure the transmitted (unabsorbed) light. Spectrophotometers differ fromphotometers in that they utilize monochromators, whereas photometers use fil-ters to isolate the desired wavelength region. Filters isolate a wider band of light.In spectrophotometric titrations, the cell containing the analyte solutionis placed in the light path of a spectrophotometer. Titrant is added to the cellwith stirring, and the absorbance is measured. The endpoint is determinedgraphically. Applications of this titration include the analysis of a mixture ofarsenic and antimony and the analysis of copper with ethylene diamine tetraacetic acid (EDTA).The possibility of errors in spectrophotometric analyses is increased whennumerous dilutions are required for an analysis.Colorimetry involves comparing the color produced by an unknown quantityof a substance with the color produced by a standard containing a known quantityof that substance. When monochromatic light passes through the coloredsolution, a certain amount of the light, proportional to the concentration of thesubstance, will be absorbed. Substances that are colorless or only slightly coloredcan be rendered highly colored by a reaction with special reagents.In the standard series colorimetric method, the analyte solution is diluted toa certain volume (usually 50 or 100 ml) in a Nessler tube and mixed. The colorof the solution is compared with a series of standards similarly prepared. Theconcentration of the analyte equals the concentration of the standard solutionwhose color it matches exactly. Colors can also be compared to standards via acolorimeter (photometer), comparator, or spectrophotometer.The possible errors in colorimetric measurements may arise from the followingsources: turbidity, sensitivity of the eye or color blindness, dilutions, photometerfilters, chemical interferences, and variations in temperature or pH.Photometric methods are available for the analysis of the following analytes:Anodizing solutions: Fe, Cu, agents.

Atomic Absorption

Metals in plating and related solutions can be readily determined by AA spectrophotometry.Optimum ranges, detection limits, and sensitivities of metals varywith the various available instruments.In direct-aspiration atomic absorption (DAAA) analysis, the flame (usually air-acetyleneor nitrous oxide-acetylene) converts the sample aerosol into atomic vapor,which absorbs radiation from a light source. A light source from a hollow cathodelamp or an electrodeless discharge lamp is used, which emits a spectrum specificto the element being determined. The high cost of these lamps is a disadvantageof the AA method. A detector measures the light intensity to give a quantitativedetermination.DAAA is similar to flame photometry in that a sample is aspirated into a flameand atomized. The difference between the two methods is that flame photometrymeasures the amount of emitted light, whereas DAAA measures the amount oflight absorbed by the atomized element in the flame. In DAAA, the number ofatoms in the ground state is much greater than the number of atoms in any of theexcited states of the spectroscopic methods. Consequently, DAAA is more efficientand has better detection limits than the spectroscopic methods.Spectral interferences occur when a wavelength of an element being analyzedis close to that of an interfering element. The analysis will result in an erroneouslyhigh measurement. To compensate for this interference, an alternate wavelengthor smaller slit width is used.When the physicalproperties (e.g., viscosity) of a sample differ from those of thestandard, matrix interferences occur. Absorption can be enhanced orsuppressed.To overcome these interferences, matrix components in the sample and standardare matched or a release agent, such as EDTA or lanthanum, is added.Chemical interferences are the most common interferences encountered in AAanalysis. They result from the nonabsorption of molecularly bound atoms in theflame. These interferences are minimized by using a nitrous oxide-acetylene flameinstead of an air-acetylene flame to obtain the higher flame temperature neededto dissociate the molecule or by adding a specific substance (e.g.,lanthanum) torender the interferant harmless. Chemical interferences can also be overcome byextracting the element being determinedor by extracting the interferant fromthe sample.The sensitivity and detection limits in AA methods vary with the instrumentused, the nature of the matrix, the type of element being analyzed, and the particularAA technique chosen. It is best to use concentrations of standards andsamples within the optimum concentration range of the AA instrument. WhenDAAA provides inadequate sensitivity, other specialized AA methods, such asgraphite furnace AA, cold vapor AA, or hydride AA, are used.In graphite furnace AA (GFAA), the flame that is used in DAAA is replacedwith an electrically heated graphite furnace. A solution of the analyte is placedin a graphite tube in the furnace, evaporated to dryness, charred, and atomized.The metal atoms being analyzed are propelled into the path of the radiationbeam by increasing the temperature of the furnace and causing the sample tobe volatilized. Only very small amounts of sample are required for the analysis.GFAA is a very sensitive technique and permits very low detection limits. Theincreased sensitivity is due to the much greater occupancy time of the groundstate atoms in the optical path as compared with DAAA. Increased sensitivity canalso be obtained by using larger sample volumes or by using an argon-hydrogenpurge gas mixture instead of nitrogen. Because of its extreme sensitivity, determiningthe optimum heating times, temperature, and matrix modifiers is necessaryto overcome possible interferences.Interferences may occur in GFAA analysis due to molecular absorption andchemical effects. Background corrections compensate for the molecular absorptioninterference. Specially coated graphite tubes minimize its interaction withsome elements. Gradual heating helps to decrease background interference, andpermits determination of samples with complex mixtures of matrix components.The GFAA method has been applied to the analysis of aluminum, antimony,arsenic, barium, beryllium, cadmium, chromium, cobalt, copper, iron, lead,manganese, molybdenum, nickel, selenium, silver, and tin.Cold vapor atomic absorption (CVAA) involves the chemical reduction of mercuryor selenium by stannous chloride and its subsequent analysis. The reduced solutionis vigorously stirred in the reaction vessel to obtain an equilibrium betweenthe element in the liquid and vapor phases. The vapor is then purged into anabsorption cell located in the light path of a spectrophotometer. The resultantabsorbance peak is recorded on a strip chart recorder.The extremely sensitive CVAA procedure is subject to interferences from someorganics, sulfur compounds, and chlorine. Metallic ions (e.g., gold, selenium),which are reduced to the elemental state by stannous chloride, produce interferencesif they combine with mercury.Hydride atomic absorption (HAA) is based on chemical reduction with sodiumborohydride to selectively separate hydride-forming elements from a sample.The gaseous hydride that is generated is collected in a reservoir attached to ageneration flask, and is then purged by a stream of argon or nitrogen into anargon-hydrogen-air flame. This permits high-sensitivity determinations of antimony,arsenic, bismuth, germanium, selenium, tellurium, and tin.The HAA technique is sensitive to interferences from easily reduced metalssuch as silver, copper, and mercury. Interferences also arise from transitionmetals in concentrations greater than 200 mg/L and from oxides of nitrogen.

Ion Chromatography

In ion chromatography (IC), analytes are separated with an eluent on a chromatographiccolumn based on their ionic charges. Because plating solutionsare water based, the soluble components must be polar or ionic; therefore, IC isapplicable to the analysis of plating and related solutions.Ion chromatographs consist of a sample delivery system, a chromatographicseparation column, a detection system, and a data handling system.IC permits the rapid sequential analysis of multiple analytes in one sample. Thevarious detectors available, such as UV-visible, electrochemical, or conductivity,allow for specific detection in the presence of other analytes. IC is suitable for theanalysis of metals, anionic and cationic inorganic bath constituents, and variousorganic plating bath additives. It is also used for continuous on-line operations.Interferences arise from substances that have retention times coinciding withthat of any anion being analyzed. A high concentration of a particular ion mayinterfere with the resolution of other ions. These interferences can be greatlyminimized by gradient elution or sample dilution.IC has been applied to the analysis of the following analytes in plating andrelated solutions:Metals: Aluminum, barium, cadmium, calcium, trivalent and hexavalent chromium,cobalt, copper, gold, iron, lead, lithium, magnesium, nickel, palladium,platinum, silver, tin,zinc.Ions: Ammonium, bromide, carbonate, chloride, cyanide, fluoborate, fluoride,hypophosphite, nitrate, nitrite, phosphate, potassium,sodium, sulfate, sulfide,sulfite.Acid Mixtures: Hydrofluoric, nitric, and acetic acids.Organics: Brighteners, surfactants, organic acids.

ELECTROANALYTICAL METHODS

Electroanalytical methods involve the use of one or more of three electrical quantities—current, voltage, and resistance. These methods are useful when indicatorsfor a titration are unavailable or unsuitable. Although trace analysis may be donequite well by spectroscopic or photometric methods, electroanalytical methodsoffer ease of operation and relatively lower costs of purchase and maintenance.

Potentiometry

Potentiometry involves an electrode that responds to the activity of a particulargroup of ions in solution. Potentiometric methods correlate the activity of anion with its concentration in solution.In potentiometric titrations, titrant is added to a solution and the potentialbetween an indicator and reference electrode is measured. The reaction mustinvolve the addition or removal of an ion for which an electrode is available.Acid-base titrations are performed with a glass indicator electrode and a calomelreference electrode. The endpoint corresponds to the maximum rate of changeof potential per unit volume of titrant added.Advantages of potentiometric titrations include its applicability to colored,turbid, or fluorescent solutions. It is also useful in situations where indicatorsare unavailable.The sensitivity of potentiometric titrations is limited by the accuracy of themeasurement of electrode potentials at low concentrations. Solutions thatare more dilute than 10-5 N cannot be accurately titrated potentiometrically.This is because the experimentally measured electrode potential is a combinedpotential, which may differ appreciably from the true electrode potential. Thedifference between the true and experimental electrode potentials is due to theresidual current, which arises from the presence of electroactive trace impurities.The direct potentiometric measurement of single ion concentrations is donewith ion selective electrodes (ISEs). The ISE develops an electric potential inresponse to the activity of the ion for which the electrode is specific. ISEs areavailable for measuring calcium, copper, lead, cadmium, ammonia, bromide,nitrate, cyanide, sulfate, chloride, fluoride, and other cations and anions.Cation ISEs encounter interferences from other cations, and anion ISEsencounter interferences from other anions. These interferences can be eliminatedby adjusting the sample pH or by chelating the interfering ions. ISEinstructions must be reviewed carefully to determine the maximum allowablelevels of interferants, the upper limit of the single ion concentration for the ISE,and the type of media compatible with the particular ISE.Some of the solutions that can be analyzed by potentiometric methods are:Anodizing solutions: Al,

Anodizing solutions: Al, H2SO4, C2H2O4, CrO3, Cl

Brass solutions: Cu, Zn, NH3, CO3

Bronze solutions: Cu, Sn, NaOH, NaCN, Na2CO3

Chromium solutions: Cr, Cl

Cadmium solutions: Cd, NaOH, NaCN, Na2CO3

Acid copper solutions: Cl

Alkaline copper solutions: NaOH, NaCN, Na2CO3

Gold solutions: Au, Ag, Ni, Cu

Lead and tin/lead solutions: Pb, Sn, HBF4

Nickel solutions: Co, Cu, Zn, Cd, Cl, H3BO3

Silver solutions: Ag, Sb, Ni

Acid tin solutions: Sn, HBF4, H2SO4

Alkaline tin solutions: Sn, NaOH, NaCO3, Cl

Conductometry

Electrolytic conductivity measures a solution’s ability to carry an electric current.A current is produced by applying a potential between two inert metallicelectrodes (e.g., platinum) inserted into the solution being tested. When othervariables are held constant, changes in the concentration of an electrolyte resultin changes in the conductance of electric current by a solution.In conductometric titrations, the endpoint of the titration is obtained from aplot of conductance against the volume of titrant. Excessive amounts of extraneousforeign electrolytes can adversely affect the accuracy of a conductometrictitration.Conductometric methods are used when wet or potentiometric methodsgive inaccurate results due to increased solubility (in precipitation reactions)or hydrolysis at the equivalence point. The methods are accurate in both diluteand concentrated solutions, and they can also be used with colored solutions.Conductometric methods have been applied to the analysis of Cr, Cd, Co, Fe,

Ni, Pb, Ag, Zn, CO3, Cl, F, and SO4.

Polarography

In polarography, varying voltage is applied to a cell consisting of a large mercuryanode (reference electrode) and a small mercury cathode (indicator electrode)known as a dropping mercury electrode (DME). Consequent changes in currentare measured. The large area of the mercury anode precludes any polarization.The DME consists of a mercury reservoir attached to a glass capillary tube withsmall mercury drops falling slowly from the opening of the tube. A saturatedcalomel electrode is sometimes used as the reference electrode.The electrolyte in the cell consists of a dilute solution of the species beingdetermined in a medium of supporting electrolyte. The supporting electrolytefunctions to carry the current in order to raise the conductivity of the solution.This ensures that if the species to be determined is charged, it will not migrateto the DME. Bubbling an inert gas, such as nitrogen or hydrogen, through thesolution prior to running a polarogram, will expel dissolved oxygen in order toprevent the dissolved oxygen from appearing on the polarogram.Reducible ions diffuse to the DME. As the applied voltage increases, negligiblecurrent flow results until the decomposition potential is reached for the metalion being determined. When the ions are reduced at the same rate as they diffuseto the DME, no further increases in current occur, as the current is limited bythe diffusion rate. The half-wave potential is the potential at which the currentis 50% of the limiting value.Polarograms are obtained by the measurement of current as a function ofapplied potential. Half-wave potentials are characteristic of particular substancesunder specified conditions. The limiting current is proportional to the concentrationof the substance being reduced. Substances can be analyzed quantitativelyand qualitatively if they are capable of undergoing anodic oxidation orcathodic reduction. As with other instrumental methods, results are referred tostandards in order to quantitate the method.Advantages of polarographic methods include their ability to permit simultaneousqualitative and quantitative determinations of two or more analytes inthe same solution. Polarography has wide applicability to inorganic, organic,ionic, or molecular species.Disadvantages of polarography include the interferences caused by large concentrationsof electropositive metals in the determination of low concentrationsof electronegative metals. The very narrow capillary of the DME occasionallybecomes clogged.Polarographic methods are available for the following solutions:

Anodizing solutions: Cu, Zn, Mn

Brass solutions: Pb, Cd, Cu, Ni, Zn

Bronze solutions: Pb, Zn, Al, Cu, Ni

Cadmium solutions: Cu, Pb, Zn, Ni

Chromium solutions: Cu, Ni, Zn, Cl, SO4

Acid copper solutions: Cu, Cl

Alkaline copper solutions: Zn, Fe, Pb, Cu

Gold solutions: Au, Cu, Ni, Zn, In, Co, Cd

Iron solutions: Mn

Lead and tin-lead solutions: Cu, Cd, Ni, Zn, Sb

Nickel solutions: Cu, Pb, Zn, Cd, Na, Co, Cr, Mn

Palladium solutions: Pd, Cr3+, Cr6+

Rhodium solutions: Rh

Silver solutions: Sb, Cu, Cd

Acid tin solutions: Sn4+, Cu, Ni, Zn

Alkaline tin solutions: Pb, Cd, Zn, Cu

Acid zinc solutions: Cu, Fe, Pb, Cd

Alkaline zinc solutions: Pb, Cd, Cu

Wastewater: Cd, Cu, Cr3+, Ni, Sn, Zn

AMPEROMETRY

Amperometric titrations involve the use of polarography as the basis of an electrometrictitration. Voltage applied across the indicator electrode (e.g., DME orplatinum) and reference electrode (e.g., calomel or mercury) is held constantand the current passing through the cell is measured as a function of titrantvolume added. The endpoint of the titration is determined from the intersectionof the two straight lines in a plot of current against volume of titrant added.Polarograms are run to determine the optimum titration voltage.Amperometric titrations can be carried out at low analyte concentrations atwhich volumetric or potentiometric methods cannot yield accurate results. Theyare temperature independent and more accurate than polarographic methods.Although amperometry is useful for oxidation-reduction or precipitationreactions,few acid-base reactions are determined by this method.Some of the reactions that can be analyzed by amperometric methods aregiven in Table II.

ELECTROGRAVIMETRY

In electrogravimetry, the substance to be determined is separated at a fixed potentialon a preweighed inert cathode, which is then washed, dried, and weighed.Requirements for an accurate electrogravimetric analysis include good agitation,smooth adherent deposits, and proper pH, temperature, and current density.Advantages of electrogravimetry include its ability to remove quantitativelymost common metals from solution. The method does not require constantsupervision. Disadvantages include long electrolysis times.Some of the metals that have been determined electrogravimetrically arecadmium, cobalt, copper, gold, iron, lead, nickel, rhodium, silver, tin, and zinc.

SAMPLING

Analyses are accurate only when the sample is truly representative of the solutionbeing analyzed. Each tank should have a reference mark indicating the correct levelfor the solution, and the bath should always be at this level when the sample istaken. Solutions should be stirred before sampling. If there is sludge in the tank,the solution should be stirred at the end of the day and the bath allowed to standovernight, taking the sample in the morning.Solutions should be sampled by means of a long glass tube. The tube isimmersed in the solution, the thumb is placed over the upper open end, and afull tube of solution is withdrawn and transferred to a clean, dry container. Thesolution should be sampled at a minimum of 10 locations in the tank to ensure arepresentative sample. A quart sample is sufficient for analysis and Hull cell testing,and any remaining solution can be returned to its tank.

STANDARD SOLUTIONS, REAGENTS, AND INDICATORS FOR WET

METHODS

Standard solutions, reagents, and indicators can be purchased ready-made fromlaboratory supply distributors. Unless a laboratory has the experience and highdegree of accuracy that is required in preparing these solutions, it is recommendedthat they be purchased as prepared solutions. Preparations for all the solutions aregiven here to enable technicians to prepare or recheck their solutions.A standard solution is a solution with an accurately known concentration ofa substance used in a volumetric analysis. Standardization of standard solutionsrequires greater accuracy than routine volumetric analyses. An error in standardizationcauses errors in all analyses that are made with the solution; therefore,Primary Standard Grade chemicals should be used to standardize standard solutions.The strengths of standard solutions are usually expressed in terms of normalityor molarity. Normalities of standard solutions and their equivalent molarities arelisted in Table III. The methods to standardize all the standard solutions requiredfor the analysis of plating and related solutions are listed in Table IV.Indicators are added to solutions in volumetric analyses to show color changeor onset of turbidity, signifying the endpoint of a titration. The indicators requiredfor all of the analyses and their preparations are listed in Table V. Analytical Gradechemicals should be used in preparing analytical reagents (Table VI) and ReagentGrade acids should be used (Table VII). When chemicals of lesser purity are used,the accuracy of the results will be diminished.Tables VIII through XII provide specific methods for testing the constituentsof electroplating, electroless, and anodizing baths, as well as acid dips and alkalinecleaners.

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Fig. 1. Test setup for determination of cathode efficiency. Use 500-ml beakers and

1 ´ 2-in. brass cathodes. The anodes for the test solution should match that used in the

plating bath. Use copper anodes for the coulometer.

SAFETY

As with any laboratory procedure, the accepted safety rules for handling acids,bases, and other solutions should be followed. Acids are always added to water,not the reverse. Mouth pipettes should not be used for pipetting plating solutions.Safety glasses should always be worn, and care should be exercised to avoidskin and eye contact when handling chemicals. A fume hood should be usedwhen an analytical method involves the liberation of hazardous or annoyingfumes. Laboratory staff should be well versed in the first-aid procedures requiredfor various chemical accidents.

DETERMINATION OF CATHODE EFFICIENCY

The procedure for determining cathode efficiency, using the setup pictured inFig. 1, is as follows:

1. Connect the copper coulometer in series with the test cell.

2. The copper coulometer solution should contain 30 oz/gal copper sulfatepentahydrate and 8 oz/gal sulfuric acid.

3. Use the same anodes, temperature, and agitation in the test solution thatare used in the plating bath.

4. Plate at 0.4 A (30 A/ft2) for a minimum of 10 minutes.

5. Rinse both cathodes, dry in acetone, and weigh.% Cathode Efficiency =weight in grams of test metal X valence of test metal in bath X 3177weight in grams of copper metal « atomic weight of test metal

*Editor’s note: To view this article in its entirety, including corresponding tables,

please consult the online Guidebook archive.

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