Interference gravimetric analysis

In some situations the rate at which a precipitate forms can be used to separate an analyte from a potential interferent. For example, due to similarities in their chemistry, a gravimetric analysis for Ca + may be adversely affected by the presence of Mg +. Precipitates of Ca(01T)2, however, form more rapidly than precipitates of Mg(01T)2. If Ca(01T)2 is filtered before Mg(01T)2 begins to precipitate, then a quantitative analysis for Ca + is feasible. 

The properties of precipitates that are used in chemical analysis are described in this chapter. The techniques for obtaining easily filterable precipitates that are free from contaminants are major topics in this chapter. Such precipitates are not only used in gravimetric analysis but also employed in the separation of interferences for other analytical procedures. 

Occlusion is not very important in flow turbidimetry because, although water trapped in the interior of the crystal may contain some interferents, precipitate growth is limited and the precipitate is not dried as in gravimetric analysis. 

Once a sample is in solution, the solution conditions must be adjusted for the next stage of the analysis (separation or measurement step). For example, the pH may have to be adjusted, or a reagent added to react with and mask interference from other constituents. The analyte may have to be reacted with a reagent to convert it to a form suitable for measurement or separation. For example, a colored product may be formed that wUl be measured by spectrometry. Or the analyte will be converted to a form that can be volatilized for measurement by gas chromatography. The gravimetric analysis of iron as FeaOa requires that all the iron be present as iron(in), its usual form. A volumetric determination by reaction with dichromate ion, on the other hand, requires that all the iron be converted to iron(II) before reaction, and the reduction step will have to be included in the sample preparatioii. 

The first step in perfonning gravimetric analysis is to prepare the solution. Some form of preliminary separation may be necessary to eliminate interfering materials. Also, we must adjust the solution conditions to maintain low solubility of the precipitate and to obtain it in a form suitable for filtration. Proper adjustment of the solution conditions prior to precipitation may also mask potential interferences. Factors that must be considered include the volume of the solution during precipitation, the concentration range of the test substance, the presence and concentrations of other constituents, the temperature, and the pH. 

Although preliminary separations may be required, in other instances the precipitation step in gravimetric analysis is sufficiently selective that other separations are not required. The pH is important because it often influences both the solubility of the analytical precipitate and the possibility of interferences from other substances. For example, calcium oxalate is insoluble in basic medium, but at low pH the oxalate ion combines with the hydrogen ions to form a weak acid. 8-Hydroxy-quinoline (oxine) can be used to precipitate a large number of elements, but by controlling pH, we can precipitate elements selectively. Aluminum ion can be precipitated at pH 4, but the concentration of the anion form of oxine is too low at this pH to precipitate magnesium ion. 

Nickel forms a red chelate with dimethylglyoxime (DMG), which is quite suitable for gravimetric analysis. Precipitation of the chelate is complete in an acetic acid-acetate buffer or in an ammoniacal solution. Acetate buffer is generally used when Zn, Fe, or Mn is present in the alloy. The sample given to you is a nichrome alloy that has Ni (approximately 60%), Cr, and Fe as the major constituents. Interference from Cr and Fe is removed by complexation with tartrate or citrate ions. Precipitation is then carried out in an ammoniacal solution. The Ni content is calculated from the weight of the precipitate (see Table 10.2 for the formula). 

In gravimetric analysis, the solution-preparation step has its own special significance. The analyte must be separated from interfering species, or the interferents must be masked. For example, if iron(III) is to be estimated as its hydrated oxide in the presence of chromium(III), then the mixture is initially treated with perchloric acid so as to oxidize chromium(III) to chromate (chromium(VI), Cr04 ), followed by addition of ammonia to precipitate the hydrated iron oxide. Sometimes it is necessary to remove interferents, for example, when calcium is to be estimated as calcium sulfate in the presence of barium. The barium is removed as its chromate and the calcium is precipitated quantitatively as its sulfate. 

A variety of methods have been used for the analysis of inorganic and organic cations traditional spectroscopic techniques such as colorimetry, wet chemical methods such as gravimetric analysis, turbidimetry, and titrime-try, and electrochemical techniques such as use of an ion-selective electrode and amperometric titrations. Some of these methods suffer from interferences and limited sensitivity they can be labor-intensive and are often difficult to automate. 

Usually, fluid inclusion oils are measured by GC-MS as whole oils (i.e., without prior fractionation), because extraction yields are usually very low (too low for gravimetric analysis). However, fluid inclusion oils can be fractionated into total hydrocarbon fractions and nitrogen-sulfur-oxygen (NSO) fractions to avoid unwanted interference of NSO compound peaks with aliphatic and aromatic hydrocarbon peaks. Furthermore, samples that have significant n-alkane interference, in particular in their biomarker distributions, can be further separated into n-alkanes and branched and cyclic hydrocarbon fractions by using molecular sieves (e.g., ZSM-5 sieve such as silicalite, a zeolitlic form of silica). 

A wide range of chemical methods and procedures is found in the treatise of Kolthoff and Elving (289). They also give a summary of the chemistry and solubility of silica. Preparation of solutions for analysis, the silicomolybdic acid yellow and blue colorimetric methods including interferences, gravimetric procedures, and special procedures for biological materials are discussed in concise detail. 

As with the free acids, the pharmacopoeias have relied on gravimetric analysis as a means of quantitatively assaying the salts. The N.N.R. uses ultraviolet light and this is an improvement, but, unfortunately, the decomposition products cause some interference here too. 

Polymerization-grade chloroprene is typically at least 99.5% pure, excluding inert solvents that may be present. It must be substantially free of peroxides, polymer [9010-98-4], and inhibitors. A low, controlled concentration of inhibitor is sometimes specified. It must also be free of impurities that are acidic or that will generate additional acidity during emulsion polymerization. Typical impurities are 1-chlorobutadiene [627-22-5] and traces of chlorobutenes (from dehydrochlorination of dichlorobutanes produced from butenes in butadiene [106-99-0]), 3,4-dichlorobutene [760-23-6], and dimers of both chloroprene and butadiene. Gas chromatography is used for analysis of volatile impurities. Dissolved polymer can be detected by turbidity after precipitation with alcohol or determined gravimetrically. Inhibitors and dimers can interfere with quantitative determination of polymer either by precipitation or evaporation if significant amounts are present. 

Fluoride ion, and weak acids and bases do not interfere, but nitrate, nitrite, perchlorate, thiocyanate, chromate, chlorate, iodide, and bromide do. Since analysis of almost all boron-containing compounds requires a preliminary treatment which ultimately results in an aqueous boric acid sample, this procedure may be regarded as a gravimetric determination of boron. 

Sample preparation of pharmaceutical products is an important step in the analysis methodology and must be carefully carried out to avoid contamination, loss of metal(s) or addition of interferences that could result in errors in the measurements. Modem versions of the international pharmacopoeias contain methods involving atomic emission methods, and some replace tedious titration, spectrophotometer or gravimetric methods. 

The sample preparation method must not only deliver a measurable amount of sample but the compounds accompanying the analyte must not interfere with the analysis. As an illustration, consider a gravimetric assay in which the detection step has no discriminating power and the sample preparation provides all of the specificity. In contrast, an enzyme assay can be performed on a very complex sample without any sample preparation or isolation of the analyte, because the changes in substrate concentration can be linked directly to the activity of the enzyme. 

The concentration-distance curves method is based on the measurement of the distribution of the diffusant concentration as a function of time. Light interference methods, as well as radiation adsorption or simply gravimetric methods, can be used for concentration measurements. Various sample geometries can be used, for example semiinflnite solid, two joint cylinders with the same or different material, and so on. The analysis is based on the solution of Pick s equation. 

To increase the precision of quantitative analysis, the plasticizer sample is diluted with carbon disulfide, its infrared absorption measured, and compared with absorptions standard of standard samples prepared also in CS2 to cover the range of concentrations from 0.5 to 3 mg/ml. For each suspected (identified) plasticizer, a series of standards has to be prepared and measured. It is also important to select a suitable wavelength for quantitative analysis. For dioctyl phthalate bands at 1725 and 1121 cm" are usually used. For tricresyl phosphate band at 1191 cm is used. Similar to gravimetric method, the results are subject to various interferences when mixtirres of plasticizers or mixture of plasticizers with other additives ate used. 

Like cements, the elemental composition is determined by XRF or AAS techniques. The XRF bead is made using lithium tetraborate at 1050°C. Sulfide content cannot be determined by XRF. Sulfite, SO3 , and sulfate, S04 , are safely analyzed by XRF. Na2C03 -I- K2CO3 fusion is carried out for Ca, Mg, Fe, and A1 analysis by AAS. Lanthanum chloride is used as a sulfate interference suppressant. Gravimetric sulfate determinations are also carried out by precipitation as barium sulfate. The Leco Carbon-Sulfur Analyzer can also be used for quality control purposes. The fluoride is determined by XRF or a pyrohydrolysis method. The measurement of particle size distribution is carried out in a manner similar to that for cements and clays. 

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