Quantitative Gravimetric Methods

The ammonium phosphomolybdate precipitation (1) is subject to interference by V, As, W, Ti and Si which can form isomorphous compounds (Chapter 5.3). Under some conditions, heavy metals such as Sn, Bi, Zr and Pb can also interfere with the precipitation of MgNH4P04-6H20. For this and other reasons, the exact composition of a phosphate precipitate can vary, and a carefully specified procedure, appropriate to the type of mixture being quantitatively analysed, should always be followed. Many such detailed procedures, each capable of giving accurate and reproducible results, have been worked out and are available in specialised monographs on analytical methods. 

A very accurate and reliable gravimetric method is to precipitate the phosphorus as quinolinium phosphomolybdate, (C9H7N)3PMOi204q in the presence of citric acid. This is done from a boiling acid solution of the sample, to which has been added a reagent solution of quinoline + molybdate -I- citric acid. The quinolinium compound is less soluble and after drying at 250°C has a more constant composition than the ammonium salt. 

Instead of weighing the ammonium phosphomolybdate from Eqaution 14.1, it may be titrated (and hence estimated) with a solution of NaOH of known strength (14.9). A more practical technique is to dissolve the ammonium phosphomolybdate in a known excess of NaOH, and back-titrate the residual alkali with standard acid to a phenolphthalein end point. A procedure similar to Equation 14.1 can be adopted with precipitated quinolinium phosphomolybdate (14.10). 

Another procedure is to precipitate the phosphate as MgNH4P04 ffljO, dissolve it in dilute HCl, add an excess of EDTA and adjust to pH = 10. The ranaining EDTA is then titrated with MgCl2. 

If not easily hydrolysed to soluble orthophosphates, condensed phosphates are usually best identified and estimated by the techniques of paper chromatography or x-ray diffraction (XRD) (see below). Several purely chemical methods are, however, known. 

---

Gravimetric and volumetric methods are practicable for the quantitative determination of the a-sulfo fatty acid esters. Using gravimetric methods the surfactant is precipitated with p-toluidine or barium chloride [105]. The volumetric determination method is two-phase titration. In this technique different titrants and indicators are used. For the analysis of a-sulfo fatty acid esters the quaternary ammonium surfactant hyamine 1622 (p,f-octylphenoxyethyldimethyl-ammonium chloride) is used as the titrant [106]. The indicator depends on the pH value of the titration solution. Titration with a phenol red indicator is carried out at a pH of 9, methylene blue is used in acid medium [106], and a mixed indicator of a cationic (dimidium bromide) and an anionic (disulfine blue VN150) dye can be used in an acid and basic medium [105]. 

Gravimetric methods involved the quantitative precipitation of kojic acid as the copper salt,87 63 or careful extraction, drying, and weighing of kojic acid proper.86... 

The lead content of biological samples is usually very small, rendering gravimetric methods impracticable, and methods have often relied upon the formation of coloured complexes with a variety of dyes. More recently, the development of absorption spectroscopy using vaporized samples has provided a sensitive quantitative method. Oxygen measurements using specific electrodes offer a level of sensitivity which is unobtainable using volumetric gas analysis. 

Nickel may he measured quantitatively hy several microanalytical gravimetric methods that include (l)formation of a red precipitate with dimethyl-glyoxime, (2) precipitation as a hlack sulfide with ammonium sulfide, (3) precipitating as a complex cyanide hy treating with alkali cyanide and bromine, and (4) precipitation as a yellow complex hy treating an ammoniacal solution of nickel with dicyandiamide sulfate (Grossman s reagent), followed hy the addition of potassium hydroxide. All of these methods can separate nickel from cobalt in solution. 

The accuracy and utility of the normal-phase separation is evident, but the gravimetric method of quantitation has several major drawbacks ... 

C) Quantitative Determination, i. Gravimetric method. This consists in adding paranitrophenylhydrazine and thus precipitating the ketones (dimethyl and methyl ethyl) as hydrazones, which are collected and weighed. 

Gravimetric methods are based on the quantitative measurement of an analyte by weighing a pure and insoluble compound from the analyte. Analytical balances, which provide accurate and precise data, are used for this purpose. Detailed information on gravimetric methods and their applications can be found in the literature.3... 

The quantitative determination of chlorine may be carried out either by a volumetric or a gravimetric method. 

The quantitative amount of the tars was determined based on the contents of the 22 most abundant PAHs in the gas. The procedures of methods used for calibration and optimising the GC-MS is described elsewhere [9]. Content of the heavy fraction of the tars that could not be analysed with GC-MS, was determined by gravimetric methods. 

Coulometric methods are performed by measuring the quantity of electrical charge required to convert a sample of an analyte quantitatively to a different oxidation state. Coulometric and gravimetric methods share the common advantage that the proportionality constant between the quantity measured and the analyte mass is derived from accurately known physical constants, which can eliminate the need for calibration with chemical standards. In contrast to gravimetric methods, coulometric procedures are usually rapid and do not require the product of the electrochemical reaction to be a weighable solid. Coulometric methods ai-e as accurate as conventional gravimetric and volumetric procedures and in addition are readily automated. ... 

As IR spectroscopy is a secondary method of analysis, the development of quantitative analysis methods requires calibration with a set of standards of known composition, prepared gravimetrically or analysed by a primary chemical method, to establish the relationship between IR band intensities and the compositional variable(s) of interest. Once a calibration has been developed, it can then be used for the prediction of unknowns, provided two general conditions are met i) the spectra of the unknowns are recorded under the same conditions as employed in the calibration step (i.e., same instrumental parameters, identical means of sample handling, etc.) and ii) the composition of the calibration standards is representative of that of the unknowns. 

Coulometry encompasses a group of analytical nieih-(Xl.s that involve measuring the quaniiiv oi electricily (in coulombs) needed to convert the analyte quantitatively to a different oxidation state. 1 ike gravimetric methods, coulometry has the advamage ihai the... 

In nearly every experiment in this manual, tests are made on the product to study its properties, and in several instances directions are given for analyzing the product quantitatively to determine its purity. Volumetric methods of assaying are given, since they are quicker than gravimetric methods and often just as accurate. 

In the conventional gravimetric method, it would have been necessary to quantitatively precipitate the I" present and to weigh the precipitate (less than 0.1 mg). The only precautions in the present method are that all of the Ag added should be used to precipitate Agl, and that all of the Agl be collected. If other species are precipitated (for instance, AgCl), the precipitate should be separated and purified before measuring its activity. 

Gravimetric method determines the content of CD by precisely measureing the weight change of the sample before and after immobilization. This method is based on different content of CD which when immobilized on the carrier has no effect on quantity of crosslinking agent which was grafted on the carrier. The determination result is not very accurate and is suitable for applications which have low expectations on quantitative analysis. 

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. 

---