Adsorption spectrophotometric methods

Haywood and Riley [14] have described a spectrophotometric method for the determination of arsenic in seawater. Adsorption colloid flotation has been employed to separate phosphate and arsenate from seawater [15]. These two anions, in 500 ml filtered seawater, are brought to the surface in less than 5 min, by use of ferric hydroxide (added as 0.1 M FeC 2 ml) as collector, at pH 4, in the presence of sodium dodecyl sulfate [added as 0.05% ethanolic solution (4 ml)] and a stream of nitrogen (15 ml/minutes). The foam is then removed and phosphate and arsenate are determined spectrophotometrically [16]. Recoveries of arsenate and arsenite exceeding 90% were obtained by this procedure. 

The change in concentration in solution can be measured by the analytical technique most suitable for the particular substance being studied. Under favorable conditions, if the change in concentration is monitored continuously (e.g., by a spectrophotometric method), it may be possible to follow the kinetics of adsorption, but this is rarely possible. 

A charge-transfer spectrophotometric method has been developed for unit-dose assay of the tropane alkaloids and some of their synthetic analogues. The high molar adsorptivities of the charge-transfer bands of the alkaloids with iodine in ethylene chloride give high precision even at low doses, such as in hypodermic and pediatric tablets. 

Complexation with PAN immobilized on a Dowex 50Wx4 cation exchanger makes a basis of the determination of Zn in human hair, natural water and pharmaceutical and cosmetic formulations [11]. Flow-injection solid-phase spectrophotometry using TAN immobilized on silica gel was proposed for the determination of Zn in pharmaceutical preparations [12]. Adsorption of the complex with PAR on Sephadex anion exchanger makes a basis of solid phase spectrophotometric method for the determination of Zn in environmental samples [13]. 

Surface acidity and basicity were measured by adsorption of organic bases such as pyridine (PY, pk,=5.3), morpholine (MP, pk, = 8.33), piperidine (PP, pk, = 11.1) and acidic substrates like acrylic acid (AA, pk = 4.2), phenol (PH, pk, = 9.9), respectively by spectrophotometric method[15,16]. Redox properties (one electron donor and one electron acceptor) were determined by the same method using meta dinitrobenzene (DNB, electron affinity, EA = 2.21eV) and phenothiazine (PNTZ, ionisation energy, IE = 7.13eV) as the adsorbates. 

This field provides a brief description of the suggested monitoring and analysis method for quantitative determination of a particular substance. For example, a method for quantitative determination has been developed for cadmium, copper, manganese, and lead in water by means of co-precipitation with zirconium hydroxide followed by subsequent analysis by atomic adsorption spectrometry. An Inductively Coupled Plasma-Atomic Emission Spectrophotometric method has been employed by the Environmental Protection Agency (EPA Method 200.7) for the determination of dissolved, suspended, or total elements in drinking water, surface water, and domestic and industrial wastewaters. 

Leon-Gonzalez et al.[31] proposed an FI spectrophotometric method for the determination of Triton-type non-ionic surfactants based on their reaction with alizarin fluorine blue. An on-line ion-exchange column was incorporated in the system to eliminate interferences from ionic and amphoteric surfactants. In case of interferences from non-ionic surfactants, an on-line Amberlite XAD-4 adsorption column was used to retain selectively the Triton-type surfactant, which was subsequently eluted by ethanol. However, no information was given regarding interferences from refractive index effects at the ethanol/aqueous interface and their elimination. 

Uric acid was isolated from urine by adsorption on Darco-G60 at pH 4.2 overnight at 4 C, eluted with hot O.IN NaOH, and repeatedly recrystallized from Li2C03 solution. Uric acid analyses were performed by the differential spectrophotometric method [23] employing purified uricase purchased from Worthington Biochemical Corporation. 

The extent of dehydration during temperature treatment of tin(IV)oxide powder is Mcertained by thermogravimetry (Simultaneous Thermal Analysis STA 409, Netzsch). Surface are l8 are determined by nitrogen adsorption using a standard volumetric BET apparatus. The tin(IV)oxide content of the loaded crtrriers and the catalysts is quantified gravimetrically. The platinum content of the catalysts is obtained by a spectrophotometric method which has been described elsewhere (11). 

The zone and adsorbent are scraped off the plate and transferred quantitatively to an extractor. Methanol is generally used for the extraction, the volume being then made up to a definite amount. The substance itself is determined polarographically, colorimetricaUy (after conversion into a coloured derivative) or, easily the best, spectrophotometricaUy at the wave length in the UV where the substance has a characteristic adsorption maximum. A blank on the adsorbent is necessary in the spectrophotometric method. The amount of substance can then be calculated frpm the extinction coefficient. This combination of TLC and spectrophotometric determination of the eluted substance, is a method of wide analytical potentiality in biology and chemistry. 

See J.Chem.Technol.Biotechnol.S9( 993)3B7. Initial results on the adsorption of nitrite by the same anatase sample using a spectrophotometric method (Sec.6.3.4) was incomplete. Air should be excluded to prevent oxidation of nitrite. 

A second method is the adsorption of a dye from a solution, e.g. methylene blue, fuchsine, methyl violet and malachite green. A certain grain fraction of a dried clay is stirred for a certain time in a dye solution with a specific concentration. The reduction of the solution s colour intensity is a measure for the adsorbed amount of dye and it is measured spectrophotometrically. The dye molecules are much larger than nitrogen molecules and so they are only adsorbed and will not enter the pores. 

Separation of mono- and di-ester pyrrolizidine alkaloids has been achieved by ion-pair adsorption t.l.c., using chloride (or iodide) as the counter-ion.48 Chloranil has been used to oxidize pyrrolizidine alkaloids on t.l.c. The pyrrole derivatives that were formed were then detected with Ehrlich s reagent49 or sulphuric acid.50 Mixtures of pyrrolizidine alkaloids have been separated by h.p.l.c. on a reversed-phase styrene-divinylbenzene resin.51 In a sensitive method for the detection of pyrrolizidine alkaloids, the protonated alkaloids were complexed with aqueous methyl orange. The dye was then released from the complex and estimated spectrophotometrically.52... 

Other test methods are available. Content of benzene and other aromatics may be estimated by spectrophotometric analysis (ASTM D-1017) and also by gas-liquid chromatography (ASTM D-2267, ASTM D-2600, IP 262). However, two test methods based on the adsorption concept (ASTM D-2007, ASTM D-2549) are used for classifying oil samples of initial boiling point of at least 200°C (392°F) into the hydrocarbon types of polar compounds, aromatics, and saturates and recovery of representative fractions of these types. Such methods are unsuitable for the majority of naphtha samples because of volatility constraints. 

Determination of solubility by headspace analysis offers several advantages over spectrophotometric techniques. First, because of the selectivity of chromatographic analysis, compound purity is not a critical factor second, absolute calibration of the gas chromatographic detector is not necessary if the response is linearly related with concentration over the range necessary for the measurements and finally, this method does not require the preparation of saturated solutions, since a partition coefficient, not a solubility, is actually measured. However, headspace methodology would probably not be applicable for determining PAH solubilities for three reasons. First, there is little data in the literature on the vapor pressures of PAHs. Second, the aqueous solubilities of most PAHs are too low to be measured by this procedure. Finally, adsorptive losses of PAHs to glass surfaces from the vapor phase would cause errors. 

The total concentration of free fatty acids is usually determined by extrac-tion/titration methods or spectrophotometrically as Cu soaps. Early attempts to quantify the concentration of individual short-chain fatty acids involved steam distillation and adsorption chromatography. Complete separation and quantitation of free fatty acids can be achieved by GC, usually as their methyl esters, for which several preparative techniques have been published. Free fatty acids are major contributors to the flavor of some varieties, e.g., Romano, Feta, and Blue in the latter, up to 25% of the total fatty acids may be in the free form. Short chain fatty acids are important contributors to cheese aroma, while longer chain acids contribute to taste. Excessive concentrations of either cause off-flavors (rancidity) and the critical concentration is quite low in many varieties, e.g., Cheddar and Gouda. 

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