Phenol colorimetric analysis

In colorimetric analysis, a reagent is selected that would form a colored complex or derivative with the analyte. Often, the analyte is extracted from the aqueous sample into an organic solvent before adding color-forming reagent. Such extractions become necessary, especially for organic analytes such as phenols, lignin, and tannin. 

The rapid formation of the phenols during biodegradation has been shown by the colorimetric analysis. 

Phenolic compounds in aqueous and solid samples may be estimated as total phenols by colorimetric analysis. The method involves the reactions of phenols with 4-aminoantipyrine in the presence of potassium ferricyanide at pH 8 to form a yellow antipyrine dye (soluble in chloroform), the absorbance of which is measured by a spectrophotometer at 500 nm. 

The results of the colorimetric analysis of total phenolics expressed as Gallic Acid Equivalents (GAE) and those of total flavonoids expressed as Catechine Equivalents (GE) are given in the table 2. 

One of the major applications of Fischer carbene complexes is the formation of substituted phenols, known as Dotz benzannulation. The first example of this reaction on solid phase was reported by the group of Martinez [35]. The Fischer carbene complex 16 was obtained with a loading of 95% (Scheme 10), and the reaction could be monitored by colorimetric analysis. Microwave-assisted D5tz benzannulation allowed a sixfold decrease in the reaction time. Investigations into various reaction conditions showed that the reaction was almost independent of solvent, and that elevated temperature and prolonged reaction time did not have a major influence on the yield. 

In the past, the reduction of metallic ions and certain diazotised organic compounds by hindered phenols has served as the basis for colorimetric methods of analysis [112-114],... 

Phenolic antioxidants in rubber extracts were determined indirectly photometrically after reaction with Fe(III) salts which form a red Fe(II)-dipyridyl compound. The method was applicable to Vulkanox BKF and Vulkanox KB [52]. Similarly, aromatic amines (Vulkanox PBN, 4020, DDA, 4010 NA) were determined photometrically after coupling with Echtrotsalz GG (4-nitrobenzdiazonium fluoroborate). For qualitative analysis of vulcanisation accelerators in extracts of rubbers and elastomers colour reactions with dithio-carbamates (for Vulkacit P, ZP, L, LDA, LDB, WL), thiuram derivatives (for Vulkacit I), zinc 2-mercaptobenzthiazol (for Vulkacit ZM, DM, F, AZ, CZ, MOZ, DZ) and hexamethylene tetramine (for Vulkacit H30), were mentioned as well as PC and TLC analyses (according to DIN 53622) followed by IR identification [52]. 8-Hydroquinoline extraction of interference ions and alizarin-La3+ complexation were utilised for the spectrophotometric determination of fluorine in silica used as an antistatic agent in PE [74], Also Polygard (trisnonylphenylphosphite) in styrene-butadienes has been determined by colorimetric methods [75,76], Most procedures are fairly dated for more detailed descriptions see references [25,42,44],... 

Emmet [301] developed a colorimetric method involving chlorination of the urea with hypochlorite, followed by condensation with phenol. The limit of detection for this method was 0.2 p,g/l as nitrogen. The method was easily adaptable to automatic analysis. 

Refs on Analysis of NG 1) Marshall 2(1917), pp 739—40 (Estimation of small quantities by reaction with phenol disulfonic acid to form. PA and colorimetric measurement against K nitrate standards) 2) Koehler et al, AnnChimAnal-ChimAppl 2, 271-2(1920) CA H 3794(1920) (Devarda method) 3) W.W. Becker, lECAnalEd 5, 152-4(1933) CA 27, 2909U933)(Redox method for NG and NGc in the presence of DNT)... 

The colorimetric method based on the reagents of Folin and Denis or of Folin and Ciocalteu has been generally preferred over other methods to determine total phenols in complex natural materials such as wines and fruits (1, 2, 3, 4, 5). This method is relatively simple, convenient, reliable, generally applicable, and it is accepted as an official analysis in several countries for total phenols in wines and a number of other products. Although it is a preferred method, it can be even better than is commonly recognized. 

The total phenolic compounds in an aqueous sample can be determined by a colorimetric method using 4-aminoantipyrine. This reagent reacts with phenolic compounds at pH 8 in the presence of potassium ferricyanide to form a colored antipyrine dye, the absorbance of which is measured at 500 nm. The antipyrine dye may also be extracted from the aqueous solution by chloroform. The absorbance of the chloroform extract is measured at 460 nm. The sample may be distilled before analysis for the removal of interfering nonvolatile compounds. The above colorimetric method determines only ortho- and meta-substituted phenols and not all phenols. When the pH is properly adjusted, certain para-substituted phenols, which include methoxyl-, halogen-, carboxyl-, and sulfonic acid substituents, may be analyzed too. 

An automated flow injection analysis (FIA) system for quantifying ethanol was developed using alcohol oxidase, horseradish peroxidase, 4-amino-phenazone, and phenol. A colorimetric detection method was developed using two different methods of analysis, with free and immobilized enzymes. The system with free enzymes permitted analysis of standard ethanol solution in a range of 0.05-1.0 g of ethanol/L without external dilution, a sampling frequency of 15 analyses/h, and relative SD of 3.5%. 

To improve the quality control process of gasohol and hydrated ethanol, an automated FIA system was developed using AOD and HRP enzymes, and addition of 4-aminophenazone and phenol. A colorimetric detection method was used in two different methods of analysis, with free (4) and immobilized enzymes. Both systems have shown good results when compared with established methods such as gas chromatography (GC) and high-performance liquid chromatography (HPLC) (4,7). 

Esterases. Acetyl esterase (EC 3.1.1.6) removes acetyl esters from acetylated xylose and short-chain xylo-oligomers. It s polymeracting counterpart, acetyl xylan esterase (EC 3.1.1.72), has a similar activity, but prefers polymeric xylan.244 In addition to acetate-specific enzyme detection kits, HPLC or GC analysis of acetate release from native extracted xylan and chemically acetylated xylan, colorimetric substrates, such as p-nitrophenol acetate and /3-napthyl acetate, or the fluorometric substrate, 4-methylumbelliferyl acetate are also used to assay acetyl esterases.244,253 The third esterase, ferulic acid esterase (EC 3.1.1.73), hydrolyzes the ester bond between ferulic acid or coumaric acid and the arabinose side chain of arabinoxylan. Assays for this activity are usually carried out using starch-free wheat bran or cellulase-treated gramineous biomass as a substrate and monitoring ferulic or coumaric acid released by HPLC or TLC. When preparing enzyme-treated substrates, care must be taken to employ phenolic-acid-esterase-free cellulases.244 Other substrates include methyl and ethyl esters of the phenolic acids, as well as finely ground plant biomass.240,254,255... 

The progress of the reaction was followed by monitoring the decrease in formaldehyde concentration with time. Previous studies used the hydroxylamine hydrochloride method of analysis (5 -55), but this was avoided in the current study as it requires tedious pH titrations. Instead, a colorimetric method was used that was first developed by Nash (55), involving formation of 3,5-diacetyl-1,4-dihydrolutidine, by reaction of formaldehyde with ammonia and acetyl acetone at neutral pH. The cyclic product absorbs at 412 nm with a molar extinction coefficient of 8,000 (55). Other colorimetric methods cannot be used as they all involve very strongly acidic or basic media (55), which would force the phenol-formaldehyde reaction to completion. 

DPS and ethanol contents were determined by gas chromatography and the phenol content by a colorimetric method. DPS analysis was done in a column (4 mm x 3 m)... 

To create a fast and simple method for monitoring of human exposure to neuropathic OPs, a principal new approach to NTE activity analysis has been developed in joint study of the Institite of Physiologically Active Compounds Russian Acad. Sci. and Chemical Department of Moscow State University [88,91,92], Recently, a new biosensor for NTE assay was introduced using a tyrosinase carbon-paste electrode to detect phenol produced by the hydrolysis of phenyl valerate. In this type biosensor phenol is quantified by measuring electroreduction of the generated o-quinone on a graphite electrode (Fig. 6) [88,91 ]. The tyrosinase carbon-paste electrode improved the sensitivity of the NTE assay 10-fold compared to the colorimetric method or an earlier amperometric technique based on oxygen detection [92]. Moreover, the new electrode operates in a... 

Several flavonoids isolated from tea have been analyzed and their structure determined using NMR. There are several problems with the classical methods of analysis of flavonoids in tea. Due to the presence of complex mixtures of flavonoids in tea, they are often characterized as total polyphenols . The colorimetric method for analysis of total phenols can interfere with other reducing compovmds. LC can well resolve peaks for individual flavonoids however, there are only a few standards available commercially, making the assignment of peaks vmcertain in many cases. Thus, the structure of flavonoids giving rise to peaks in LC is often determined using various ID- and 2D-NMR experiments. 

The colorimetric Nesslerization method uses alkaline phenol and hypochlorite to react with ammonia to form indophenol blue in an amount proportional to the ammonia concentration. The blue color is intensified with sodium nitroprusside, and the concentration is measured using a calibrated colorimeter. The titration of basic ammonia is accomplished with standard sulfuric acid using a mixed indicator. Determination of ammonia may also be based upon the indophenol reaction adapted to automated gas-segmented continuous flow analysis. Potentiometric determination of ammonia is performed by ion-selective ammonia electrodes. 

Walker (1993) has described an HPLC method for lanthanide analysis based on phenol red (fig. 4c) as the stationary phase complexant and colorimetric indicator in die mobile phase. Lanthanide separation is based on the hiba conqtlexes. The presence of the indicator in the mobile phase eliminates the need for post-column derivatization, as the free indicator absorbance at 490 nm can be monitored in the column effluent to detect the existence of the lanthanide complexes. Overall, the technique is not as effective as... 

The samples preparation and extraction of phenolics from source materials is the first step involved in their analysis. While colorimetric methods are used for determination of different classes of phenolics, chromatographic and... 

---