Acidity value colorimetric determination

Colorimetric procedures used In steroid assays are often subject to drug Interference. In the determination of 17-Ketosterolds by the Zimmerman reaction, drugs with the 17-Keto basic structure such as ascorbic acid, morphine and reserplne will cause Increased values. In the determination of 17,21 -dlhydroxysterolds by the Porter-Sllber reaction the dlhydroxy-acetone chain Is the reactive unit. Drugs like meprobamate, chloral hydrate, chloropromazlne and potassium Iodide will Interfere with this reaction and cause elevated values. In the colorimetric determination of vanlllylmandellc acid (VMA) by a dlazo reaction, drugs like methocarbamol and methyl dopa cause... 

Table II. Carbohydrate compositions (weight percentage) of individual oligomer peaks purified (QAE-Sephadex or HPLC ion-exchange separation, respectively) from mixtures of citrus pectin oligomers or B fruit extracts Compositions shown are for peaks whose biological activity is described in Figure 4. Uronic acid values are based on colorimetric assay. Proportions of neutral sugars were determined by GC and adjusted so that totals equal 100%. In fact, some oligomers (G7 peaks 8, 9 and 10. B extract peak 10) produced small (less than 1 % of the total integrated area), unknown peaks in the GC chromatograms.

The phenoxymethyl penicillin dye complex with methyl green can be extracted from a pH 3 buffer into benzene and determined colorimetric-ally 73. Penicillin V and phenoxyacetic acid can be determined separately. The procedure is claimed to be simple, rapid and accurate, and the values obtained agree with potencies obtained by biological tests. 

Litmus (azolitmin) is the classical indicator for acid-base titrations, although by now it has been supplanted by much better indicators. Neither litmus nor azolitmin should be used in colorimetric determinations of pH because of their salt and protein errors. Litmus is of value only in the form of indicator paper (cf. Chapter Eleven). 

I.4. Acetoacetic Acid. Whereas the results discussed previously were obtained by chromatographic techniques, acetoacetic acid cannot be determined by this means because of its instability. Besides older colorimetric or manometric methods for its determination, Walker (Wl) has described a new technique, based on the color obtained with p-nitro-aniline, which gives a normal value of 0.31 mg% acetoacetic acid in normal whole blood. 

High voltage electrophoresis at 50 volts/cm. has been shown to separate amines and ammonia from their parent amino acid (13) and was employed to permit colorimetric determination of ammonia and the amines. The excess of ammonia observed in every case coupled with factors of 10 and 100 higher sensitivity for ammonia than methylamine or ethylamine respectively in the procedure indicates that the microdiffusion results G(NH i + amines) to a good approximation are also the G(NH3) values. 

The active acidity is the instantaneous condition of free hydrogen ions in the soil solution. It results most frequently from dissociated free acids or dissociated acid salts occurring in the soil. It is expressed by the pH value, and determined by colorimetric or potentiometric methods. It should be determined in fresh samples as far as possible, preferably in the field or as soon as possible after transporting the sample into the laboratory, to obtain the actual acidity under natural conditions. 

The analytically pure amino acids required for the measurement of physical properties of amino acids, the determination of amino acids by colorimetric and microbiolo cal methods and the study of the metabolic functions of amino acids, may be prepared by reciystallisation from water, dilute HCl, or an aqueous solution of methanol, ethanol or other alcohol. An amino acid is recrystallised und conditions such as those outlined in Table II until inorganic impurities have been removed, and the value found for a physical property (such as specific rotation or solubility) or a constituent (such as nitrogen) is in close agreement with the standard or theoretical figure. If no standard figure few a physical property is available the product is recrystallized until a constant value is obtained. 

The anhydride may also be determined by a potentiometric titration with sodium methylate in methanol.A combination of both the aqueous method and the anhydrous method gives the amount of acid impurity in MA. For samples containing less than 0.1% acid, a known amount of acid is added to get a clear inflection point in the potentiometric curve.Acid impurities can also be determined by potentiometric titration of an acetone solution with a standard solution of tripropylamine or N-ethylpiperidine. Xylene insolubles, as mentioned earlier, also measure the acid content. In systems where anhydride has to be determined, the anilic acid value may be used. Determination of trace quantities could be accomplished using compound formation with dimethylaniline. The intense orange-red addition compound (determined colorimetrically) may be used to determine 0.1% MA in some systems. 

Most published quantitative data are based on isolation of the compounds, and thus represent minimum values. Colorimetric and spectrophotometric methods for quantitative determination of a few substances (without isolation) have been described. Ramaut et al. (1966) and Rundel (1969) determined usnic acid spectrophotometrically. Laasko and Gustafsson (1952) determined usnic acid as the FeClg-complex, and Jayasankar and Towers (1968) used the reaction product of usnic acid with Ehrlich s reagent for the determination. Determination methods for parietin (Hill and Woolhouse, 1966 Richardson, 1967) and atranorin (Vainshtein and Ravinskaya, 1971)... 

Nickel also is deterrnined by a volumetric method employing ethylenediaminetetraacetic acid as a titrant. Inductively coupled plasma (ICP) is preferred to determine very low nickel values (see Trace AND RESIDUE ANALYSIS). The classical gravimetric method employing dimethylglyoxime to precipitate nickel as a red complex is used as a precise analytical technique (122). A colorimetric method employing dimethylglyoxime also is available. The classical method of electro deposition is a commonly employed technique to separate nickel in the presence of other metals, notably copper (qv). It is also used to estabhsh caUbration criteria for the spectrophotometric methods. X-ray diffraction often is used to identify nickel in crystalline form. 

Carbon paste and graphite epoxy electrodes modified with RUO2 can be used for detection of amino acids and peptides in F1A systems. Optimal conditions are in strongly alkaline solutions at +0.45 V vs Ag/AgCl electrode, with a fast and linear response. Carbon paste electrodes can be modified also with C03O4371. Colorimetric methods for the determination of amino groups attached to a solid support may give erroneous values... 

Colorimetric assays used in endocrinological procedures are also often subject to drug interference. We have observed an interesting interference in a patient with carcinoid. The patient excreted 400 mg of 5-hydroxyindoleacetic acid (5-HIAA) and when a vanillylmandclic acid (VMA) determination was performed by a nonspecific diazo method, the value was reported to be 375 mg. The catecholamines were just above normal. There was an immediate suggestion that the patient also had a pheochromocytoma. However, when a specific chromatographic VMA method was used, the value was found to be within normal limits. Subse-... 

Two methods are commonly used to determine the endpoint of an acidity titration. The potentiometric method titrates to a predetermined pH and the colorimetric method uses an indicator that changes color at a particular pH to determine the endpoint. Other methods define the endpoint as the inflection of a titration curve, i.e., plots of pH value versus milliliter of NaOH used (Sadler and Murphy, 1998 Hand et al., 1993). However, the increased precision... 

---