Acid corrections

Only in a few investigations cited here, acid corrections were applied. Schroe-der [62] gave a detailed description of the method and discussed the application to combustion heat of several materials of ecological interest (leaves, faeces, Le-pidoptera larvae, zooplankton). Moreover, he compared the results obtained with a Parr semimicro bomb with that of a Phillipson type bomb and established a negative mass dependence. 

During combustion of organic matter, acids are formed from non-metallic oxides, mainly H2SO4, HNO, and HCl. They are collected in bomb washings with methyl red saturated deionized water and titrated with 0.0725 M Na2C03 to the methyl red endpoint as recommended in the Parr manual. Investigations showed that 4 J ( 1 cal) per ml NazCOj have to be subtracted from the observed heats of combustion Schroeder determined a mean acid correction of -0.33 % with sample masses of 0.10 g and of -0.14 % at 0.30 g in the semimicro bomb and of -0.57 % (5 mg) and - 0.31 % (20 mg) in the Phillipson type bomb. Acid corrections are less pronounced for plant than for animal matter and more important with the more accurate semimicro bomb as they form 20 to 30 % of the technique variations in this instrument [62]. 

Moreover, samples with an organic content smaller than about 25 % should be burnt with additional, known amounts of benzoic acid (up to 30 %) to guarantee complete combustion and to promote ignition [37], Paine [26] recommended to add mineral oil of known combustion heat to samples to bind the pulverized material together and to increase the temperature difference after burning. Cross and coworkers [65] used equal amounts of activated charcoal to bum wasp cocoons with an extremely high ash content of 84 % (see below 5.1.2). 

Sometimes, pellets tend to break when removed from the sample press. If this happens, it is recommended to add a small amount of water to give a much greater cohesiveness to the pellets. The samples have to be dried again after the suc-eesful removal [66], 

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The simplest analytical procedure is to oxidize a sample in air below the fusion point of the ash. The loss on ignition is reported as graphitic carbon. Refinements are deterrninations of the presence of amorphous carbon by gravity separation with ethylene bromide, or preferably by x-ray diffraction, and carbonates by loss of weight on treating with nitric acid. Corrections for amorphous carbon and carbonates are appHed to the ignition data, but loss of volatile materials and oxidation may introduce errors. 

It can be concluded from the publications of Rohde [2] and Freund and Lebach [3, 4] that it was Ehrlich who first suggested the use of 4-(dimethylamino)-benzalde-hyde in the presence of hydrochloric acid for color reactions with methylketols that was found also to apply to indole derivatives [S]. Auterhoff [6] designated a reagent for urobilinogen, consisting of a solution of 2 g 4-(dimethylamino)-benzaldehyde in 20 percent hydrochloric acid, correctly as Ehrlich s solution. Hence, it is essentially correct to refer to all reagents which contmn these components as Ehrlich s reagent. 

B = average mis of 0.5N sulfuric acid required for titration of the blan ks C = normality of acid corrected for temperature F = factor for EtAcet specified in Table II as 0.0881, and... 

The following procedures were reported for the assay of Melphalan Tablets (a) hydrolysis with aqueous potassium hydroxide under reflux and potentiometric titration of the liberated chloride ions with silver nitrate in the presence of nitric acid. (Corrections for "ionisable chlorine" are made by titrating under the same conditions a sample that is not subjected to hydrolysis [25,46].) (b)... 

PAMPA is often used at various pH values in order to measure permeability pH profiles as the permeability of ionisable compounds depends heavily on the pH of the buffer. As the pH range of the intestinal tract varies between pH 6 and pH 8 this is the range of pH values that mostly is used. Kerns (2004) recommended to measure from pH 4 to pH 7,4 in order to predict both bases and acids correctly. Ruell (2003) used permeation pH profiles from pH 4 to pH 9 together with the pKa values of the compounds under investigation to establish the optimum pH value for a single pH PAMPA measurement. 

If the sample contains significant acidity or alkalinity, correct the results as follows Dissolve approximately 15 g of sample, accurately weighed, in 40 mL of undistilled pyridine, and add 60 mL of water and 0.5 mL of Phenolphthalein Indicator. If the solution is colorless, titrate with 0.1 N sodium hydroxide to a light pink endpoint, recording the volume required, in milliliters, as v. If the solution is pink, titrate with 0.1 N hydrochloric acid to the disappearance of the pink color, recording the volume required, in milliliters, as v. Calculate the acidity correction factor, A, by the formula... 

Standard Preparation Transfer an accurately weighed quantity of USP Gibberellic Acid Reference Standard, equivalent to about 25 mg of pure gibberellic acid (corrected for phase purity and volatiles content), into a 50-mL volumetric flask, dissolve in and dilute to volume with methanol, and mix. Transfer 10.0 mL of this solution into a second 50-mL volumetric flask, dilute to volume with methanol, and mix. 

Hydrogenation was carried out in glacial acetic acid. Correction to the gas phase by the method of Fuchs and Peacock (above) yields 28.4 kcal mol" = 119.0 kJ mol. ... 

Preparation method For 1 g of chemical Esterification number (%) Pyruvic acid (%) Corrected esterification number (%)... 

A recent paper (Reppert et al., 1951) has drawn attention to the remarkable speed with which ascorbic acid corrects the increased capillary fragility in scurvy. It is argued that this could hardly allow time for the repair of intercellular cement substance and suggests rather a direct chemical or enzymic response. Some experiments, reported in the barest outline, claim to show that ascorbic acid in large doses inhibits the hyaluronidase-hyaluronic reaction in rabbits and, to a small extent, in vitro. It is suggested that this inhibition of the spreading factor may account for the rapid improvement in capillary fragility. Obviously more experimental evidence is needed before this supposition can be seriously considered. 

Perchloric acid was discovered by Count Stadion. He discovered potassium perchlorate in the residue from the preparation of chlorine dioxide from potassium chlorate and concentrated sulphuric acid, correctly determined its composition by decomposing it by heat into oxygen and potassium chloride, and obtained a 70 per cent, solution of perchloric acid ( oxygenated chloric acid ) by distilling potassium perchlorate with somewhat diluted sulphuric acid. He also precipitated the first residue with baryta water to remove sulphuric acid, removed chloride with silver oxide, and evaporated the filtrate in vacuum over concentrated sulphuric acid. Stadion adopted Davy s theory of chlorine. 

Due to the complexity of organic acids correct collection of specimens is most crucial (Table C.5). Samples for organic acid analysis should be frozen as soon as possible and kept and shipped frozen. At room temperature chemical instability causes substantial losses of some important groups of acids, such as oxo acids. For prolonged periods samples should be stored at -80 °C instead of-20°C. 

The titration is made with sodium methylate in methanol using methanol as the sample solvent. Two inflection points are obtained when the reference pH values are plotted against milliliters of titrant. The difference between the inflection points corresponds to one equivalent per mole of maleic acid. In order to make the inflection points more distinguishable for less than V o of acid, a known amount of acid is added. The total acidity, corrected for the amount of free acid, gives the amount of maleic anhydride. The titration curves are rather characteristic for maleic anhydride and maleic acid. 

Table 11.1. Grape acidity correction regulations in the European Community (except Portugal)...

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