Formaldehyde method

There is a substantial group of diazonium salts to which the alkaline formaldehyde method, or any other deamination process requiring alkaline conditions, should not be applied, inasmuch as they are unstable in the presence of alkali. This instability is merely an extension of the hydrolytic cleavage reaction previously noted (pp. 274-276). Comparatively few diazonium salts are converted to the diazo oxides in aqueous mineral acid. As the acidity of the solution is decreased, however, diazo oxide formation is facilitated, and finally on the alkaline side many diazo compounds that are stable in acid media undergo hydrolytic cleavage.96 96 Thus, while 2,4-dinitrobenzenediazonium acid sulfate is stable to dilute aqueous sulfuric acid, in neutral or slightly alkaline solution it readily yields the diazo oxide.6 The reaction is not quantitative a poorly defined insoluble product is also formed in 14 to 20% yields. 

L. Hough, Periodate oxidation of neutral polysaccharides Oxidation to formaldehyde. Methods Carbohydr. Chem., 5 (1965) 370-377. 

This metiiod first described by Nash in 1952 has tiie advantage over many of the other colorimetric formaldehyde methods in tiiat tiie reaction takes place imder relahvely mild reaction conditions. One disadvantage, however, is that a reaction is gained not only with free formaldehyde but to some extent also witii bound and, although to a limited extent, polymeric formaldehyde. 

Now, experiment shows that, by reason of the formation of di-amino-acids and other nitrogenous products not correctly measurable by the formaldehyde method, one cannot in practice measure in this way more than 90% of the total nitrogen. Thus in reality only 90% of the water which has been estimated for the reaction, or 21.i X. 90 = 18.99, round numbers, 19 g. of water, has been introduced per 100 g. of albumin, a quantity which represents 67 X. 90 = 60 molecules of water. A complete disintegration would require a determined value of 38 g. of water, or 120 molecules. 

Formaldehyde Method of Sorensen. — This method, which can serve just as well for determining amino-acids as for determining the degree of hydrolysis of protein material, is based on the following prindple If, to a solution of an amino-acid, neutral from the fact of internal neutralization of the acid COjH by the basic NHa, formaldehyde, likewise neutral, is. added, the latter, reacting on the amino group, removes this basic function and the solution immediately becomes add. For example, in the case of glydn. 

To obtain satisfactory results, it is important first of all to use a quantity of substance such as shall require about 5 c.c. of N/10 NaOH, and further it is well to repeat each determination two or three times. If the solution contains ammonia, it is better first to remove it, this reacting only in an incomplete manner in the formaldehyde method. Place the liquid to be analyzed in a tall, narrow vessel, add to it i c.c. of solution of phenolphthalein, 2 g. of BaCU, and a half-volume of the saturated methyl alcohol solution of Ba(OH)2. Then for several hours cause a rapid current of air to pass through the liquid, and receive the ammonia... 

Water-soluble potassium (expressed as K2O) can be determined by manual or automated flame photometric and tetraphenylboron titration methods for all types of fertilizer samples. Atomic absorption can be used for samples below 5% K2O to maintain the highest degree of precision and accuracy. (Note Tetraphenylboron method for potassium is being used less often because of safety issues associated with formaldehyde.) Methods using ICP-OES for high concentration of potassium are currently being developed. Note Potentiometric measurements can be used to determine potassium in fertilizers but are not official approved methods and do not meet the accuracy and precision requirements as current methods. 

For the determination of the gold number hydrosols prepared by the formaldehyde method having particles l3ung between 20 and 30 nn are the most suitable. The correct degree of subdivision may be known by a faint brownish opalescence in incident light in transmitted light the solution must be deep red and clear. If the protective effect of the colloid in question is approximately known, it is wise to dilute imtil a few tenths of a cubic centimeter will prevent the R. Zdgmondy Zeit. f. analyt. Chemie, 40, 697-719 (1901). 

Histamine, a substance with profound effects on the mammalian organism, is not demonstrable with the formaldehyde method of Falck and Hillarp. However, it was recently shown that it will give a fluoroplu e with a different aldehyde, orthophtaldialdehyde (OPT) With the OPT method, histamine occurs in mast cells, as well as in certain epithelial cells of the murine (but only the murine) stomach. These are identical with cells that can be induced to produce and contain other amines, and presumably also identical with cells that contain the anti-pemi-cious principle. No nerves containing histamine have been detected, but the sensitivity of the method is less than the Falck and Hillarp method for catecholamines. 

Estimation of Ammonia.— Neutralise 10 ml. of urine to phenol-phthalein, using N/10 NaOH, as in the estimation of the total acidity of urine. Neutralise 2-3 ml. of commercial formalin solution (30-40 per cent, formaldehyde) in the same manner. Mix the two neutral solutions. The mixture becomes acid, and the pink colour of the indicator is discharged. This is because the H-ions previously combined with the ammonia have been liberated by the formaldehyde. Titrate this increase in acidity as before with N/10 NaOH. Each ml. of alkali required corresponds to 1 7 mg. of NHg in the original urine. The ammonia value obtained by the formaldehyde method is usually 10-25 per cent, too high owing to the fact that the —NHj groups of other urinary solutes also interact. The discrepancy is usually neglected in clinical estimations. 

Embryos stored in methanol must be rehydrated before staining. Rehydration methods differ slightly for embryos fixed in methanol and embryos fixed in formaldehyde (Methods 1 and 2, below). If a new probe or antibody is being used, a gentle rehydration method is recommended (Method 3, below). 

Heikes, B., B. McCully, X. Zhou, Y.-N- Lee, K. Mopper, X. Chen, G Mackay, D. Kareeki, H. Schiff, T. Campos, and E. Atlas (1996), Formaldehyde methods comparison in the remote lower troposphere during the Mauna Loa Photochemistry Experiment 2, J. Geophys. Res., 101, 14741-14756. 

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