2.4-ジニトロフェニルヒドラジンを用いる直流ポーラログラフ法によるアルデヒドの間接定量法
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概要
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The solubility of 2, 4-dinitrophenylhydrazine (abbrev. as 2, 4-DNPH) in water-ethanol mixed solvent increases by the addition of mineral acids. At the condition of the concentration of 0.81 <I>M</I> phosphoric acid (supporting electrolyte), 30 vol% ethanol and 0.03 wt% gelatin (maximum suppressor), the two step-reduction potential wave, which is incompletely separated, appears in the direct current polarography of 2, 4-DNPH, and these two waves are supposed to be due to the two NO<SUB>2</SUB> groups. At the dropping mercury electrode <I>E</I><SUB>1/2</SUB> of the first wave is-0.330 V and <I>E</I><SUB>1/2</SUB> of the second wave is-0.580 V (<I>vs</I>. Hg pool). By the treatment of two-step wave as one (<I>i. e</I>. total wave), <I>E</I><SUB>1/2</SUB> is-0.465 V <I>vs</I>. Hg pool at the dropping mercury electrode. In this case applies the linear relation between the square root of mercury reservoire hight and the limiting current value. Accordingly, the diffusion is the rate determining step of this electrode reaction at least at <I>E</I><SUB>1/2</SUB>=-0.465 V, the Ilkovic equation holds, and consequently the total wave hight is proportional to the concentration of 2, 4-DNPH. The calibration curve of 2, 4-DNPH concentration and total wave hight, at the condition mentioned above, is a straight line passing through the origin. The limiting current is very stable and unaltered during a measurement. In the condition of the concentration of 15.14×10<SUP>-4</SUP><I>M</I> 2, 4-DNPH, 0.81 <I>M</I> phosphoric acid, 30% ethanol, less than 9.90×10<SUP>-4</SUP><I>M</I> aldehyde, reaction temperature 0°C and reaction time larger than 120 min, a reducing value of the reduction wave-hight reaches the maximum value and is proportional to an amount of aldehyde in a sample. Accordingly, the quantitative analysis of aldehyde is possible based on this reducing value of reduction wave hight in the condition to maintain the constant concentrations of phosphoric acid and of ethanol during all experiments.<BR>In ordinary direct method, a sample escape causes frequently an error in the analysis of volatile materials such as acrolein, acetaldehyde and formaldehyde. But in the present method, a sample is poured into the reagent solution containing a definite amount of 2, 4-DNPH, which is previousry prepared, and the reaction is completed by shaking the mixture occasionally during 120 min in a water-ice bath. By this procedure, the concern about a sample-escape is unnecessary and from the reducing value in polarogram of the 2, 4-DNPH, drawn under the condition mentioned above, the aldehyde at such a low concentration as 0.709.90×10<SUP>-4</SUP><I>M</I> can be determined in good precision.<BR>In the condition mentioned above, the gradients of calibration curve of acrolein, acetaldehyde and formaldehyde are quite the same and 1.33×10<SUP>4</SUP>μA/<I>M</I>. So that when a sample containing more than two sorts of those aldehydes is analysed, it is unable to determine them separately, but able to obtain the total molar concentration of aldehydes.<BR>In the concentration range of total aldehyde, each of which is different in mixing ratio, from 0.95×10<SUP>-4</SUP><I>M</I> to 7.66 × 10<SUP>-4</SUP><I>M</I> a quantitative analysis can be done precisely. At the concentration 6.48 × 10<SUP>-4</SUP><I>M</I> of aldehyde, the relative standard deviation calculated by five our analyses is 0.005%.
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