This paper is not concerned with the practical difficulties in making pH measurements, such as corrections for liquid junction potentials or the errors inherent in certain electrode systems, since these matters are to be considered in other papers on this symposium. The principal purpose of this paper is the consideration of a number of theoretical difficulties in the definition of the term which place a practical limit on the precision of the pH measurements or on the precision of calculations made from the measurement. The latter part of the paper will consider the even greater difficulty of correlating acidity measurements in nonaqueous solvents with those in water. The majority of industrial processes in which pH measurements are important permit a rather wide tolerance in the pH for successful operation. In these cases, a precision of ±0.3 pH units is usually ample. Since the magnitude of the corrections applied to observations in order to obtain precise pH values seldom exceed 0.3 pH units, it is obvious that for control work the operator need not concern himself with the “fine points” of pH measurement. On the other hand, he should know enough about the process and measuring apparatus to avoid a positive misapplication of the latter and should have a fair knowledge of the inherent errors in order to avoid placing a false significance on the measurements obtained.
Three bituminous coal samples were extracted at 350°C. with phenanthrene, labeled in one series with tritium, in another with C 14 . The extract yield was 80-90%. Results: converting coal to soluble products breaks C-H bonds; 10-15% of the hydrogen content of the coal exchanged with that of the phenanthrene during extraction; 3-8% chemically linked phenanthrene was found in the extracts; less than 1% hydrogen exchange took place between pure model compounds (anthracene, p-benzyl--phenol) and tritiated phenanthrene under similar experimental conditions. Thus, chemical reactions take place during phenanthrene extraction of bituminous coals. The role of phenanthrene in this process is probably an interaction with the free radicals which are formed in the thermal decomposition of coal.jy|ost solvents which have been used for coal extraction can be divided into two groups. Extraction with the first group of solvents is based on purely physical effects and gives low extract yields. Most monocyclic aromatic compounds belong to this group. On the other hand, the extract yield is high when coal is treated with hydroaromatic solvents at higher temperatures. In that process chemical interactions take place, and hydrogen is transferred from the solvent to the coal substance. The higher yields with hydroaromatic solvents result from the hydrogenative cleavage of carbon-carbon and carbonoxygen linkages in the coal polymer and the formation of lower molecular weight, more soluble materials. The results of Orchin and co-workers (I, 2) on the extraction of coal with phenanthrene and other polynuclear aromatic compounds do not fit into this classification. They reported that at a temperature of about 350°C. phenanthrene and other related angular condensed aro-1
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