5'4 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



TABLE 2. Acid Thresholds in Man {in .Normal Concentrations) 



f.g 



This table is based on values cited in von Skramlik C198) and certain more recent studies. Data from earlier literature in 

 other compilations were not incorporated because of frequent errors of computation observed therein or uncertainties of 

 method or technique of e.xperimentation. Values shown are the median of several values, the number being shown in column 

 n. The ranges of values are also reported. Hahn's (90) values of .003 .V for all acids (except butyric and malic acids) were 

 not included in this table. 



* a, Cragg (59); b, Fabian & Blum (74); c, Gibson & Hartmann (83), d, Hopkins (iio); e, Knowles et at. (123); f, Taylor 

 (195); g, (Paul and Bohnen, Corin, Richards, Heymann, Richet, Renqvist) cited by von Skramlik (198). 



centration at which a difference from water can be 

 detected or as a recognition threshold, i.e. where the 

 quahty can be recognized. The former are usually 

 lower. 



Weak organic acids appear more sovir than would 

 be expected from their degree of dissociation. At 

 threshold, the hydrogen ion concentration of acetic 

 acid is less than that of hydrochloric acid. Liljestrand 

 (135) found that the pH of weak organic acids at 

 threshold ranged from 3.7 to 3.9, for strong mineral 

 acids from 3.4 to 3.5. The findings of equal pH (ap- 

 pro.ximately 4.4) for organic and inorganic acids (59) 

 or equal normality of .003 n for all acids (89) are at 

 variance with the more common result of different 

 pH and different normalities at threshold (12, 26, 

 114, 158, 174, 195). 



Cragg C58) noted that subjects with a more alkaline 

 saliv'a required more concentrated hydrochloric acid 

 solutions to match an acetic acid standard. The sour 

 taste of buffers and of solutions of the monobasic 

 salts of organic acids can be detected at pH values 

 which are lower than those of inorganic acid solutions 

 (12, 135, 158}. Buffer solutions held in the mouth 

 retain the sour taste longer than does plain acid. The 

 pH of acetic acid changes less than the pH of hydro- 

 chloric acid after being held in the mouth. 



The relative stimulating efficiency of suprathresh- 

 old concentrations studied by means of equal sourness 

 matches between different acids and the standard, 

 hydrochloric acid, is shown in figure 8. On the basis 

 of hydrogen ion concentration, the organic acids 



acetic, carbonic, tartaric, lactic and acetylactic acids 

 were all more sour than hvdrochloric acid (16, 103, 



158). 



These effects are not clue solely to the buffering 

 action of saliva. When acid solutions are applied by 

 a flow system applicator so that the saliva is 

 thoroughly rinsed off, equal afferent nerve discharge 

 was not achieved with equal pH, ecjual normality or 

 molarity (20). Figure 9 shows the magnitude of 

 response in the chorda tympani discharge produced 

 by different acids of the same pH. 



Thus, some basic physiological mechanism com- 

 plicates the relation between sourness and acidity. 

 Richards (174) suggested that the hydrogen ions 

 might react with some substance on the receptor 

 surface so that, as these ions in a solution of the organic 

 acid were taken up, further dissociation would re- 

 place them. Others (103) refer to the potential as well 

 as actual hydrogen ion concentration as a deter- 

 miner of sourness. Kenrick (116) and Beatty & Cragg 

 (16) noted that the amount of phosphate buffer 

 (pH 7) necessary to bring equisour concentrations of 

 different acids to a pH 4.4 was proportional to the 

 sourness defined by the normality of an equisour 

 hydrochloric acid solution. Ostwald & Kuhn (151) 

 noted a parallel between the sourness and the swell- 

 ing of gelatin in different acids. Sourness has been 

 attributed to the rate at which the acid penetrates 

 the cell or intracellular spaces (61) or to adsorption 

 on the cell surface (195)- 



Acid stimulation of the integument of lower organ- 



