THE MEASUREMENT OF ph AND ACID-NEUTRALIZING POWER OF SALIVA

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1 THE MEASUREMENT OF ph AND ACID-NEUTRALIZING POWER OF SALIVA BY BASIL C. SOYENKOFF AND CLAUS F. HINCK, JR. (From the Department of Chemistry, College of Dentistry, New York University, New York) (Received for publication, January 9, 1935) The reaction of human saliva has been determined by a large number of investigators. Indeed, the earlier experiments in this direction antedate the quantitative formulation of acidity in terms of ph. The hydrogen electrode was found to give results which differed by 0.1 to 0.5 ph unit from those obtained with the aid of indicators (1). There appears to be no other published evidence regarding the precision of any of the methods used. Measurements of greater precision than the indicators afford would be desirable, if the behavior of such saliva constituents as proteins, phosphates, etc., is to be studied. The above considerations led us to investigate the applicability to saliva of those electrometric methods which have been successfully employed in the case of blood serum and plasma. The subjects were 20 to 45 years of age. Active caries was present to a slight extent in a few cases. Otherwise the oral conditions were normal. The mouth was rinsed with water before collecting the sample. The saliva was allowed to accumulate in the mouth (by normal flow unless otherwise specified) for a sufficiently long period, then transferred to a suitable vessel. In reporting the results of our measurements, we shall use the term ph in its empirical sense only (2) ; i.e., without any implication concerning its relationship to H ion concentration or activity. EXPERIMENTAL Hydrogen Electrode Checked against Quinhydrone Electrode-The measurements were performed at 25 f In using the capillary quinhydrone electrode, we followed the procedure described by Laug (3). 467

2 468 Acidity of Saliva The saliva was collected for 25 to 30 minutes and transferred to the solution cup of the Clark electrode vessel. The cup was closed with a rubber stopper which carried two glass tubes. One communicated, through a stop-cock, with a pressure reservoir. The other, a capillary dipping into the saliva, was also provided with a stop-cock. The saliva was mixed, in so far as possible, by rocking the vessel. A quinhydrone cell was then attached to the capillary and filled. The measurement with the quinhydrone electrode was repeated at intervals until the difference between two consecutive results did not exceed 2 millivolts; then a check determination was made with the hydrogen electrode. In calculating the ph values, the quinhydrone electrode was assumed to be more positive than the hydrogen electrode by volt at 25. The quinhydrone electrode showed a drift of about 1 millivolt during 3 to 8 minutes after the filling; equilibrium readings were taken as the correct, ones. Check determinations with the hydrogen against quinhydrone electrode weremade on f?ft,een saliva samples from the same individual and on five samples from four other subjects. Only in a single case, the values by the two methods differed by as much as 0.06 ph unit. The average difference was ph unit. No further experiments were made with the hydrogen electrode since the quinhydrone electrode proved more convenient to use. Quinhydrone Electrode Checked against Glass Electrode-The quinhydrone cells were the same as previously used, except that a small stop-cock was attached at the lower end in order to prevent flow and diffusion within the capillary. The glass electrodes were of the type developed by MacInnes and Dole (4). Fig. 1 shows one of those electrodes supported in a glass vessel, C, which contains saliva (or some other solution used in the measurement). B is one arm of a U-tube filled with saturated KCl. Suspended in the other arm (not shown) is the reference half-cell (Ag-AgCl electrode in 0.1 N HCl). The same salt bridge and reference half-cell were used in the measurements with the quinhydrone electrode. The measurements were made in a shielded oil bath whose temperature was maintained at 25 f A Lindemann electrometer served as the null point instrument; the smallest

3 B. C. Soyenkoff and C. F. Hinck, Jr. 469 deflection which could be observed corresponded to 0.5 millivolt. The compensating electromotive force was supplied by a type K potentiometer. Both the quinhydrone electrode and the glass electrode were calibrated against 0.05 M phthalate (ph 3.982) and a phosphate buffer of ph (measured with the hydrogen electrode). FIG. 1. The glass electrode vessel. C represents the electrode vessel; B, the salt bridge. The saliva used in the check measurements was thoroughly mixed in the vessel shown on Fig. 2. The electrode vessels were then filled through the capillary attached to its side. Seven check determinations were made. The values obtained by the two methods differed, on the average, by 0.01 ph unit; the largest difference was 0.02 ph unit. The drift observed with the glass electrode was 0.5 millivolt or less; successive readings on samples of the same saliva or buffer solution checked within 0.5 millivolt. The glass electrode was therefore adopted for use in subsequent work. ph of Resting and Parafin-Stimulated Saliva-The measure-

4 470 Acidity of Saliva ments were made in an oil bath at 37 (cf. above). The glass electrodes were calibrated against 0.05 M phthalate whose ph was assumed to be the same at 37 as at 25 (5). The saliva was obtained between 3 and 4 p.m. After a sample of resting saliva (about 8 minutes required) was collected, the subject was asked to rinse his mouth and was given a piece of parawax to chew. In about 3 minutes a sufficient quantity of the stimulated saliva accumulated in the mouth. In order to minimize the loss of COZ, the samples were transferred from the mouth to the electrode vessel C (Fig. 1) with the aid of a funnel (with a FIG. 2. The mixing cell. The saliva was introduced through the capillary tube attached to the side. wide cone) which was attached to C with a short piece of rubber tubing. With the funnel held between the lips and the teeth, saliva was allowed to flow into the electrode vessel until its level reached 1 cm. above the glass membrane. Nine samples of resting saliva from nine subjects ranged from 6.37 to 6.89 ph. The ph of nine samples of stimulated saliva, collected immediately after, varied between 6.97 and In the case of four of the subjects, additional samples were taken between 11 a.m. and 12 noon and between 5 and 6 p.m. (The subjects had completed their breakfast at 8 a.m., lunch at p.m.) The ph values of those samples were 6.40 to 6.88 for resting saliva,

5 ES. C. Soyenkoff and C. F. Hinck, Jr to 7.27 for stimulated saliva. The average of all of the above results for resting saliva is ph 6.64; for stimulated saliva, ph Change of Saliva on Addition of Acid and Alkali-Neither the quinhydrone electrode (in its usual form) nor the glass electrode proved suitable for the potentiometric titration of saliva, probably because of the presence of proteins which precipitated on the electrode surface during the addition of acid. Preliminary experiments with the hydrogen electrode showed that it, too, was 0 4 I P 6 CC,HCl+-+CCNadH FIG. 3. The titration curves of saliva. The amounts of 0.1 N HCI or NaOH added to a 2 cc. sample of resting saliva are given as the abscissae. The titration curve of 1:2 saliva is represented by a solid line; that of 1:lO saliva, by a dotted line. subject to a similar influence. The quinhydrone electrode gave readings reproducible to 0.15 ph unit when used in the titration of dilute (1: 10) saliva with 0.1 N HCI. The titration curves thus obtained, however, were nearly identical regardless of the origin of the saliva sample. The curve plotted from the average values for ten samples from ten individuals is shown on Fig. 3. In the case of more concentrated (1:2) saliva solutions, each titration curve was plotted from ph measurements on a number

6 472 Acidity of Saliva of saliva samples from the same individual, but with different amounts of acid or alkali added. The procedure was as follows: Five men, aged 20 to 45 years, served as the subjects. The saliva was obtained at various hours between 9 a.m. and 9 p.m., but always at least 2 hours after the last meal. Moreover, the time of the day did not vary by more than 1 to 2 hours for a given subject. The sample was received in a cylindrical funnel which was calibrated to deliver 2 cc. It was introduced, through the side tube Acid or base added 0.1 N HCI 0.1 N NaOH TABLE ph of Saliva on Addition of Acid and Alkali - - Amount of acid M base added to 2 cc. saliva cc I -7 Difference between maximum and minimum values of the mixing cell (Fig. 2), under a measured quantity of 0.1 N HCl or NaOH made up to 2 cc. with water. The cell contents were stirred, and a portion of the mixture forced up for ph measurement with the glass electrode. Whenever large amounts of precipitate formed on the addition of acid, measurements were made on two portions from the same sample; the first one consisting of clear liquid and the second containing as much as possible of the precipitate. Such duplicate measurements checked within 0.02 ph unit. The saliva samples differed markedly in their acid-neutralizing

7 B. C. Soyenkoff and C. F. Hinck, Jr. 473 power. The difference between the lowest and the highest ph reading obtained on the addition of a given amount of acid (or alkali) is found in the fourth column of Table I. The average values for each point on the titration curve are found in the third column. For the curve plotted from those values, one is referred to Fig. 3. DISCUSSION In beginning a general study of saliva, we were faced with the need for a rapid, precise, and reliable method of ph measurement. The ordinary calorimetric procedure involves dilution with water and the addition of a foreign substance (an indicator). Hence its usefulness is, at best, limited. Besides, the previously published results (1) by the calorimetric and electrometric (hydrogen electrode) methods do not check well enough to recommend either. From the experiments reported above, the MacInnes glass electrode would appear to be reliable to at least 0.02 ph unit. Readings made with two glass electrodes on the same saliva sample usually check within 0.01 ph unit. Some of the electrodes, made half a year ago, have since been used in more than 50 measurements each. The capillary quinhydrone electrode gave results reproducible to 1 millivolt only after considerable practice in its manipulation. It was very sensitive to temperature changes and, occasionally, to other influences which were less easy to trace. A series of measurements with the glass electrode of the ph of resting and paraflin-stimulated saliva at 37 gave results in approximate agreement with the data found in the literature. The readings obtained with the resting and stimulated saliva form two well separated groups, the ph values of stimulated saliva being higher by an average of 0.5 unit. The range of variation within each group appears smaller than that previously reported, but it may possibly increase when additional subjects become available for study. None of the electrodes which we tested was suitable for use in a potentiometric titration of saliva. Each one of the titration curves was therefore plotted from ph readings on a series of samples from the same subject, taken in a similar manner and, in so far as was known, under similar conditions. The data thus obtained permit certain conclusions regarding the behavior of saliva as a buffer.

8 474 Acidity of Saliva If we assume the ph figures in Table I to represent, at least approximately, hydrogen ion activities, it will be seen that the acid-neutralizing power of saliva decreases markedly between ph 4 and 2.5. On the alkaline side of ph 4, all but a very small fraction of the added hydrogen ion is removed by chemical combination; on the acid side of ph 2.5, more than half of the added hydrogen ion remains uncombined. This interpretation of the ph data is supported by results obtained by conductometric titration of saliva. Several samples of (1:3) saliva were titrated with 0.1 N HCl. The conductivity increase, at first very slight, became rapid and uniform beyond the point where sufficient acid had been added to obtain ph 4. One may thus conclude that the buffer range of saliva constituents lies, mainly, on the alkaline side of ph 4. Variations in the amounts of these substances would be reflected in relatively small ph changes of saliva as such. On the addition of acid, however, a point would be reached where individual variations of the buffer content of the sample would lead to large ph differences. Further addition of the acid would cause the titration curves of several samples again to approach each other. The figures in the fourth column of Table I (difference between maximum and minimum ph value for every point on the titration curve) demonstrate this effect. In comparing the acid-neutralizing power of several samples of resting saliva, one or two ph measurements on each sample will hence be sufficient provided that suitable amounts of acid have been previously added (for instance, 0.3 or 0.4 cc. of 0.1 N HCI to 2 cc. of saliva). The ability of saliva to neutralize acids is thought by some to have an important bearing on the etiology of caries. Experiments designed to establish a definite relationship between caries susceptibility and the ph and buffer action of saliva have led to contradictory conclusions. In so far as the lack of agreement among the investigators might be due to their methods of procedure, it is hoped that our results may, in part, contribute to useful knowledge in the field of caries research. A grateful acknowledgment is made of the advice given by Dr. Belcher, of the Rockefeller Institute, regarding the preparation of silver-silver chloride electrodes. The authors also wish to thank

9 B. C. Soyenkoff and C. F. Hinck, Jr. 475 members of the staff of the New York University College of Dentistry and others who served as the subjects. SUMMARY The ph values of saliva obtained with the MacInnes glass electrode and capillary quinhydrone electrode differed by less than 0.02 ph unit. The glass electrode gave more reproducible results and proved easier to handle than the quinhydrone electrode. The average ph values of resting and stimulated saliva, measured with the glass electrode at 37, differed by 0.5 ph unit. Saliva acts as a buffer on the alkaline side of ph 4. Samples of resting saliva vary considerably in their acid-neutralizing power. BIBLIOGRAPHY 1. Henderson, M., and Millet, J. A. P., J. Biol. Chem., 76, 565 (1927). Marenzi, A. D., and Rossignoli, J. J., Compt. rend. Sot. biol., 99, 177 (1928). Peabody, W. A., Hall, I. C., and Lewis, R. C., Dent. Cosmos, 69, 1089 (1927). 2. Clark, W. M., The determination of hydrogen ions, Baltimore, 3rd edition, 480 (1928). 3. Laug, E. P., J. Biol. Chem., 66, 553 (1930). 4. MacInnes, D. A., and Dole, M., Ind. and Eng. Chem., Anal. Ed., 1, 57 (1929). 5. Kolthoff, I. M., and Tekelenburg, F., Rec. trav. chim. Pays-gas, 46, 37, 41 (1927).

10 THE MEASUREMENT OF ph AND ACID-NEUTRALIZING POWER OF SALIVA Basil C. Soyenkoff and Claus F. Hinck, Jr. J. Biol. Chem. 1935, 109: Access the most updated version of this article at Alerts: When this article is cited When a correction for this article is posted Click here to choose from all of JBC's alerts This article cites 0 references, 0 of which can be accessed free at tml#ref-list-1