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(5) Norman, A. G., and Jenkins, S.H., Nature, 131, 729 (1933). (6) Paloheimo, L., Biochem. Z., 165,463 (1925); 214, 161 (1929). (7) Peterson, C. J., Hixon, R. M., and Walde, A. W., ISD.ENG. CREM.,Anal. Ed., 4, 216-17 (1932). (8) Phillips, Max, and Goss, M. J., J. Assoc. Agr. Chem., 21, 140-5 (1938). (9) Ritter, G. J., and Barbour, J., IND. ENG.CHEM.,Anal. Ed., 7, 238 (1935).
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(10) Ritter, G. J., Mitchell, R. L., and Seborg, R. M., J. Am. Chem. SOC.,55, 2989-91 (1933). (11) Sherrard, E. C., and Harris, E. E., IND. ENG.CHEM.,24, 103 (1932). RECEIVED November 4, 1938. Presented before the Division of Cellulose Chemistry at the 96th Meeting of the American Chemioal Society, Milwaukee, Wis., September 5 to 9, 1938.
Testing Dentifrice Abrasives MERVYN L. SMITH Research Laboratory, John & E. Sturge, Ltd., 1 Wheeley’s Road, Birmingham, England
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ONSIDERABLE emphasis is now placed on the physical properties of the insoluble materials that are almost universally used in dentifrices to assist the brush in cleaning the tooth surface. This close attention was stimulated because certain dentifrices were considered unduly abrasive, while a number of scattered observations on the effect of brushing extracted teeth with commercial dentifrices (6) indicated a danger of damage t o the various tooth structures resulting from daily use over a period of years. It has, moreover, been shown recently (2, 11) that although wear of the enamel is very slight with the majority of dentifrices in use today, the softer tissues exposed by gum recession are much more open to attack. Consequently it is important to have a means of testing the abrasiveness of fine powders. Such tests are necessary for control in choosing suitable types of powder bases and for testing finished dentifrices in order to exclude those having excessive abrasive effects. For these routine purposes biological surfaces that are variable and require tiresome repetition in the test are obviously unsuitable and some standard surface is essential. Several instruments have been described which measure the abrasiveness of fine powders in arbitrary units against such surfaces, but so far their readings have not been standardized in terms of powders of characteristic physical properties nor have the abrasivenesses of the powders tested been related to the amount of wear produced by them on tooth structures. A systematic study should aim first a t grading a series of defined powders on the abrasion apparatus and then attempting to establish limits of particle size and hardness-the two important characteristics involved-liable to cause serious damage to the several tooth structures involved. In this connection Ray and Chaden (6) have pointed out that different individuals probably require dentifrices of different abrasiveness. An experimental approach of this type is simple when one considers a single substance, since it is self-evident that the grading order of a series showing increasing particle size must be the same against all surfaces. (The actual range of abrasiveness of the series will, of course, vary with the different surfaces according to the degree of penetration of the particles.) Difficulty must, however, be anticipated in comparing chemically different powders. Thus a series of samples each containing the same number of identically sized particles could conceivably be graded in different orders by different surfaces because of the specific interaction of the physical factors involved, such as hardness, particle shape, ductility, etc. At the same time it should be possible to assess different powders closely enough for practical purposes, especially as the range of hardness and particle size likely to be used is small.
Grading of Fine Abrasive Powders SCRATCH versus ABRASIONTEST.The effect of the abrasive can be estimated either by examining the scratches left on the surface after a small amount of rubbing or by continuing the treatment until a measurable amount of the surface has been removed. The two methods are complementary, each having certain advantages. The scratch test is selective, different sized scratches being made by different particles (8), and is ideal for finding small amounts of added adulterant such as pumice or emery. Even the very simple form of test described in Federal Specification FFF.D.191 for dentifrices is fairly sensitive (IO). The abrasion test gives a quantitative figure for the amount of abrasion but it is not possible to distinguish scratches of different depth. For the purpose of grading powders, however, a quantitative abrasion test is essential, though discrimination must be used in assessing the results when heterodisperse powders are being tested. Experimentally considerable difficulty is involved in these tests, as may be appreciated by the experience of Souder and Schoonover (IO) who found with a rotating table test that “the addition of 10 per cent of fine emery to a paste of minimum abrasiveness did not increase appreciably the loss in weight of the disk.” The apparatus of Ray and Chaden (6) is, however, very sensitive, while the modification with glass bed (8) can distinguish an addition of 0.020 gram per cent of a coarse ingredient in a fine dentifrice. Incidentally, the explanation of Souder and Schoonover that “a film of soap or some other ingredient prevents the two surfaces coming together” is not tenable, since results have been obtained in substantial agreement in comparing powders alone and made up in commercial dentifrices. I n previous work on grading the powders have not been sufficiently characterized and the reason for the wide variation in tests with different samples does not appear. Thus Ray and Chaden using commercial dentifrices found figures for calcium carbonate from 1.5 to 10 (milligrams of weight loss from an antimony block after 5,000 revolutions) and for calcium phosphates from 0.7 to 17, and these differences are presumably due to differences in particle size distribution and the presence of impurities. For the results, however, to have more than a restricted value referring to the particular sample, each series should be characterized from this point of view. Wright and Fenske (11) gave ranges of a similar order (with the same apparatus) using powders, but again no physical description was attempted. An earlier communication (8) described tests on a series of experimentally precipitated chalk powders by which a relation between particle size and abrasiveness was established.
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tendency to a decrease of the abrasion values in successive runs, and to correct for this tendency and also for variation in other experimental conditions on different occasions such as the thickness of the film of suspension and setting of the plate, frequent checks are made with a standard powder and the results for the unknown “bracketed” with these figures. The antimony plate also is reground a t intervals in a standard manner (rubbed down on a plate-glass sheet with successively fine grades of emery and finally with chalk, all in glycerol susDension) . MODIFICATIONS IN ABRASION APPARATUS. Since the original description was published (8), certain modifications have been made in the light of recent experience with the test which materially improve the ease of working.
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110 Y
,+ c
E * $ 6 OI
2?4 a
i
8 2 d Y
Q L A v a a g e p r t i c l e diameter in micram
FIGURE1. ABRASIVENESSOF SIZEDPOWDERSMEASURED WITH ANTIMONY PLATE ABRASIONAPPARATUS Dotted lines indicate probable curves based on analogy with calcium carbonate series which has been studied in detail. Figures in brackets are approximate Mohs hardnesses of the different materials.
Hardness was also shown to be extremely important where very hard particles such as those of the abrasive industry are concerned but of much less importance with the usual run of dentifrice abrasives whose hardness has the small range of 2 to 3.5 on the Mohs scale. Figure 1 shows the general character of the results, but only the calcium carbonate series was investigated in detail since homodisperse sized samples of other materials were not available. It is evident that small increments of particle size are much more important with the harder than with the softer powders. The curve for precipitated chalk is of interest, showing that the abrasive effect of built-up aggregates is less for a given particle size than that of calcite rhombs, the difference being most marked with the finer size grades. ANTIMONY PLATEAS STAXDARD ABRASIVESURFACE.An important consideration in a test involving the relative motion of solid surfaces is the possibility of surface flow. When comparing a series of powders of different abrasiveness the finer ones, since they renew the surface less rapidly, might allow an amorphous and more resistant layer to build up. Providing care is taken that the tests all refer to the surface in a standard condition, this would merely alter slightly the relative abrasiveness of the coarse and fine powders on the arbitrary scale used. Thus there would be a slightly different arbitrary scale in which, if the fine powders are taken as standard, the abrasiveness of the coarser ones would be exaggerated. Antimony was originally chosen (6) as an abrasion standard because it has a hardness between that of enamel and dentine but approximating the latter (4). Other advantages which minimize the extent of surface flow are its brittleness and its high melting point (630’ C.) for a metal of low hardness. [The experiments of Bowden and Hughes ( I ) have shown that the attainment of a local momentary temperature approaching the melting point plays an important part in the formation of the polish layer.] The conditions of the test will determine how rapidly such surface temperatures are reached. In the apparatus described below the loading is 100 grams per sq. cm. and the relative speed 42 cm. per second; as the test lasts only 10 minutes and there is ample lubrication the temperature rise should not be excessive. Thus, the conditions are such that little flow should occur. There is, however, a
FIGURE2. ANTIMONYPLATEABRASIONAPPARATUSFOR FINEPOWDERS Modified form of apparatus as now used
Figure 2 shows that the spreader, A , has been made more rigid and controllable, so that the film of sus ension can be set to a repeatable thickness. The position of tfe antimony plate on the glass bed is adjusted by means of the slotted holes at the base of the pillar, B. It is important that the trailing arm, C, should be horizontal (and thus parallel t o the glass surface) and this is set by the slot at the pillar pivot, D. The stirrup pivots, E, at the lead block should be on a diameter of the bed. These adjustments are made until the abrasion over the whole surface of the antimony plate is uniform as judged visually after a few hundred revolutions. Differences between the leading and trailing edges of the plate are corrected by altering the position of the pillar pivot, C; differences between the two sides by adjusting the spreader screws, or, possibly, altering the set of the glass bed, Streaks of no abrasion on the plate are due t o bumps in the rubber spreader and may be smoothed away with sandpaper. The speed of rotation has now been standardized at 80 r. p. m. during a test which lasts for 1000 revolutions. The individual tests are repeatable to within about 20 per cent,
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ANALYTICAL EDITION
Influence of Particle Size on Abrasion Miller (6) on the basis of hand-brushing experiments stated that three different sizes (unspecified) of calcium carbonate had similar abrasiveness. This can, however, apply to only a very limited particle size range or to sizes above a certain minimum, since there must be a decreasing abrasiveness with smaller sizes; in the limit, colloid particles can have no possible abrasive action. Further, from the nature of the abrasion process such a relation must be a pure coincidence. On the other hand, the character of the backing or bed has an important effect on abrasiveness, the use of a hard bed of glass exaggerating the abrasive effect of the coarser and harder particles especially. This is a useful feature from the point of view of grading powders, as it provides a more stringent and sensitive test. Two hard surfaces (nickelcopper alloy and glass) are used in the federal test (10) for dentifrices. Consideration of the mechanism of abrasion shows that even were the gross abrasion loss per given weight of powder independent. of particle size, the condition approximated with a soft bed, the actual scratch depth must increase considerably to compensate for the decreased number of particlesi. e., it is necessary to consider both particle size and abrasion in appraising the powder. Previously this has been overlooked. The total abrasion loss depends on both the number and volume of the individual scratches. It is fair to assume that the number of these and hence the "total scratch length" are proportional to the number of particles per gram of material. Thus for two homodisperse powders with particles of diameter x and 22 the abrasivenesses are proportional, respectively, to u1/z3 and vz/8x3 where and v2 are the volumes of the scratches per unit length. For these quantities to be equali. e., for abrasion to be independent of particle size-vz must equal 8vl. Evidently the scratch must be much deeper in the case of the larger particles to fulfill this equality, the actual relation between scratch depth and volume depending on the assumed shape of the hollow and the fraction of the particle buried. The simplest case is that of a cylindrical scratch which would be made by a spherical particle. Figure 3 shows the relative scratch volumes for two particles of radii z and 22, respectively. From the relation between the scratch depth, s, and the area of the segment, ABC, it is possible to calculate the scratch depths. For the condition of scratch depth independent of particle size (assuming s = 2/20) the scratch depth increases about 3.25 times when the particle size doubles; for the experimental figures quoted (8) the factor is 4.5. These ratios are a minimum, since the assumed penetration is probably too high under the experimental conditions concerned. Clearly, since scratch depth is an important factor in assessing the possible damage of a dentifrice abrasive, the particle size as well as the measured abrasion figure must be considered.
Relation between Abrasiveness Determined against Antimony Plate and Wear on Tooth Structures Having established a satisfactory means of grading powders, can the results be related to the wear of the teeth? Here the variability of the biological material provides the biggest difficulty, the enamel itself ranging in microhardness units from 330 to 2050 and the softer tissues from 85 to 165 (6). Thus there is no such thing as a standard tooth surface so an exact correlation cannot be expected. A direct experimental comparison between the weiglit losses with the antimony block abrasion meter of Ray and
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Chaden and the wear on tooth structures has recently been described by Wright and Fenske (11),using extracted teeth. Unfortunately, the powders they used were not characterized according to their physical properties, and the results lack the significancewhich they would have had if a few graded series of different powders had been compared. The authors also did not take sufficient account of the variation in their biological material and the experimental errors involved and concluded that the agreement was not good. However, a critical examination of their data (7) showed that if such variation was allowed for there was satisfactory agreementi. e., the indication of the abrasion meter was a useful guide to the amount of wear produced in the average tooth structure,
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FIGURE3. RELATIVE SCRATCH DEPTHS For spherical particles with diameters in the ratio 1 t o 2
It has been assumed that the wear on such extracted teeth parallels the wear that would occur with the same abrasive in the mouth. This seems reasonable for the surface enamel, though less sound for the softer tissues in which, as the experiments with radioactive indicators have shown, the interchange of ions is more frequent. Any closer correlation must involve clinical experience, although experiments with extracted teeth exaggerate considerably the degree of wear to be expected in vivo, because the extracted teeth lack the natural protective film which in the mouth is constantly reformed after cleaning. It is also known that the outermost layer of enamel is hypercalcified and so may act as an additional protective layer. Obviously in tests on extracted teeth in which some depth of tissue is removed, the abrasion measured is the mean for several successive layers of imperceptibly graded characteristics. (For these reasons the type of statement which equates so many strokes in an abrasion machine with the effect of a certain number of years of tooth cleaning must be viewed with caution.) These considerations clearly will complicate the application of the abrasion results. However, the fact that Wright and Fenske found a reasonable correlation, taking the mean of 12 to 40 tests on different teeth, which probably vary in hardness to an extent overshadowing the effect of the protective layers, indicates that the tests with the antimony plate are of real significance to the probable wear in vivo. I n any case the establishment of a standard of abrasion is essential and the measure of agreement obtained is distinctly encouraging support for the antimony plate grading of dentifrice powders. On the basis of the tests with sized suspensions heterodispersity in the powder is a disadvantage from this point of view, since the effect is that the larger particles bear a disproportionate amount of the load and so bite more deeply, while the finer ones do little work. The depth of the cut required by each single particle should presumably be of the same order as the thickness of the mucin film investigated by IIaeseler and co-workers (3, 9). This will be a very variable quantity, but wlieu. visible the film niust have a mininium thickness of the order of 0.5 micron.
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Summary I n view of the possibility of damage, especially to the softer tissues, by regular brushing, a standard test for grading the fine powders used in dentifrices is important. An test is more satisfactory than a scratch test, although the latter is useful for detecting coarse adulterants and is more selective. Antimony has several advantages as a standard surface. Surface flow does not appreciably interfere with the test, providing proper precautions are taken. Some modifications of a convenient abrasion apparatus are described. Results obtained with this apparatus using sized powders show that particle size distribution is important, since for the same abrasion loss per unit weight of powder larger particles will give fewer but deeper scratches. The measure of experimental agreement found by Wright and Fenske between the abrasion results with a variety of powders against the antimony surface and the tissues of ex-
tracted teeth is held to justify the use of the standard surface for testing dentifrices. The correlation between the abrasion test on extracted teeth and the wear involved in cleaning of teeth in vivo is discussed. The test with extracted teeth exaggerates the wear,
Literature Cited (1) Bowden and Hughes, Nature, 139,152 (1937). (2) Bryan, Drug CosmeticInd.,42,164 (1938). (3) Haeseler and Fain, Dental Cosmos, 77, 878 (1935). (4) Hodge and McKay, Ibid., 75,20,227(1933). Miller, W.D., Ibid., 49,1,109,225 (1907). (5) (6) Ray and Chaden, Ibid., 75,1070 (1933). (7) Smith, M.L., J. Am. Dental Assoc. (in press). (8) Smith, M. L.,J . SOC.Chem. Ind., 54,269T (1935). CHEM., Anal. Ed., 8, 191 (1936). (9) Snell and Haeseler, IND.ENCI. (lo) Souder and Schoonover, J. Am. Dental Assoc. 24,1817(1937). (11) Wright and Fenske, Ibid., 24, 1889 (1937). R~~~~~~~ Ootober 4, 1938.
A Simple Hydrogen Electrode Outfit Wa HEINLEN HALL Bowling Green State University, Bowling Green, Ohio
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HYDROGEN electrode outfit capable of meeting established requirements for the accurate determination of the hydrogen-ion concentration of buffers is a frequent necessity in many types of work. The extensive use of glass electrodes has increased this need, because the absolute accuracy of the glass electrode technique is actually limited to the accuracy of the method used in standardizing the calibrating buffer ( 3 ) . The apparatus shown in the accompanying sketch has been useful in checking buffers according to the criteria suggested by Beans and Hammett (1). It was constructed from the following pieces: A , 180-cc. electrolyzing beaker; B, 60-cc. wide-mouthed bottle; C, ordinary 12.5-cm. (5-inch) test tube. The two electrodes, D and D’, are similar in design to the Wilson type. The saturated calomel half-cell, B, and agar gel salt bridge, E , were patterned after E. Muller’s design described by Kolthoff and Furman (2). The trap, C, should be filled with water to about 1 cm. above the end of the tube leading into the electrode chamber. The electrode chamber is made gas-tight by fitting a short piece of rubber tubing around the salt bridge a t the point where i t leaves the chamber. The position of the holes in the No. 10 rubber stopper is shown in the upper-right corner of the diagram.
Literature Cited (1) Beans, H.T.,and Hammett, L. P., J. Am. Chem. Soc., 47, 1225 (1925). (2) Kolthoff, I. M.,and Furman, N. H . , “Potentiometric Titrations,” 2nd ed., pp. 79,80,New York, John Wiley & Sons, 1931. (3) MacInnes, D. A., and Longsworth, L. G., TTans. Electrochem. Sac., 71,79-91 (1937). RECEIVED November 12, 1938.
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