. : VAPOR PRESSURE OF AMMONIA By Carl S. Cragoe, Cyril H. Meyers, and Cyril S. Taylor CONTENTS Page I. Introduction i II. Previous measurements 3 III. General description of apparatus and method 10 IV. Purification of samples and description of manometer fillings 13 V. Description of preliminary experiments 14 1 Hysteresis in an impure sample 14 2 Lag in coming to equilibrium 16 VI. Measurements by the static method 19 VII. Determination of the normal boiling point by the dynamic method 26 VIII. Form of empirical equations 27 IX. Discussion of results 28 X. Summary 31 Appendixes 1. Vapor pressure of ammonia (degrees centigrade, mm of mercury and atmospheres) ^^ 2. Vapor pressure of ammonia (degrees Fahrenheit, pounds per square inch and atmospheres) 34 3. Rate of change of vapor pressure with temperature ( -^j 35 I. INTRODUCTION The measurements presented in this paper form a portion of the work undertaken by the Bureau of Standards in the determination of the thermal properties of materials used as refrigerating media. The existing data on the vapor-pressure-temperature relation for ammonia are undoubtedly sufficiently accurate to meet the requirements of refrigeration engineering. The Clapeyron equa- tion, however, offers a means of correlating the measurements of the latent heat of vaporization (already published),* with the data on specific volumes of saturated liquid and vapor (to be published shortly), provided the slope of the saturation line can be determined with sufficient acciu^acy. On account of the large errors which may be introduced into the calculated values of the * Osborne and Van Dusen, B. S. Bulletin, 14, p. 439; 1917 (Scientific Paper No. 315). I 2 Scientific Papers of the Bureau of Standards Voi. i6 slope by relatively small errors in the pressiire or temperature, it appeared that existing data were deficient either in the range or the precision required. The accuracy of any vapor-presstire measurements is determined, in general, by foiur factors; namely, (a) purity of the material^ (6) certainty of equilibrium conditions, (c) precision of the pressure-measiuring instrument, and (d) temperature measurement and control. (a) The extent to which factors (a) and (h) may affect the results of the vapor-pressure measurements will depend upon the methods used. Nonvolatile impurities present in solution would affect measurements by the static method, and also by the dynamic method, if measurements were made of the temperature of the boiling liquid, while their effect on the temperature of the con- densing vapor is relatively unimportant. Noncondensing gases have but little effect on measiurements by the dynamic method, while in the static method a small amount of noncondensing gas may affect the measured pressin-e to an extent out of all proportion to the amount of gas present. It is worthy of note that the non- condensing gas does not notably affect the vapor pressure, but causes the total pressure, as measured by the static method, to differ from the true vapor pressure. (6) In measurements by the static method a very considerable lag in the attainment of equilibrium between the vapor and its liquid may be encountered even with a liquid well freed from impurities, especially if the liquid is not agitated. An example of this is furnished later. The presence of a small amount of permanent gas, such as air, greatly increases the lag in coming to pressure equilibrium. This was found to be the case at low tem- peratures, as illustrated in an attempt to measure the boiling point of a commercial sample of ammonia by the static method. (c) The sensitivity of the pressure-measuring instruments used in the present work was such as to permit readings of pressure to I part in 5000, or better, except for pressures below i atmosphere. Pressures below 5 atmospheres were measured with mercury manom- eters; pressures between 5 and 15 atmospheres with a mercury manometer and with a piston gage; pressures above 15 atmos- pheres with a piston gage only. {d) Temperature control plays an important role in any vapor- pressure measurement, particularly in the establishing of equi- librium. A change in the temperature of 0.1° C in the case of 5 9457 93 916981 541 Cragoe, Meyers,! Vapor Pressure Ammonia Taylor J of ammonia is equivalent to a change in the vapor pressm-e of about 2 mm, 12 mm, and 40 mm of mercury at — 50^,0°, and +5o°C, respectively, or a percentage change in pressure of about 0.7, 0.4, and 0.25, respectively. The aim in the present experiments was to maintain temperatures constant to 0.01° C, or better, for very long time intervals. Platinum resistance thermometers were em- ployed for the temperature measurements and temperatures were read to thousandths of a degree. II. PREVIOUS MEASUREMENTS The measurements of various observers are given in Table i, which also includes for comparison, in the columns designated p calc. the final results obtained in the present investigation. Vari- ous determinations of the normal boiling point of ammonia are given in Table 2. TABLE 1.—Previous Measurements of Vapor Pressure of Ammonia Compared with Present Results p calc. p calc. p calc. Temp. p obs. present Temp. /.obs. present Temp. p obs. present work work work "C mm mm °C mm mm °C mm mm Ilunsen (183 5) Elegnault (18 62) Regnault (1862) -33.7 749.3 746.6 First series Second series - 5.0 3040.0 2661. 5 3207. 7 3221, + 6.93 4140. 4143.0 3610. 3221. 3212. 3221. + 7.32 4199. 7 4200. 5 + 5.0 4260. 3868. 3207. 3221. + 7.34 4198. 4204. +10.0 4980. 4612.0 3214. 7 3221.0 + 8.45 4368. 4370. 5 +15.0 5780. 5462.5 3198. 3221. +11.42 4835. 4842.5 +20.0 6670. 6428.5 -18. 03 1524. 9 1555. 9 +13.52 5335. 3 5199. Faraday (184! -16. 79 1614. 9 1641.8 ) +18. 15 6134. 6057.0 -17. 78 1889. 8 1572. 9 -16. 58 1630. 5 1656. 7 + 19.29 6399. 6284. -12.61 2286. 1959. 6 -13. 09 1900. 1920. 8 +19. 29 6382. 7 6284. - 7.78 2667. 2385.3 -12. 15 1980. 1997. +25. 35 7676. 3 7602. - 6.11 2834.6 2548.3 3.97 3615. 7 3727. + +32. 70 9569. 9473.0 - 3.33 3078. 5 2839. + 4.72 3812.1 3829.5 R«jgnault (1862) 3383.3 3221. R jgnault (1862) 1rhird series + 0.56 3429.0 3288. 5 Second serie s Op«m manometer + 5.00 3886. 2 3868. -30.96, 838.4 855.6 + 6.67 4084.3 4105. 5 -31. 37 823.6 838.5 -25. 70 1117.7 1100. 7 + 7.22 4152. 9 4186. -31.48 815.1 834.0 -25. 84 1097.2 1093. + 9.44 4442.5 4523. 5 -27. 36 1006. 7 1017. -23.92 1187. 3 1195. +10. 78 4572. 4737. 5 -27.47 1002.9 1012. 6 -18. 31 1518. 7 1537. +11.11 4648.2 4791. -22. 71 1244.5 1263.3 -18. 10 1529.9 1551. + 12.78 4861. 6 5071. -22.60 1251.7 1269.6 -16. 17 1670. 1686. + 13.61 4953. 5215. -18. 37 1518. 2 1533. -13.51 1867.9 1887. 3 + 15.56 5257.8 5564.5 -18.35 1513. 5 1534.3 -13. 55 1859. 1884. 2 +16.28 5334.0 5698. 5 -10.42 2119. 4 2144.4 - 9.21 2222.4 2252. 3 + 18.67 5715. 6159. 5 -10.52 2133. 8 2135. 7 - 5.03 2599.6 2658.4 +19.44 5814.1 6314.5 3203. 7 3221. - 0.10 3159. 3208. 8 +28.33 7620. 8323.0 3206.7 3221. + 6.24 3963. 7 4043.0 1 1 Scientific Papers of the Bureau of Standards Vol. i6 TABLE 1—Continued p calc. p calc. />calc. Temp. pohs. present Temp. pohs. present Temp. pobs. present work work work »C mm mm °C mm mm »C mm mm Regnault (1862) Brill (1906) Burrell and Robertson (1915) Third series -54.4 239.5 234.9 -39.3 600.0 558.8 Closed manometer -50.7 309.3 294.1 -37.7 650.0 608.0 + 9.98 4550. 4 4509. -46.2 403.5 382.2 -35.3 700.0 653.9 +14. 38 5302. 4 5351. -45.0 437.1 409.1 -35.4 730.0 684.9 +19. 70 6318. 6367. -41.5 521.9 496.5 -34.6 760.0 713.4 +30. 49 8802. 9 8877. -39.8 568.2 544.1 Keyes and Browalee (1916) +38.90 11 236. 11305.0 -38.2 610.4 592.3 -33. 22 760.8 764.8 +48. 93 14 670. 14 826. -33.0 761.0 773.3 3255. 5 3221. +55. 47 17 333.9 17 529. Davies(1906, + 20 6480. 4 6428. 5 +54. 35 21 619.7 21 770. -49.8 298.0 310.2 + 25 7574. 7520. 5 +73.32 26 766. 4 26 793. -41.0 531.0 510.1 +30 8808. 8749. +81. 72 32171.0 32 238. -30.0 867.0 896.7 +35 10 196.0 10 124. B umcke(1888> -20.0 1392. 9 1426. 8 + 40 11722.0 11658.0 -18.5 1451.6 1524. 3 -15.0 1726. 2 1772. 4 +45 13 429. 13 361.0 3207. 2 3221. -10.0 2145. 9 2181.4 +50 15 292. IS 245. +34.0 9728. 9837. - 5.0 2616. 9 2661. 5 +55 17 393. 17 323. +63.5 21 310. 4 21 334. Hoist (1915) + 60 19 641.0 19 606. Brill (1906) -43. 15 449.7 453.6 + 55 22 187.