GUESTAUTHOR F. 1. Swinton
University of Strathclyde Glasgow, c.I., Scotland
Textbook Errors, 78
The Triple Point of Water
A
survey of eight, recently published. standard physical chemistry t,extbooks on the aut,hor's bookshelves has indicated that there is little agreement. among them as to t,he equilibrium temperature for water a t its triple point. The following values werc quoted: +0.OO9QoC, +0.0098'C, +O,Ol°C OII one page and O°C on another, while the remaining five texts settled for +0.0075T. In one of these latter books (boasting no fewer than five authors!) appears the completely erroneous, indeed nonsensical, statement, "The triple point (of water) is a t +0.007:i°C in the absence of air at. 1 atmosphere." In view of thc great importance that the triple point temperat,urc i ~ f water now has in defining the International Pract,ieal Temperature Scale it is surely essential that niiseonceptions should be avoided, especially when the undergraduate meets this topic for the first time. The scale of temperature which is now in general uso by all physical scientists is the International Praet,ical Temperature Scale of 1948. This scale was not finally approved by the International Committee on Weights and Measures until Oetobw. 1960, and an excellent, account of this scale and its relationship to t,he Thermodynamic Celsius Temperatnre Scale has been give11 by Stimson.' The triple point of water has uow been chosen as the basic fixed point on both Practical and Kelvin or = Thermodynamic Scales, I ~ ' ~ , , ~=, , 273.16' . +O.O1° Celsius. The reason for the choice of tht: triple point temperature rat,her than the freezing point, is that the triple point can he reproduced with high precision (*0.0001"C) using a relatively simple triple point cell, whereas it is difficult tn reproduce the freezing point of water (the temperature of equilibrium between ice and air-saturated water under a prmsurr of one standard atmosphere) to better than +0.001"C. There is then nothing r~hsrure ahout the facts, namely
and
D i c u l t i e s only start to arise when an attempt is made to calculate the difference between the two tempel.atures. The five textbooks quoting a value I I tH'Ov,,,, ~ = +O.O075"C attribute the difference between the freezing point a t one atmosphere pressure and the triple point a t 4.58 mm Hg pressure to t.he effect of pressure alone and use the Clansius-Clapeyron rquation to calculate this difference. Given that the molar volume of ice and of water and are 19.63 cmz and 18.00 em" at O°C re~pectively,~
f AH, is fin02 .I. that t.he ent,halpy of fusion i ~ ire, mole-',
,
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1 lie freezing point of !vatel. at one atmosphere prvssnre should t,herefore be 0.0075"C below the triplr point,, i.e., a t +0.O02.S0C. This indeed would he t h t ~ freezing point of water if air rould he excluded from thr irelwater mixture. Under normal rondit,iorrs t,here is, of course, another effect t,ending to depress the freezing point of water, namely, the presence of disdved air. Given that the solnhilit,ies of nitrogen and of oxygen in water a t 0°C are 1.051 X 1 0 P moles g-' and 2.183 X 10Fmoles g-' respectively, and t,hat air consist,s of approximately 79% nitrogen and 21% oxygen, t,he t.otal molality of dissolved air a t this temperature is 1.289 X l W 3 moles/1000 g. The molal er.vosa)pic constant for of water is 1.8fil deg g molr-' so that t,he depressio~~ the freezing point is
The t,~~ttll difference (I~'~),,,,.I,- I"'~,.~,L.)betweell t l ~ o t.riple point, t,emperature of water and its normal freezing point is thus (0.0075 0.0024) = 0.0099'C E O.Ol°C. In t,he above calcnlation we have ignored thc presence of gases such as CO, and SO?that normally occur in industrial atmospheres and, because of their high solubility, would dqwess the freezing point t , ~an ) even grrater extent. The calculatioi~of the difference between triple point and freezing point is a good illustration of the use of two basic physico-chemical principles, and it is surprising that the majority of d e m e n t a ~ ytextbooks ignore one of t,hese. It is to be hoped that in the future, anthors will show more respwt to their nndergraduate readers and give a little more thought tn t,he presentat,ion of this fundamental topic.
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Suggest,ionsof material suitable for this column and guesl coll m n a suitable for p~~hlication directly should be sent with as many details as possible, and particnlnrly with references to modern textbooks to W. H. Eberhardt, Soh001 of Chemistry, Georgia Institute of Technology, Atlanta, Ga. 30332. STIMSON, H. F.,J . Res. .Vat. RILT. S l a d a ~ d 8 , 6 5 A ,139 (1961). a All the numerical data ,wed in l.he present calculations are liken fvom "Handbook of Chemist,yy and Physias," Chemical Ruhhrr Publishing Co., 4fith Edition (1965).
Volume 44,
Number
9, September 1967
/
541
