LETTERS Buffer Capacity To the Editor: A recent article' discusses the buffer capacity of a weak monobasic acid being titrated with a strong base. Though the conclusion reached as to the region of greatest buffer effect is correct, several of the intermediate steps are apt to confuse the student. In the first place, it should be pointed out that the letter C as used in the article mentioned does not denote the coricentration of acid a t any time but, rather, "milliequivalents of acid originally present," and, similarly, (A-) denotes "milliequivalents of base added." The equation, PH = log
1
+ log (A-)
slope of the titration curve a t any point. this by the relation:
We can do
Since (A-) in the example given = V/5,
At the point of maximum buffer capacity, when half the acid has been titrated, the slope of dpH/d(A-) = 0.868 and the slope of the titration curve dpH/dV = 0.174.
- log [C - (Ad)]
which is derived from the law of mass action, depends upon concentrations, but it holds also when the terms are defined as above, since the varying amount of water present dilutes (A-) and C alike. On differentiating we find
There's Money in It
To the Editor: There is one branch of metallurgy which has not y e t received adequate attention, a t least as far as my reading extends-namely, alloys for coins. Today, as since the beginning of history, gold and silver are alloyed with copper. Bronze coins also come from antiquity and nickel is the only modern addition Inclusion of the factor (logme) does not change the except that for a short time platinum coins were in minimum as found in the cited article, but i t should be circulation in Russia. taken into account in computing the slope of the With the great recent increase both in the knowlfunction. edge of alloys and their properties and fabrication, Finally, it will. be instructive to obtain dpH/d V there should be new alloys for coins and I suggest the where V is the volume of base added. so as to have the following specifications : Coins should be of distinctive colors as a t present, PARK."The region of maximum buffer capacity," J. CBM. yellow for gold, white for silver, brown for bronze, and EDUC.,19,171 (1942).
possibly some shade of blue or green for token coins. They should be hard enough to stand hand-to-hand wear for thirty years and should not be attacked by perspiration or H a or HzC03. Available for alloys are V, W, Mg, Be, Zu. Cd, Ce, Cr, Mo, Mn. Perhaps some coins could be made to glow in ultraviolet light. F. W. SMITH VIA NOGALES ARIWNA
GUASAVE SINALOA, MEXICO English versus Metric System
To the Editor: An inexplicable puritanism seems to characterize most writers of textbooks in general chemistry when a question is raised concerning the use of the English system of weights and measures. "The metric system," they say, "must be emphasized to the exclusion of any system which was based originally upon the size of someone's foot, or the length of his a m , or the height of his hone." "Scientific measurements must be kept undefiled by using only a scientific system"-ven if it results in such curiously logical figures as 273 and 22.4. Believing possibly that the metric system is the only "scienti6c system," the academician exhibits a certain missionary zeal in spreading this gospel to the innocent undergraduates in the hope that hie students will never know anything else or be required to use any other units of weight, length, and volume. Our colleges are supposed to train students in the fundamentals of chemistry, so that they may eventually take their places in the industry. What a shock it is to many of them to learn tbat liters and kilograms give way to gallons, cubic feet, and pounds. Especially is this true in the plant; less obvious in the research laboratory. Many new graduates are still more amazed to learn that simple calculations of volumes and weights do not require the use of conversion tables from the metric to the English system. The figure 22.4 is no more mysterious than is the number 359. The same concepts which govern the fundamental volume and weight relationships, as expressed in liters and grams, can be worked out for cubic feet and pounds. Why then do we not teach in our courses in general chemistry the relationship between the pound mol and the pound molecular volume (in cubic feet)? Why not have students work out the absolute zero on the Fahrenheit scale and calculate volume changes of gases with variations in degrees Fahrenheit? Possibly we are wrong in accusing our teachers of general chemistry of trying to jam the metric system down the throats of their students because they feel that it is the only system which should be used. We suspect instead that the average chemistry teacher is entirely unaware that such simple relationships exist within the English system. Price lists of chemicals as quoted in trade and professioual journals are practically always specified in so much per pound or gallon. Capacities of industrial
equipment, of reactors, of autoclaves, and of most large scale chemical apparatus are given in cubic feet or gallons. Steam pressure tables give pressures in pounds per square inch or inches of mercury. Why, in the face of this overwhelming use of the English system, do we disregard and neglect i t so utterly in our general courses in chemistry? It would, of course, be most desirable if a uniform system were adopted and maintained. There is little question as to the superiority of the metric system over the English system. But as long as the research worker and the academician use the metric system while the chemical en~ineer. the ~ l a n tchemist, the chemical salesman, and many engineering specialists use the English system, it would seem desirable to give aU chemists training in both. There are a few very simple instances where the English system may properly be introduced in a course in general chemistry to supplement the metric system. Students become quite familiar with the fact that one gram mol of a gas a t standard temperature and pressure occupies a volume of approximately 22.4 liters. They should also know that one pound mol (the molecular weight in pounds) of a gas occupies a volume of 359 cubic feet a t 32'F. and a pressure of one atmosphere. There is nothing difficult or unusual about this statement. In teaching students that a balanced chemical equation represents actual as well as relative weights why not make use of the pound mol and the pound molecular volume? This relationship involving units of the English system makes it possible to calculate the weight in pounds of a given volume of a gas expressed in cubic feet, the volume in cubic feet of a given number of pounds of a gas, and the volumes in cubic feet of gases, either as reactants or products, involved in any chemical reaction which can be expressed by a chemical equation and in which weights are given in pounds. It should be clearly pointed out that 359 cubic feet is not identical with 22.4 liters. If the volume of a gas increases '/am for a rise in temperature of one degree Centigrade, i t is perfectly obvious that the volume should change regularly with variations in degrees Fahrenheit. Since one degree Fahrenheit is a unit equal to 6/s of a degree Centigrade, one would expect the volume change to be only 6/s as great for a rise in temperature of one degree Fahrenheit (tbat is, a change of l / m of the original volume per degree Fahrenheit). In effect the absolute zero (-273'C. or O°K.) is 492O Fahrenheit below 32OF., or approximately -460°F. This relationship is expressed by the absolute Fahrenheit scale or the Raukine scale. OR. = O F . 460 (approximately). Volumes of gases can theiefore be calculated just as easily whether temperatures are expressed in degrees Centigrade or degrees Fahrenheit. In the latter case conversion into degrees Raukine is all that is necessary. L. F. AUDRIETH H. F. JOHNSTONE
-
+
UNIv, URBANA. ILLINOIS
