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).
