letters Trading Safety for Economy?
To the Editor:
An entire history and sociology of chemistry might be taught from nitration procedures [YBeautyor the Beast", J. Chem. Educ. 1990,67,8011; I fear, however, that students would be left with little respect for the learning or the intellectual powers of academic chemists. "Fair is foul, and foul is fair", but teach not what no business dare!
At some time, students must learn that skill and application will win shining crystals from many a black tar. But, even on a small scale, beginning students should not be taught profoundly unsafe procedures if these can easily be modified to a safe procedure. There may come a time when they consult their notes to scale-up. The Fine Chemical Industry practices nitration with acetic anhydridelnitric acid mixtures on ton scale, but most certainly not according to your "fair" procedure. Were they to do so, they would frequently exchange plants for holes in the ground. The premixed anhydridelnitric acid anhydride recommended is [Dingle, L. E. Chem. in Brit. 1968,4, 1361 only one drop outside the range believed to be detonable on standing (according to t h e "Explosives" chapter in Ullmann's Encyclopaedia of Industrial Chemistry i t i s within this). It is certainly well within the range of compositions capable of explosion by autocatalytic oxidation of acetate by nitrate. In industry, we incline to limit the concentration of nitric acid (or equivalent) to a maximum 10%; this orocedure has 37%. The above reference is easilv traced through Hrethwick's Hnlnndhouk ofReacriue ~ h e r n i cu/ Huzurds. This. iurelv. ". should be consulted bv all teachers devising experiments, and, dare I suggest, by reviewers and vour editorial team before vou print them. In thts case, the safe procedure would be one where the nitrlcacld 1s added to the orzanlc components (phenol does not acetylate that fast in acetic anhydride, a i d it would deacetylate on addition of wet nitric). Doing this, the student could observe stepwise nitration, first to a mixture of mono-nitro compound, then to almost pure di-nitro, and, possibly, if a drop of sulfuric acid be used as catalyst (which improves both the performance and safety of this type of nitration, which otherwise sometimes shows induction periods), finally picric acid might result. If the method much as it stands is insisted upon, the acetic anhydride should he diluted with a t least a n equal volume of acetic acid. But we may ask why the nitration mixture is premixed, desoite the oractice of the earlier (and wiser?) reference cited. The answer, I suggest, is that nitrations were performed by adding substrate to nitration mix, usually in excess, in the early 19th century. In academe they still are, for following great-grandfather's footsteps is there a popular pursuit, whereas thought, especially about such trivial and life-preserving matters as experimental detail, is no more popular in colleges than elsewhere. Ingold (floreat @ 1930)elucidated the mechanism of nitration but the subsequent 60 years of academic literature affords little evidence that his insights have been allowed to influence exoerimental orocedures. In fact, many of the defects and dangers of nitration are the consequence of adding substrate tonitrate, rather than 344
Journal of Chemical Education
conversely. This encourages over-nitration and oxidation of substrate (which is the more dangerous reaction) by the excess nitrate present through most ofthe procedure^ It is alsocommon to see reported mishaps, which may beattnbuted to failure to consider the acid catalvsis so lone established or to neglect of a completely balanced equation (includine those reaeents commonlv dismissed a s mere cataly&. I t is not llways the safest procedure that is the most economic. or oroduces theleast effluent.hut these are of less import in university than industry. ltksually is the safest procedure that produces the highest conversions and purity. The traditional method is only advisable when the stability of the substrate demands it. P. G. Urben 2 Upper Rosemary Hill Kenilworlh CV8 2PA. England
Solving Equilibrium Problems To the Editor:
In your recent feature "A Problem for Buffer Buffs"by J. R. Leis et al. [J. Chem. Educ. 1991, 68, 1411, the authors imply that the solution to the equation [H~o+P+ (Cb+ K ~ ) [ H ~ O- (K, + ] ~+ KaCJH3Ot1 -If&',
=0
requires the use of a computer or laborious calculation. This is unjustified pessimism. Letting x = [H3Of1and plugging in the values of the constants (C. = Ca = 0.01 M, K. = 1x 10-12;K, = 1x 10-14),yields the equation
2 + 0 . 0 ~ -2 2
1 0 - l ~-~1
=o
Since the authors alreadv established that the value ofx is in the neighborhood of 1k 10 12, this value can be plugged into each term of the eauation with the followinrr result: Examination of the terms indicates that the first is negligible with respect to the others. Dropping the first term simplifies the equation to a quadratic: 0 . 0 2 - 2 x 10-14%- 1 x
=o
which, incidentally, is the same equation you would get using
K, = [OHI[HAI/[A7 plugging in K,JH3Ot1 for [OH-] and KJK* for Kb, and doing a bit of algebra. When a rough solution is plugged into each term of a higher order equation, it is not uncommon for one or two terms to "drop out". Were the last term to pmve negligible, this cubic equation could again be simplified to a quadratic. Were the second or third term to drop out, the calculation would still be easier, if repetitive. An equation with more than three terms is no cause for despair. Ben Ruekberg University of Rhode Island Kingston, RI 02881
