ON COMMERCIAL PHOSPHORIC ACID. 
267 
work I believe, a similar sketch of apparatus, but using a largo carboy and 
an aspirator. The product of this operation, when dissolved in water, is a 
mixture of meta- and pyropliosphoric acids, and, if sufficient air has not been 
admitted, of phosphorous acid also ; it is therefore necessary to boil it with a 
little nitric acid, as in the pharmacopoeia process. On the large scale, how¬ 
ever, this might easily be made into a pure glacial acid, and if such an article 
were commercially attainable, I am quite of opinion that plain directions for 
its dilution, etc., as given in the U. S. P., would be the best pharmacopoeia 
process, placing the glacial acid as they do in the list of materia medica. 
If not trespassing too much on your time, I will shortly mention one or 
two processes I have tried and the results arrived at with regard to them 
Neustadt (Chem. News, vol. iv. p. 13) publishes a method for obtaining phos¬ 
phoric acid pure on the large scale from bone-ash, converting it first into a 
soda salt, then phosphate of baryta. The quantity of the latter from one 
part of bone-ash he directs to be mixed with one part of sulphuric acid diluted 
with three parts of water and stirred occasionally for two or three days, until 
no sulphuric acid is found in the liquor. This reads very well, but, like some 
other nice things, at least in my hands, does not work as it reads. I com¬ 
menced with phosphate of soda, wffiich is easily obtained pure in commerce, 
and made the phosphate of baryta from it. This I digested with its equiva¬ 
lent of sulphuric acid, diluted as directed; considerable heat was evolved, 
showing that action was taking place, after three days, however, with fre¬ 
quent shaking, sulphuric acid was still present in the liquid. I left it another 
week, shaking occasionally, and still there was sulphuric acid. I then boiled 
it for three or four hours, and still there was sulphuric acid. I then filtered 
off and washed the precipitated sulphate of baryta, etc., and added more phos¬ 
phate of baryta to the filtrate; then I certainly had no sulphuric acid, but, 
what was worse, phosphate of baryta was dissolved in the free phosphoric 
acid. To this I cautiously added dilute sulphuric acid while hot, until no 
further precipitate was produced. I had then certainly pure phosphoric acid, 
but, as it seemed to me, at too much trouble and risk of danger from the pre¬ 
sence of the poisonous baryta compound (or, on the other hand, from excess 
of sulphuric acid) for it to be a good pharmacopoeia process. 
I then tried decomposing phosphate of lead, suspended in water, by means 
of sulphuretted hydrogen. Phosphate of lead is easily prepared from the 
commercial phosphate of soda, and for some time I thought this would prove 
a satisfactory and easy process. Repeated experiments showed, however, 
that small particles of the phosphate were apt to escape decomposition unless 
the recently precipitated salt were used and it was rubbed up perfectly smooth 
with the water, and also an excess of sulphuretted hydrogen ensured for 
some time (which is an objection of itself); and in this case, as in that of the 
baryta process, the acid, unless immediately filtered off, contained varying 
quantities of dissolved lead salt, the mere possibility of which unfits it as a 
process for inexperienced hands. If properly carried out, however, I believe 
this to be a good and easy method. 
With regard to the use of the glacial acid as the source of the dilute phos¬ 
phoric acid, it is stated in the ‘ Lancet ’ (1864, vol. i. p. 361) that such an acid 
will not do for Syr. Ferri Phosph. The objection to it for this purpose 
entirely, as I believe, consists in the presence in it of meta- and pyrophos- 
phoric acids, which, as Maisch has shown (Pharm. Journ. vol. iii. p. 278), 
remain under certain cir<?umstances much longer undecomposed than had 
previously been imagined. Boiling with a small quantity of nitric acid, as 
ordered in the U. S. P., effectually removes them, and the acid is then (if 
made from pure glacial) pure and fit for any of the purposes to which it 
may be applied. 
