Seven hundred and twenty grams (3.8–4 moles) of sirupy arsenic acid (75–80 per cent) is boiled in a beaker until the temperature of the acid is 150°; about 120 g. of water is driven off, leaving a sirup containing approximately 95 per cent of orthoarsenic acid, which is then added to 300 g. (3.2 moles) of phenol in a 1-l. round-bottomed, three-necked Pyrex flask provided with an efficient stirrer, a thermometer, and a connection with a downward condenser. The flask is set in an oil bath which is heated at once to 155–160°, and the stirrer is run at a rate high enough to insure thorough mixing.

When the inside temperature reaches 140°, boiling commences and water with a very little phenol begins to distil. The distillation is allowed to continue until 60 cc. (one molecular equivalent) of water has been collected; this usually requires one hour, and the inside temperature rises to 146°. The downward condenser is then replaced by a reflux condenser (Note 1) and the reaction is allowed to continue until a total of four hours has elapsed from the time the contents of the flask first reached 140°. After the return condenser has been attached, the inside temperature declines slowly to 141° or 142° and the reaction mixture becomes thicker and somewhat tarry. After the contents of the flask have been partially cooled, they are poured into 4 l. of water and mechanically stirred; the agitation is continued for a short time in order to break up the tarry material and enable the water to dissolve the hydroxyphenylarsonic acids completely.

Finely ground barium hydroxide is added gradually to the well-stirred water solution until the material is slightly alkaline to litmus, in order to remove the excess of arsenic acid; when this point is reached, the solution becomes pink. If the procedure is carried out properly, 700–800 g. of Ba(OH)₂·8H₂O should suffice (Note 2). The time required by this method is greater than when a hot solution of barium hydroxide is used, but the method is more convenient and does not cause such a great increase in volume. After removal of the barium arsenate by filtration, the mother liquor and washings are treated with sulfuric acid until the solution contains neither barium nor sulfate ions. When the barium sulfate has been separated and thoroughly washed (Note 3), the filtrate is concentrated on a steam bath to about 3 l., neutralized to litmus with sodium hydroxide, filtered, evaporated until the solution becomes well coated with crystals, and then treated with 2.5 volumes of alcohol. After the mixture has cooled in an ice box, the sodium p-hydroxyphenylarsonate is separated, washed with alcohol, and dried in an oven at 80°. A second crop may be secured from the filtrate by concentrating it further and precipitating with alcohol. The total yield of anhydrous sodium p-hydroxyphenylarsonate is 252 g. (33 per cent of the theoretical amount). By proper manipulation, it is possible to obtain as much as 240 g. in the first crop and to have it free from sulfate, arsenate, and sodium o-hydroxyphenylarsonate which is one of the by-products formed in this reaction (Note 4).