[Note 1:  The end-point should be chosen exactly at the point of change; any darker tint is unsatisfactory, since it is impossible to carry shades of color in the memory and to duplicate them from day to day.]

[Note 2:  While variation of two parts in one thousand in the values obtained by an inexperienced analyst is not excessive, the idea must be carefully avoided that this is a standard for accurate work to be !generally applied!.  In many cases, after experience is gained, the allowable error is less than this proportion.  In a few cases a larger variation is permissible, but these are rare and can only be recognized by an experienced analyst.  It is essential that the beginner should acquire at least the degree of accuracy indicated if he is to become a successful analyst.]

STANDARDIZATION OF HYDROCHLORIC ACID

SELECTION AND PREPARATION OF STANDARD

The selection of the best substance to be used as a standard for acid solutions has been the subject of much controversy.  The work of Lunge (!Ztschr. angew.  Chem.! (1904), 8, 231), Ferguson (!J.  Soc.  Chem.  Ind.! (1905), 24, 784), and others, seems to indicate that the best standard is sodium carbonate prepared from sodium bicarbonate by heating the latter at temperature between 270 deg. and 300 deg.C.  The bicarbonate is easily prepared in a pure state, and at the temperatures named the decomposition takes place according to the equation

2HNaCO_{3} —­> Na_{2}Co_{3} + H_{2}O + Co_{2}

and without loss of any carbon dioxide from the sodium carbonate, such as may occur at higher temperatures.  The process is carried out as described below.

Procedure.—­Place in a porcelain crucible about 6 grams (roughly weighed) of the purest sodium bicarbonate obtainable.  Rest the crucible upon a triangle of iron or copper wire so placed within a large crucible that there is an open air space of about three eighths of an inch between them.  The larger crucible may be of iron, nickel or porcelain, as may be most convenient.  Insert the bulb of a thermometer reading to 350 deg.C. in the bicarbonate, supporting it with a clamp so that the bulb does not rest on the bottom of the crucible.  Heat the outside crucible, using a rather small flame, and raise the temperature of the bicarbonate fairly rapidly to 270 deg.C.  Then regulate the heat in such a way that the temperature rises !slowly! to 300 deg.C. in the course of a half-hour.  The bicarbonate should be frequently stirred with a clean, dry, glass rod, and after stirring, should be heaped up around the bulb of the thermometer in such a way as to cover it.  This will require attention during most of the heating, as the temperature should not be permitted to rise above 310 deg.C. for any length of time.  At the end of the half-hour remove the thermometer and transfer the porcelain crucible, which now contains sodium carbonate, to a desiccator.  When it is cold, transfer the carbonate to a stoppered weighing tube or weighing-bottle.