silver sulphate and pumice must be placed in tube E. Reconnect the apparatus and open all cocks except the stop-cock in the dropping funnel, leaving the clamp k set for the proper rate of gas flow, as previously determined. Slowly open the cock of the dropping funnel, allowing acid to drop just fast enough to evolve carbon dioxide at the prescribed rate. The constant attention of the operator is necessary at this point, for by causing too rapid evolution of gas some moisture may escape absorption in the small tube of the absorption bulbs and the experiment be rendered worthless.

The acid should be allowed to run in until about 1 cc is left above the stopcock, this acting as a seal during the subsequent boiling. After the decomposition of the carbonate is complete the solution in the flask is slowly heated until it boils, always with due regard to the rate at which the gas is made to flow through the absorption bulbs. The boiling is continued for one minute, when the flame is withdrawn, the cock of the dropping funnel being opened at the same time to allow air to enter so that no back suction occurs, due to the cooling effect. Air is now drawn through the apparatus until 1000 cc of water has flown from the aspirator. This amount of air should be sufficient to sweep all of the carbon dioxide into the absorption bulbs.

The clamp f is now closed, and the absorption bulbs are removed, plugged and placed in the balance case. After 15 minutes they are weighed, the increase in weight being the weight of carbon dioxide. From this and the weight of sample the percent of carbonic anhydride (combined carbon dioxide) is calculated.

For the duplicate or any subsequent determination the generating flask and the dropping funnel are washed absolutely free from acid, so that no decomposition of the next carbonate sample may occur before the bulbs are in place. The first U-tube should also be emptied and recharged with absorbent, if such is to be used for the next determination. If a large number of determinations is to be made with the same apparatus much time will be saved by providing two decomposition flasks and two absorption bulbs. While one determination is being made another sample may be weighed into the duplicate flask and the second absorption bulb may be weighed. The next determination may then be started while the first bulbs are standing in the balance case, preliminary to the final weighing. It is also necessary to determine when the various absorbents have become so saturated as to be inefficient for further work. Soda lime in the tube C is good until the lumps have fallen into a powder. Silver sulphate in the pumice of tube E may become inefficient through absorption of hydrochloric acid or through the accumulation of water in the tube. The solubility of silver sulphate in water is much less than in concentrated sulphuric acid. If the acid solution becomes diluted the silver salt crystallizes and will not there

after readily absorb hydrochloric acid. As the silver sulphate becomes saturated with hydrochloric acid it darkens, on account of the action of light. When the darkening effect has proceeded as far as the middle of the tube the material should be replaced. Calcium chloride must be replaced when it becomes visibly moist for the first third of any absorbing tube.

A method has already been given for the determination of the amount of carbon dioxide which can be absorbed by the solution in the bulbs.

CHAPTER IV

ELECTRO-ANALYSIS

We have here to deal with a class of work that, while also gravimetric in most cases, is sufficiently different from what has already been considered to be treated as a separate division. In all of the preceding exercises the element or radical to be determined was precipitated from a solution by chemical reactions produced by other substances which were added for the purpose. In the cases now to be considered the precipitation will be brought about by electrical action, the passage of a current through the solution causing the deposition of a metal upon a cathode in such a form that it can be weighed, or the accomplishment of some change which makes possible the determination of a substance not a metal. The electrolysis of silver sulphate will serve as an example. When a solution of this salt is electrolyzed at platinum electrodes the metal is plated on the cathode and sulphuric acid is produced at the anode, thus:

The silver can then be weighed and the sulphuric acid determined volumetrically.

While the electrolysis of a simple salt is frequently a tolerably simple and well understood process, the practical accomplishment of such a process for the purpose of a quantitative analysis is usually possible only when a certain set of conditions is maintained. The principal reasons for failure to attain accuracy are three: (1) Deposition may not occur upon passage of a current. (2) The deposit may be contaminated by other products of electrolysis. (3) The deposit may not have the proper physical character, so that it will not adhere to the electrode but crumbles off during the electrolysis or during the process of washing. We have thus to consider the nature of salt to be used, solvents, temperature, electrolytic pressure (voltage), current density and nature and kind of electrode.

Nature of Electrolyte. Electrolytic methods are more frequently applied to the determination of metals than of nonmetals, although methods have lately been perfected for the determination of the latter. If the metal alone is to be determined it will usually be possible to obtain it in the form of whatever salt gives the best results. Certain anions must be excluded in specific cases, either because they yield substances that attack the anode or because the acids that are produced by their electrical discharge cause the metal to deposit in an undesirable physical form. As an example of corrosive action upon the anode it is sufficient to here mention the formation of nascent chlorine at a platinum anode when a chloride is electrolyzed. With regard to the effect of the acid that accumulates in the solution as electrolysis proceeds it may be stated that there is little known, at present, of the reasons for the effect of acids, bases and other substances that may be in the solution, upon the nature of the deposit. Experiment shows, however, that such substances often exert a very important influence upon the physical character of a deposited metal and they are often added for this reason, although they may be objectionable for other reasons. A solution of copper sulphate, if electrolyzed without the addition of another substance, usually gives a dark red or brown deposit of finely divided copper which is liable to powder and be lost during washing. If a small amount of sulphuric acid is first added the deposit is improved, while nitric acid causes a still better deposit of bright red, firm and adherent metal. For this reason nitric acid is usually added although it gives rise to more or less danger of resolution when the cathode copper is being washed. On the other hand, a silver salt is best electrolyzed in the absence of nitric acid. If silver nitrate is electrolyzed from water solution with or without the addition of nitric acid (the latter is formed by the electrolysis) the plate of silver on the cathode is so decidedly crystalline that it is very easily detached. The addition of potassium cyanide in quantity sufficient to redissolve the precipitate of silver cyanide first formed gives a solution from which silver will deposit as a white firm plate. The solution in potassium cyanide has a comparatively high electrical resistance so that more energy is consumed in the accomplishment of its decomposition, nevertheless potassium cyanide is generally added.

Other examples of similar effects will appear in the exercises. It is desirable to note that little is known of the cause of such effects, also to guard against a very common misconception regarding the purpose of adding other electrolytes to solutions. that are to be electrolyzed. It is frequently stated that such substances are added in order to increase the conductivity of the solution. If such substances could increase the ionization of the salt that is to be electrolyzed, or in any manner diminish the frictional resistance to the passage of the ions, such an effect would be desirable. It is evident, however, that the addition of a foreign electrolyte can usually increase the conductivity only by itself acting as a carrier of current, in which case it has accomplished no desirable effect since the prime object is not to use a large current but to make the minimum current do the maximum work in discharging an ion already in solution.

Solvent. Very little work has been done in any solvents other than water. The use of organic solvents may, in some cases, prove advantageous in producing good deposits where other conditions fail to do so.

Temperature. Conductivity of solutions usually increases with rise in temperature. This is not due to increased ionization (ionization usually decreases with rise in temperature) but to reduced viscosity and consequent reduction in frictional resistance to ionic migration. If the complete electro-decomposition of a substance requires considerable time it is not convenient to heat to any definite elevated temperature. In most cases, therefore, the temperature of the solution is not raised above that of the laboratory except at the beginning.

Electrolytic Pressure. For every electrolyte in solution there is a definite minimum voltage, below which no decomposition will take place. If but one electrolyte is present and the voltage lies below this minimum, a continuous current cannot flow. The minimum voltage necessary to produce a continuous flow of current is called the "decomposition voltage" for the substance in question. If salts of more than one metal are present in solution, the deposit on the cathode will consist of any metals, the decomposition voltage of whose salts has been exceeded. If there is sufficient difference in the values of decomposition voltage for the different salts, separation may be made. It is only necessary to so adjust the voltage that it shall exceed the