should be performed while the solution is in the flask, and before it is filtered or transmitted to the basin.

'I will here mention for the guidance of those who may not be fully aware of the reactions of the oxides of iron, that the existence of an appreciable quantity of peroxide in the ironstone may be readily discovered by dissolving (as directed in the process) 39 or 40 grs. of the ore in hydrochloric acid, diluting with about 8 oz. of water, filtering and testing a portion of the solution with sulphocyanide of potassium. If a decided dark blood-red colour is produced, the quantity of peroxide in the stone must be determined; but if the colour is only light red or rose-pink, the proportion is exceedingly small, and for practical purposes not worth estimating. Of course, when the specimen of ironstone has an ochrey or a reddish appearance on the surface or in the fracture, the presence of a large proportion of peroxide is indicated, and its exact quantity must be determined.

In conclusion, I must not omit to notice one or two circumstances which appear at first to militate against the accuracy of this process. It may be questioned whether solutions of the protosalts of iron do not absorb oxygen so rapidly from the air as to influence the results obtained by this method. Marguerite has shown (see ante), and my own observations completely confirm his statement, that protosalts of iron, in an acid solution, become peroxidised very slowly; and, in a particular experiment, I found that contact with the air during several hours caused no diminution in the quantity of bichromate of potash required. As the process may be completed in a few minutes, it is certain that no inaccuracy can arise from this cause.

'It is also important to inquire whether the chromic acid in the chromates of potash may not be partially deoxidised by hydrochloric acid alone without the presence of a protosalt of iron. Such a reaction would obviously give rise to a serious error. It is well known that concentrated hydrochloric acid rapidly decomposes the chromic acid of the chromates when aided by the application of heat. But I have satisfied myself, by numerous experiments, that this acid exerts very little appreciable action upon dilute solutions

of the chromates of potash, either cold or warm, and that the action is only partial even after continued ebullition; so that the present method is free from inaccuracy on this account.'

M. Mittenzwey's Process.-M. Moritz Mittenzwey has described a very good process for estimating iron by means of tannic acid. The estimation can be conveniently made in the simple apparatus here figured and described.

FIG. 76.

The air in a bottle, A, fig. 76, capable of holding about a litre and a half, communicates with the atmosphere by the bent tubes, B and C, the latter being drawn out at the end D to the diameter of about one or one and a half millimètres. The two glass tubes are united by means of a moderately long piece of india-rubber tubing, E, provided with a pinchcock, F, to close it; and the lower glass tube is fixed in the neck of the bottle by a bored cork, or, better, a caoutchouc stopper.

B

As soon

In executing the analysis it is absolutely necessary that the air in the bottle should be perfectly renewed, and the temperature of all reaching the fluid be the same as that of the laboratory. as the absorbing liquid (which should amount to 150 or 250 c.c.) is prepared, the bottle should be perfectly closed, and then the pinchcock opened just for a moment, so that the pressure of the internal and external air may

FIG. 77.

B

be equalised. The absorption of the oxygen is then hastened by strongly shaking the bottle, which must be wrapped in a cloth to avoid raising the temperature by the warmth of the hand. After each shaking, water must be

allowed to flow into the bottle, A, from a weighed quantity in a beaker, B, fig. 77, so that the fluid in the two vessels may attain the same level, as shown in the drawing. The experiment is ended when, after repeated shakings, no more water runs from B to A, and the difference in the weight of the water in the beaker in grammes gives the amount of oxygen absorbed in cubic centimètres, which can be corrected for the standard temperature and pressure.

In order to apply this to the estimation of iron compounds these must be reduced to the state of protoxide by means of zinc, and the excess of acid neutralised with caustic potash or soda. (Ammonia and the carbonated alkalis must be avoided.) The solution is then poured into the absorptionflask, and pieces of potash wrapped in paper are then dropped in. The absorption is complete in a very short time. For accuracy this process is second to none, and may be recommended in preference to that of Marguerite and Fuchs, since it requires fewer precautions. 50 c.c. of a solution of protoxide which contained 1.395 Fe absorbed in three experiments 148.0 c.c., 148·44 c.c., and 148.4 c.c. of oxygen at 19° C.; the mean = 148.28 c.c., which at this temperature weigh 0.1987 gramme, answering in 1.391 grammes of iron.

6

4. Titration of Iron by Protochloride of Tin.-Mr. Sutton, in his excellent Volumetric Analysis' before quoted, gives the following directions for the direct titration of iron by protochloride of tin.

The principle involved in this reaction is, in fact, simply a reversion of the ordinary process by permanganate and bichromate. In the case of these two reagents, the amount of oxygen given up by them is the measure of the quantity of iron, whereas with protochloride of tin, it is the amount taken up by it that answers the same purpose.

Fresenius (in his Zeitschrift für Analytische Chemie,' part 1, page 26) has recorded a series of experiments made on the weak points of this process, and gives it as his opinion that it is most accurate and reliable with proper care, without which, of course, no analytical process whatever is worth anything. The summary of his paper is as follows:

a. A solution of peroxide of iron of known strength is first prepared, by dissolving 10-03 grm. fine pianoforte wire (=10 grm. pure iron) in pure hydrochloric acid, adding chlorate of potash to complete oxidation; boiling till the excess of chlorine is removed, and diluting the solution to 1 litre.

b. A clear solution of protochloride of tin, of such strength that about equal volumes of it and the iron solution are required for the complete reaction.

c. A solution of iodine in iodide of potassium, containing about 0.005 grm. iodine in 1 c.c. (if the operator has the ordinary decinormal iodine solution at hand, it is equally applicable.) The operations are as follows:

1. 1 or 2 c.c. of the tin solution are put into a beaker with a little starch liquor, and the iodine solution added from a burette till the blue colour occurs; the quantity is recorded.

2. 10 c.c. of the iron solution=0.1 grm. iron, are put into a small flask with a little hydrochloric acid, and heated to gentle boiling (preferably on a hot plate), the tin solution is then allowed to flow in from a burette until the yellow colour of the solution is nearly destroyed, it is then added drop by drop, waiting after each addition until the colour is completely gone and the reduction ended. If this is carefully managed there need be no more tin solution added than is actually required; however, to guard against any error in this respect, the solution is cooled, a little starch liquor added, and the iodine solution added by drops until a permanent blue colour is obtained. As the strength of the iodine solution compared with the tin has been found in 1, the excess of tin solution corresponding to the quantity used is deducted from the original quantity, so that by this means the volume of tin solution corresponding to 0.1 grm. iron is found.

The operator is, therefore, now in a position to estimate any unknown quantity of iron which may exist, in a given solution, in the state of peroxide, by means of the solution of

tin.

If the iron should exist partly or wholly in the state of

protoxide, it must be oxidised by the addition of chlorate of potash, and boiling to dissipate the excess of chlorine, as described in 2.

Example: 10 c.c. of iron solution, containing 0·1 grm. iron, required 15 c.c. of tin solution.

A solution, containing an unknown quantity of iron, was then taken for analysis, which required 12 c.c., consequently, a rule of three sum gave the proportion of iron as follows:15 01 grm.: 12: 0·08 grm.

:

It must be remembered that the solution of tin is not permanent, consequently it must be tested every day afresh. Two conditions are necessary in order to ensure accurate results.

1st. The iron solution must be tolerably concentrated, since the end of the reduction by loss of colour is more distinct; and, further, the dilution of the liquid to any extent interferes with the quantity of tin solution necessary to effect the reduction. Fresenius found that by diluting the 10 c.c. of iron solution with 30 c.c. of distilled water, of a c.c. more was required than in the concentrated state. This is, however, always the case with protochloride of tin in acid solution, and constitutes the weak point in Streng's method of analysis by its means; it would seem that dilution either predisposed it to rapid oxidation, or that water had the power within itself to communicate a certain proportion of oxygen to it.

2nd. The addition of the tin solution to the iron must be so regulated that only a very small quantity of iodine is necessary to estimate the excess-if this is not done another source of error steps in, namely the influence which dilution, on the one hand, or the presence of great or small quantities of hydrochloric acid on the other, are known to exercise over this reaction; practically it was found that where the addition of tin, to the somewhat concentrated iron solution, was cautiously made so that the colour was just discharged, the mixture then rapidly cooled, starch added, and iodine till blue, that the estimation was as reliable as by any other method.