alkaline, if not already so, by addition of pure NaHO) to dryness in a porcelain dish, scrape the residue off the sides of the dish and mix it with three or four times as much powdered K2Cr2O7 by rubbing the two substances together in the dish with a pestle (260). Transfer this mixture into the small tubulated flask (fig. 38, p. 132) by pouring it down the perfectly dry neck on the side opposite to that at which the tube enters, and taking the greatest care to let none of the powder enter the delivery-tube. Then pour into the flask strong H2SO4 sufficient to cover the powder to the depth of about a quarter of an inch, at once close the neck with an india-rubber or glass stopper, and dip the end of the delivery-tube an inch into water contained in a testtube. Mix the contents of the flask by cautious shaking, and warm the mixture gently, carefully guarding against allowing any of the mixture to get into the delivery-tube and thence into the water in the test-tube.* As soon as no more reddish-brown vapour is visible in the flask or its delivery-tube, discontinue the heating, and immediately withdraw the test-tube. If no coloured vapour has been evolved and the water in the test-tube is colourless, chloride and bromide are absent. If the water is coloured pour into it CS, sufficient to form a layer at the bottom of the tube about as large as a small nut; close the mouth of the tube with the thumb and shake vigorously: the CS2 on settling shows a brown colour, indicating the presence of Br from a bromide.

Separate the water from the CS, by pouring it through a wetted filter, add to the filtrate AmHO in excess, then HA in excess, and PbÄ, solution. A yellow precipitate indicates the presence of a chromate, which proves indirectly that a chloride was present.

273 a. (s) The above method will also serve for the examination for chloride, bromide, and iodide in the AgNO, precipitate. The precipitate is dried, and fused for about ten minutes in a porcelain crucible with three or four times as much fusion mixture; the cool mass is boiled with HO for some time

* See the precautions in the note at the end of par. 260..

and filtered, the filtrate being examined as directed above (273).

274. (s) If only an iodide and a bromide have to be tested for, the following method may be employed. Pour into the solution CS, sufficient to form a large drop at the bottom; add Cl-water drop by drop, shaking thoroughly after each addition: an iodide, if present, shows first by its liberated iodine colouring the CS, purple, but on continuing the addition of Cl-water this colour is destroyed; a bromide is then detected by a brown coloration of the CS2, which, however, is also destroyed if too much Cl-water is added.

GROUP V.-PHOSPHATE GROUP.

Phosphates and arsenates resemble one another closely in many reactions; the points of difference they present from one another and from other salts will, however, if carefully attended to, render their detection a simple matter.

PHOSPHATES ("PO).-Use Na,HPO.H2O.

275. MgSO4 solution to which some AmCl has been added and then a little AmHO, gives with Na,HPO4 a white crystalline precipitate (MgAmPO4): this precipitate is almost insoluble in AmHO, but easily soluble in acids. If very little Na2HPO4 is present, the precipitate appears only on warming, stirring, or shaking well, and letting stand.

276. FeCl (a few drops only) added after a little HA and some Naà solution gives, especially on being heated, a yellowish-white precipitate of Fe""'PO·

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277. AmHMo0, dissolved in HNO, gives a yellow precipitate if a drop of Na2HPO4 solution is added to it; the formation of the precipitate is hastened by gentle warming, stirring, or shaking, but it often appears only after a time. Pour off a part of the liquid and show that the precipitate is soluble in alkalis (KHO, NaHO, AmHO), and insoluble in acids, especially in HNO3.

Note. This reaction, if properly performed, is the most delicate test known for a phosphate, but careful attention must be paid to the following precautions:-The AmHM0O4 solution must be prepared according to the directions given in par. 526, remark 40; the solution to be tested must not be alkaline to test-paper, it is best made distinctly acid with HNO,; it is then to be added in small quantity only to some of the AmHMOO, solution in a test-tube, more of the liquid being added only if after gently warming and stirring or shaking well no yellow precipitate forms after a few minutes. This last precaution must be particularly observed, as an excess of phosphate altogether prevents the formation of the precipitate: show that this is the case by pouring into some Na,HPO4 solution, contained in a test-tube and made acid with HNO, a few drops of AmH MoO4 solution; it will be found that no precipitate will form even on heating and shaking the liquid, since the phosphate is present in large quantity compared with the AmHM004; but on adding a few drops of this liquid to some fresh AmHM0O4 solution the precipitate appears. It must also be remembered that HCl retards or prevents the formation of this precipitate, and therefore an HNO3 solution should always be used when possible.

278. AgNO3: yellow precipitate (Ag,PO4): pour off a portion and show the precipitate to be soluble in AmHO and in HNO。.

278 a. A very delicate test for the presence of P in the form of a phosphate or otherwise, consists in strongly heating the dry solid substance with a small piece of Na in a little tube of hard glass closed at one end. On breaking the tube and breathing upon the powdered substance a smell of onions, due to H,P, is perceived.

A phosphate made into a paste with strong H2SO and heated strongly at the tip of the inner blowpipe flame, gives a bluish-green colour to the flame.

Hydrogen phosphate, or phosphoric acid (HPO4), is a colourless crystalline substance; its solution is strongly acid, but does not char paper and organic substances. (Diff. from H2SO). It yields reactions (275 and 277.)

ARSENATES ("AsO1.)—Use Na¿HASO4.12H2O solution. 279. The precipitates formed in reactions (275, 276, 277) for phosphates are precisely similar in appearance and general properties to those formed with the same reagents by arsenates. The precipitate, however, which is formed by an arsenate with AgNO, differs from that given by a phosphate by being brown in colour; also AmHM004 gives with an arsenate a precipitate only on boiling.

280. The following differences serve to detect and separate a phosphate and an arsenate :

1. The fact that an arsenate only yields a precipitate with AmHMOO, when the liquid is boiled, whereas the corresponding precipitate with a phosphate is produced by a gentle heat.

2. An arsenate solution, if boiled with strong HCl, gives when HS is passed into the hot liquid, first a white precipitate of S, then yellow As,S,; a phosphate solution, under the same circumstances, gives no precipitate.

3. In a perfectly neutral solution, AgNO, gives with an arsenate solution a brown precipitate, with a phosphate a canary-yellow precipitate.

281. Hence, if occurring separately, a phosphate and arsenate may be distinguished by one or more of these differences.

281 a. The simplest method of ascertaining which of these two acids is present is to throw the precipitate yielded by AmCl, AmHO, and MgSO, upon a filter, wash it with a little cold water, and drop AgNO, upon it: if the precipitate consisted of MgAmAsO, it will become brown, if of MgAmPO4 canary-yellow (difference 3). Or the precipitate produced by AmCl, AmHO, and MgSO4 may be examined for arsenate by dissolving a part of it in dilute HNO3, adding AgNO, as long as it causes any white precipitate, and then very dilute AmHO gradually drop by drop; if arsenate is present a brown precipitate will form just before the liquid becomes alkaline to test-paper: phosphate gives a yellow precipitate. Instead of mixing the dilute AmHO it may be poured upon the top of the acid liquid when the colour appears at the surface of contact.

282. If both arsenate and phosphate have to be tested for, AmCl, AmHO, and MgSO4 are added as long as any further precipitate is caused: the precipitate is filtered off* and dis

A small portion of the precipitate may be filtered off upon another filter and tested at once for arsenate by dropping upon it AgNO3 solution. If the precipitate turns brown arsenate is present and must be separated in the other part of the Mg precipitate before testing for phosphate: if the precipitate turns yellow, phosphate alone is present.

solved in a little boiling strong HCl, H2S is passed for several minutes into this hot HCl solution, when the presence of arsenate is shown as above (par. 280, 2); the precipitate is filtered off if any has formed, and H2S passed into the boiling filtrate; if any further precipitate is produced, the gas must be passed again until it no longer causes a precipitate; the clear filtrate from this precipitate is then evaporated just to dryness, dissolved in a little dilute HNO3, and tested for phosphate by adding a few drops of it to AmHMOO, solution and gently warming. Arsenite, if present, may be detected by H2S in the filtrate from the Mg precipitate, acidified with HCl: yellow As,Sg is precipitated.

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UNGROUPED ACID-RADICLES (INORGANIC).

The salts of the following acid-radicles present no marked resemblances in their reactions to one another or to other salts, and are usually detected by special tests.

BORATES ('BO2, or "BO).-Use Na2B4O7.10H2O. 283. If some Na2B4O, be stirred with a little dilute HCl on a watch-glass and a strip of turmeric paper be half immersed in the liquid, then dried on a watch-glass in the steam-oven or on a water-bath,* the part which was moistened appears reddish-brown, and becomes bluish-black when moistened with AmHO. This is a most useful test for a borate.

284. If some methylated spirit be poured upon a little solid Na,BO, in a test-tube or porcelain dish, some strong H2SO4 added, and the mixture be heated and set fire to, the spirit burns with a green-edged flame, seen best by blowing the flame out after it has burnt for a time and then relighting it. This test is not quite characteristic, since some chlorides yield a similar green flame.

*The paper may also be dried over a small flame, if it is not heated above steam-heat (100° C.)