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London: Printed and Published for the Proprietors (the Executors of the late Sir WILLIAM CROOKES) by EDWIN JOHN DAVEY, at the Office, 16, Newcastle Street Farringdon Street, E.C. 4. July 11, '9'9. WHATMAN'S WHATMAN at free-Samle NOTICES. QUALITY TELLS. July 18, 1919 IN experiments, the quality of every detail tells. Filter Papers and Extraction Thimbles are important, therefore get the best-get "Genuine Whatman," manufactured by W. & R. Balston, Ltd., Paper Makers in Britain for over 150 years. Genuine Whatman Filter Papers are made in all grades- Genuine Whatman Extraction Thimbles are Fat-Free and Seamless. THERE are a number of schemes in the literature for the detection of the acids of Group I., but very few are given with experimental data or with specific and adequate directions for their proper execution in the laboratory. The distillation method with phosphoric acid devised by A. A. Noyes (Fourn. Am. Chem. Soc., xxxiv., 609) is sup. ported by experimental data and doubtless gives good results in the hands of experienced chemists; but it seems to us altogether too complicated for students in qualitative analysis. We have therefore attempted in this paper to provide a systematic procedure for the detection of the acids of Group I. which shall be rapid, efficient, and capable of giving good results when used by the inexperienced student. Provision has also been made in this scheme for the rough estimation of each of the acid radicals detected. Such a Except where special tests are to be made, as in the case of carbonates, the scheme of analysis proposed in this paper is intended for the detection of the acids when they are in solution in the form of their sodium salts. solution known as the "prepared solution" is generally obtained by boiling the original substance with a strong Na2CO3 solution and filtering the mixture. The filtrate is made slightly acid with acetic acid and then slightly alkaline with NaOH. Advantage is now taken of the fact that the acids of Group I. may be precipitated in a slightly alkaline solu tion with a mixture of BaCl2 and CaCl2. Since BaSO4 is insoluble in acids it may be separated from the barium salts of other acids by the addition of HCl (1 : 2), which was found to dissolve the Ba and Ca salts of all the other acids. The filtrate will now contain the following acids: -Sulphite, oxalate, fluoride, borate, tartrate, arsenite, arsenate, and phosphate as Ba and Ca salts dissolved in HCl. By the use of an oxidising agent such as H2O2, the sulphite is oxidised to sulphate, which then separates as BaSO4 in consequence of the excess of BaCl2, which is added. At the same time arsenites are oxidised to arsenates. Phosphate and arsenate go into solution, leaving a residue of CaF2, CaC2O4, and CaSO4 (see ante). The precipitate is divided into two equal portions. The first is treated with H2SO4, and tested for oxalate by the addition of KMnO4. The other portion of the precipitate is dried in a platinum or lead crucible and the etch test then applied. (For roughly estimating the relative amounts of F and C2O4 in this precipitate see Note 18 following scheme). The filtrate consisting of arsenate and phosphate is treated with H2SO3 to reduce the arsenate to arsenite. The latter is then completely precipitated with H2S. The filtrate, after the removal of the H2S, is tested for phosphate by means of (NH4)2M0O4. Special provision is made for the detection of carbonates (see Note 1a), chromates (see Note 1b), as well as for distinguishing between arsenites and arsenates (see Note 9). Notes. 1a. Carbonates are separately tested for and roughly estimated by means of the apparatus shown in the diagram. This consists of a test-tube and a small w.m. Experiment having shown that 500 mgrms. of most of the acids of Group I. in the form of Ba salts are not held completely in solution by an excess of acetic acid, (NH4)2SO4 is next added to remove the Ba. To the filtrate sufficient NH4Cl is now added to prevent borates and tartrates from precipitating upon making the solution bottle connected by a glass tube bent twice at right angles. alkaline with NH4OH. The filtrates containing the borate and tartrate is divided into two portions. The The short limb passes through a one hole rubber stopper former is tested for by means of an alcoholic solution of which tightly fits the test-tube. The small wide mouth bottle is fitted with a rubber stopper with two perforations turmeric after concentration and treatment with concentrated HCl and methyl alcohol. The tartrate is tested through one of which passes the long limb of the tube for by the formation of the blue copper complex which and extends to within inch from the bottom of the bottle. it forms with copper in NaOH solution, due provision To make test, introduce 0.5 grm. of the substance into the being made for the disturbing influence of ammonium salts the bottle. Keep away from flame and stopper imtest-tube. Put 15 cc. of saturated Ba(OH)2 solution into and arsenites, both of which give a similar test. The pre-mediately. Obtain a little absorbent cotton to form a cipitate consisting of oxalate, fluoride, arsenate, phos • Copyright, 1918, by Louis J. Curtman. Some CaSO4 always forms at this point for the reason pointed out in Note 17 following scheme. loose plug near the mouth of the tube. Now add 10 cc. 3 per cent H2O2 and 5 cc. dilute HCl, quickly introduce the cotton, and tightly stopper. Heat gently until no more bubbles pass through the Ba(OH)2; then remove stopper from the test-tube. A cloudiness or white precipitate shows the presence of a carbonate. By this procedure I mgrm. of carbonate can be detected in the presence of 500 mgrms. of sulphite. (See Test). b. Chromates are not provided for in the scheme because they are incompatible with sulphites and thiosulphates in neutral or acid solutions. If the prepared solution is colourless it is safe to conclude that chromates are absent, because as little as I mgrm. of CrO4 gives a decidedly yellow colour to the solution. In the absence of sulphites and thiosulphates, chromates fit without difficulty into the scheme, and will be found in the filtrate containing the borate and tartrate. The test for tartrate remains unchanged, since CrO4 does not interfere. However, to test for borate it will be necessary to remove the CrO4 by adding 10 cc. 2N.BaCl2 to the solution slightly acid with acetic acid. The filtrate is then tested for borate in the usual manner. 2. A "prepared solution" is one which contains the acids in the form of their sodium salts. Such a solution may be obtained by boiling I grm. of the finely powdered substance with 20 cc. 3N.Na2CO3 with constant stirring for several minutes. If NH3 is given off continue the boiling (adding more Na2CO3 solution if necessary) until no more of this gas is given off. Filter. The filtrate is the "prepared solution." 3. A mixture of BaCl2 and CaCl2 will precipitate I mgrm. each of CIO4, SO4, SO3, C2O4, F, and PO4. The least amount of the other radicals precipitated by the same reagents in a volume of 25 cc. are:-8 mgrms. AsO3, 3 mgrms. AsO4, 85 mgrms. BO3, 70 mgrms. C4H4O6, and 40 mgrms. S203. 4. In the filtration of the precipitates formed by barium and calcium salts it is best to use a double filter. 5. If the precipitate is too small to transfer to a beaker puncture the filter with a clean platinum wire and pass HCI (12) through the filter several times in small portions. 6. HCI (1:2) is the strength necessary to hold all the acids except SO4 in solution. 7. If thiosulphates are present as indicated by the precipitate of S and the oxidation of the latter on ignition to SO2, a precipitate will be obtained later with H2O2, even though sulphites are absent, due to the liberation of H2SO, from the thiosulphate by the HCl. A separate test for sulphites in the presence of thiosulphates is therefore necessary. The following procedure is a modification of that given by A. A. Noyes ("Qualitative Chemical Analysis," 1913, p. 113), which is based on the difficult solubility of strontium sulphite (see Test Exp. V.)-15 cc. of the prepared solution is slightly acidified with acetic acid and then made faintly alkaline with NH4OH. 3 cc. N.Sr(NO3)2 are added, the mixture heated to boiling, allowed to stand fifteen minutes, and filtered. 8. Borates in amounts less than 85 mgrms. are not precipitated by BaCl2. To detect less than this amount see Note 15. 9. Only amounts of tartrate greater than 70 mgrms. are precipitated by CaCl2. To detect less than this amount see Note 16. 10. The limit of sensitiveness for arsenite in this scheme is 8 mgrms. To detect less than this amount acidify a portion of the prepared solution with HCl and pass in H2S. An immediate yellow precipitate of As2O3 shows the presence of an arsenite. II. The scheme does not distinguish between an arsenite and an arsenate due to the use of H2O2 which oxidises any arsenite to arsenate. To determine the state of oxidation of the arsenic in the original substance acidify 10 cc. of the prepared solution with acetic acid and make it distinctly alkaline with NH4OH. Add 10 cc. magnesia mixture, and filter. Wash the precipitate twice with water, and dissolve it in HCI. Add 30 cc. H2SO3, and boil off the excess. Pass in H2S. A yellow precipitate of As2S3 shows the presence of an arsenate. The filtrate from the magnesia mixture precipitate is made acid with HCl, heated to boiling, and treated with H2S. An immediate yellow precipitate shows the presence of an arsenite. 12. As H2SO4 and H3PO4 are used in the manufacture of H2O2 it not infrequently happens that considerable amounts of these acids are found in certain commercial grades of H202 Hence before using, a control test for SO4 and PO4 should be made. Only when the control tests are negative or slightly positive can the peroxide be used. If at this point in the scheme a precipitate is obtained with H2O2, immediately or on boiling, a sulphite is present, and time may be saved by adding 10 cc. 2N. (NH4)2SO4 before filtering and thus save a filtration. 13. Barium is removed at this point because, contrary to the general statements made in the literature, it was found that even a huge excess of acetic acid failed to hold in solution 500 mgrms. PO4, CIO4, F, ASO4, or C204 in the form of their Ba salts (see Test Exp. III.). 14. It is necessary to add 5 grms. NH4Cl and also to dilute the solution to 100 cc. in order to keep the tartrate and borate in solution. NH4Cl is added in preference to (NH4)2SO4, because the latter would increase the quantity of CaSO4 which comes down later with the oxalate and fluoride. 15. A test for borate at this point indicates the presence of over 85 mgrms. If less than this amount is to be detected apply the procedure given in the scheme directly to 5 cc. of the prepared solution. Always compare the test obtained with a control which is made as follows:To 5 cc. of water add 3 cc. concentrated HCl, 10 cc. methyl alcohol, and I cc. of a saturated solution of turmeric powder in 95 per cent ethyl alcohol. To form an estimate of the amount of BO3 present compare test with that obtained with a known amount of BO3. 16. The double evaporation with excess of NaOH is made to ensure the removal of NH4 salts, which give a similar test. A negative test at this point would show the absence of more than 70 mgrms. C4H4O6. To detect smaller amounts of this acid apply the procedure given in the scheme under tartrates directly to a portion of the prepared solution, remembering, however, that if arsenites are present they must be first oxidised by adding 20 cc. 3 per cent H2O2 to the prepared solution before adding 10 cc. 25 per cent NaOH. The mixture is then evaporated and the test completed as given in the scheme. 17. Due to the presence of Ca and SO4 a precipitate of CaSO4 is always obtained at this point. Hence the formation of a precipitate at this stage of the analysis does not necessarily prove the presence of C2O4, F, AsO4, or PO4. No way of preventing the formation of CaSO4 at this point was found, although numerous devices and procedures were tried. 18. Under the conditions given in the scheme 4 mgrms. is the smallest amount of F which will precipitate as CaF2 |