A SYSTEM of QUALITATIVE ANALYSIS for the ANIONS J. T. DOBBINS AND H. A. LJUNG* Guilford College, North Camlina
T
HE NEED of a systematic scheme for the separation and identification of the anions, similar to that for the cations, has long been realized. A few such schemes have been devised but none have come into general laboratory use in qualitative analysis. Practically all texts use the "spot" test with small portions of a "prepared" solution to detect the presence or absence of the anions. The following scheme of analysis for the separation and identification of the anions, based upon oxidation-reduction theories, provides a method whereby the interferences commonly met are entirely eliminated or reduced to a minimum. THEORETICAL
Since there are quite a few pairs of the anions which are easily capable of undergoing oxidation and reduction, and since it is possible that such ions may exist together in the original solid, it is essential that the influence of the hydrogen-ion concentration on oxidation-reduction reactions be kept in mind. Throughout the greater part of the scheme the test solution is never allowed to acquire greater acidity than that produced by the hydrolysis of some of the salts used as precipitating agents, and is, in some instances, made decidedly alkaline. That this is necessary is apparent from the following facts: (1) The original solution may contain both oxidizing and reducing ions which, in all cases, will react if the hydrogen-ion concentration becomes suiTiciently great, and in other cases will react even in neutral and alkaline solution. If the hydrogen-ion concentration is held as low as possible, this interaction will be reduced to a minimum. The following are some of the possible mixtures in which interaction will take place when the hydrogen-ion concentration is great enough: (a) arsenate and sulfite; (b) arsenate and iodide; (c) chromate and sul6te; (d) chromate and halogens; ( e ) chroAn abstract of a thesis submitted by H. A . Ljung in partial lulfilmenr of the r c q u i r r m r n ~ sfur the degrec of Docror of Philosophy at rhr Univrrr!ty of Sorth Carulina. M a y 1.5, 1931.
mate and sulfide; Cf) nitrite and iodide; (g) chlorate and the halogens; (h) chlorate and sulfite. (2) In the removal of sulfur dioxide from the solution before testing for arsenate and arsenite, the hydrogen-ion concentration is held as low as possible to prevent interaction of arsenate and sulfite. (3) In identifying arsenate and arsenite the hydrogen-ion concentration is regulated to prevent interference. (4) The nitrite ion is identified in the presence of the nitrate ion by the ferrous ion in acetic acid solution. With these ideas and facts in view, the anions have been divided into groups according to their behavior with certain cations. The-ibns composing the various groups are then separated and identified by special tests. GROUP I
Ions: carbonate, fluoride, ok.alate, arsenate, arsenite, phosphate, and tartrate. These ions are precipitated as the calcium salts from alkaline solution. GROUP I1
Ions: sulfate and chromate. These ions are precipitated as the barium salts from alkaline solution. GROUP JJI
Ions: cyanide, borate, ferricyanide, sulfide, and ferrocyanide. These ions are precipitated as the zinc salts from alkaline solution. GROUP I V
Ions: thiosulfate, thiocyanate, iodide, bromide, and chloride. These ions are precipitated as the silver salts in solution slightly acid with respect to nitric acid, with the exception of the thiosulfate which is transformed into silver sulfide.
GROUP V
Ions: chlorate, nitrite, and acetate. These ions are identified in the filtrate from Group IV without precipitation. GROUP VI
Ion: nitrate. The nitrate ion is identified in some of the original solution. EXPERIMENTAL
Preparation of the Solution for Analysis The substance t o be analyzed must be in the form of the sodium, ammonium, or potassium salts. The "prepared" solution is made in the usual manner by treating the solid substance with normal sodium carbonate solution, boiling, and filtering. The filtrate, or "prepared" solution, is then ready to be analyzed for Group I. I n case the heavy metals are absent the "prepared" solution is made by dissolving the sample in sodium hydroxide and water. Group I-Detailed Analysis Group I must be analyzed, for calcium hydroxide is always precipitated upon the addition of the group reagent, thus prohibiting the use of a suitable group test. If the color of the "prepared" solution indicates the possible presence of chromate, the procedure according to ( a ) must be followed; if not, the procedure according to (b). (a) The "prepared" solution is diluted t o 50 cc. with water, 2 cc. of 4 N sodium hydroxide is added, and the anions are precipitated completely with 2 N calcium nitrate solution, filtered, and washed twice with water, and then with 5% ammonium hydroxide until free from the chromate ion. The filtrate is boiled t o precipitate completely any sulfite or tartrate remaining in solution. Any precipitate obtained on boiling this solution should be combined with the group precipitate. The residue is then analyzed according to the scheme for Group I. The filtrate, which contains Groups II-V, is reserved. (b) The "prepared" solution is diluted t o 50 cc. with water, 4 N sodium hydroxide is added, and the anions are precipitated completely with 2 N calcium nitrate, heated to boiling, filtered, and washed twice with water. and then until completely free from the nitrate ion. The filtrate, which contains Groups II-V, is reserved. The precipitate may consist of calcium carbonate, calcium fluoride, calcium onalate, calcium sulfite, calcium arsenate, calcium arsenite, calcium phosphate, and calcium tartrate. The precipitate is then treated with 12.5% acetic acid. diluted with 25 cc. of water, filtered, and wphed. Carbonate is identified in the group precipitate by an effervescence upon the addition of acetic acid.* In case i t was necessary to make the "prepared" solution, the test for carbonate must be made an some of the original solid as follows: t o a nortion of the solid is added a double quantity of potassium chlor&e, water is added, thoroughly mixed, and~dilutesulfuric acid is added. An effervescence shows the presence of carbonate. Separation and identification of fluoride and malate: The residue which is insoluble in acetic acid is treated on the filter with dilute sulfuric acid and washed. Fluoride is identified in the residue by its etching action on glass. Omlateis identified in the filtrate by the decolorization of 0.01N pemanganate. The filtrate from the acetic acid treatment may contain the foUawinn nhos~hate, and tar- ions: suEte. arsenate, arsenite. . . trate. Suwe is identified in the filtrate by the formation of white crystalline mercurous chloride upon the addition of hot saturated mercuric chloride. If sullite bas been shown to be present, the remainder of the filtrate is boiled t o expel the sulfur dioxide completely, a8 shown by the s a t e test. Arsenafc is identified in the boiled filtrate by the formation of a brown roloration upon the addition of concentrated hydrochloric acid and solid potassium iodide
Arsenite is identified in the filtrate after diluting t o double volume by the decolorization of a few drops of 0.01N iodine. If either arsenate or arsenite has been shown to be present, the remainder of the filtrate is treated witb solid sodium sulfite. made strongly acid with hydrochloric acid, heated to boiling. and the arsenic is completely precipitated with hydrogen sulfide, filtered, and the filtrate boiled to expel hydrogen sulfide. Phosphate is identified in the filtrate by the formation of a yellow precipitate upon the addition of ammonium molybdate solutiou. Tartrate'is identified in the filtrate after the removal of calcium with sulfuric acid. The solution is treated with 2 or 3 drops of normal cobalt nitrate, 4 N sodium hydroxide is added until the solution takes on a permanent blue color, then 5 cc. in excess; heat to boilinn and filter. A d e w blue coloration in the filtrate shows the ure&nce of tartrate. The observation must be ~- marl; on the hot filtrate, for the blue color diiapptws on couling. The scnsitivily of thc tcstsfor the ioniof Group I are as follows. ~
~
--
expressed in milligrams per 10 cc.: COs....; -SO,, 0.080; Go,, 0.004; A&, 0.010; 0.004.
~
~
~~~
F....; GO+.0.057;
P&
0.050;
~,if,~~.
Group II-Detailed A n a h i s Preliminary Test for Group 11: A precipitate upon the addition of 0.5N barium nitrate to the filtrate from Group I shows the presence of Group 11. Sulfate is identified in the filtrate from Group I by the formation of a white nrecinitate insoluble in water unon the addition of -hydrochloric acid and barium nitrate. If Group I1 was shown to be present theentire filtrate from Group I is precipitated completely with barium nitrate, heated t o boiling, filtered, and washed twice with water, and then until free from the nitrate ion. The filtrate, which contains Groups III-V, is reserved. The precipitate may consist of barium sulfate and barium chromate. Chromate is identified in t6e residue by the formation of a yellow precipitate by treating witb hydrochloric acid, neutralizing with ammonium hydroxide, and acidifying with acetic acid. The sensitivity of the tests for the ions of Group 11 are as
.,
follows. expressed in milligrams pei. 10 cc.: 0.020.
SO,,
0.018;
c%,,
Group III-Detailed Anolysis Preliminary Test for Group 111: A precipitate upon the addition of normal zinc nitrate to the filtrare from Group I1 and boiling shows the presence of Group 111. ;A change in color of the solution during precipitation indicates that the fedcyanide has been reduced. I n such cases ferricyanide must be identi6ed with the ferrous sulfate test in the filtrate before precipitating the group. There are instances when the ferricyanide will be reduced upon dissolving the sample, and such cannot be avoided by this scheme. If the preliminary test is positive, the filtrate from Group I1 is treated with 0.3 to 0.4 g. of solid Sodium carbonate, and the group .is precipitated completely with zinc nitrate in excess, heated t o boiling, filtered, and washed twice with water, and then until free from the nitrate ion. The filtrate, which contains Groups IV-V, is resenred. The precipitate may consist of zinc cyanide, zinc borate. zinc ferricyanide, zinc sulfide, and zinc ferrocyanide. Separation and identification of cyanide and borate: The group precipitate is treated with 12.5% *acetic acid on the filter and washed. The acetic acid solution is treated with 6 N ammonium hydroxide and ammonium polysulfide, acidified with hydrochloric acid, boiled, and filtered. Cyanide is identified in the filtrate by the formation of a red colorationupon theaddition of 10% ferric nitate.
1T
n c ~ ~ "Detection x. of citrates and tartrates (and malates)."
J . Ckem. Soc., 90, ii, 813 (1906).
Borate is identified in the filtrate by the turmeric paper test. Separation and idatification of ferricyanidc: The residue is treated on the filter with 6 N ammonium hydroxide, and the s o l ~ tion allowed to pass into dilute hydrochloric acid. Ferricyanide is identified in the filtrate by the formation of a blue coloration or blue precipitate upon the addition of ferrous sulfate. Sulfide is identified in the residue by the formation of a black or chocolate color upon the addition of sodium plumbite. Ferrocyanide is identified in the residue by the formation of a reddish brown precipitate after the residue has heen decomposed with sodium carbonate, acidified with h~drochloricacid, and normal copper sulfate has been added. Upon acidifying it is necessary that the solution be cool, as ferrocyanic acid is decomposed by carbonic acid in hot solution. The sensitivity of the tests for the ions of Group I11 are as follows, expressed in milligrams per 10 cc.: &,0.016; ---0.037; Fe(CN)a, 0.200.
&-63, 0,800
F~EN)., 0.063; s
Group IV-Detailed Analysis Preliminary Test for Group IV: A precipitate upon the addition of 5% silver nitrate to the filtrate from Group I11 shows the presence of Group IV. If the preliminary test shows the presence of thiosulfate, which is indicated by the formation of a white precipitate, changing color from white t o yellow to orange t o red to black, the filtrate from Group I11 is made alkaline with 6 N ammonium hydroxide, 1 cc. in excess is added, and complete precipitation is effected with silver nitrate, the.system is heated to boiling, made faintly acid with nitric acid, filtered, and the precipitate washed. If thiosulfate did not show up in the preliminary test, the treatment with ammonium hydroxide may be omitted. This treatment with ammonium hydroxide is essential for the reason that as silver thiosulfate is hydrolyzed the salutiorr becomes distinctly acid. If the solution is allowed to became tooacid, reduction of chlorate takes place. The filtrate, which contains Group V, is reserved. The precipitate may consist of silver sulfide, silver tbiocyanate, silver iodide, silver bromide, and silver chloride. Thiosulfatc is identified by the formation of a white precipitate which changes color almost immediately through yellow, orangesandredand finally to black upon the addition of the group XBgMt. Thiocyanate is identified in the residue by the formation of a red coloration in the suwrnatant liquor upon the addition of 2N ferric chloride. The remainder of the group precipitate is treated with ammonium polysulfide, heated to b d i n g , acidified with sulfuric acid. boiled, filtered, and washed. Iodide is identified in the filtrate g t e r making alkaline with ammonium hydroxide and acidifying with acetic acid by the formation of a purple coloration in an organic solvent by the action of sodium nitrite. An excess of sodium nitrite must be avoided as interaction of nitrite and tbiocyanate will produce an interference. Separation of thiocyanate and iodide: The remainder of the filtrate is made alkaline with ammonium hydroxide, then just acid with sulfuric acid; 5 cc. of acid is added in excess, the solution is diluted t o 50 cc., sodium sulfite is added. Heat to about 60", and effect complete precipitation with normal copper sulfatc.filrcr, and wash. Prelimtnory 1'e;Ifor R w m i d r ond (Illloride: A precipitate upon the addition of silrrr nitrare to the filtrate indsatcs the presence of bromide or chloride. Bromide is identified in the filtrate by the formation of a brown coloration in an immiscible solvent after oxidizing with 0.01N permanganate. The remainder of the filtrate is precipitated completely with silver nitrate, filtered, and washed. The residue is treated on the filter with ammoniacal silver nitrate which is 0.01N with respect to silver nitrate and U Z Y with rcapect to arumonium hsdruxide. Chloride is identified in the filtrate by thc fonnatiun 01 a white precipitatcupan thr.addition of nitncncid.
The sensitivity of the tests for the ions of Group IV are as fol-
--
lows, expressed in milligrams per 10 cc.: 40~.0.007; C ~ S .
I,
B;, 0,222; El, 0,066, Group V-Detailed Analysis
The ions of this group are identified in the filtrate from Group IV by special tests. ~h~ filtrate from G~~~~IV is treated with solid sodium carbonate, heated to boiling, concentrated, filtered, --A
*L.U
...".
LA
"aa"cU.
Chlorate is identified in the filtrate made acid with nitric acid by the formation of a white precipitate upon the addition f, silvernitrate after reduction with sodium nitrite. xitrite is identified in the filtrate by the of a coloration upon the addition of ferrous sulfate after acidifying with acetic acid. AcetateZis identified in the acidified filtrate by the formation of an intense blue coloration upon the addition of 5% lanthanum nitrate, a few drops of OOlN iodine, and normal ammonium hydroxide t o faint turbidity. If the blue color does not develop before or on becoming turbid the solution is boiled and allowed to stand. A blue color will develop if acetate is present. If borate has been shown to be present, the test for acetate must be performed as follows: the filtrate is distilled with sulfuric acid, and acetate identified in the distillate by the above tests. The sensitivity of the tests for the ions of Gfpup V are asfollows, expressed in milligrams per 10 cc.: C10, 0.065; NO,, 0.009; CIH~O~. 0.470.
Group VI-Detailed
Analysis
This group contains only the nitrate ion which is identified in some of the original "prepared" solution. Preparation of solution for nitrate test: The "prepared" salution is precipitated completely by successive additions of 0.5N barium nitrate and saturated silver acetate, heated t o boiling. and filtered. The filtrate is treated with solid ammonium chloride, evaporated almost to dryness, taken up with water, and the operation is repeated. In,c& nitrite has been shown to be absent this treatment with ammonium chloride may be omitted. Nitrate is identified in the acidified liltrate by the usual ferrous sulfate test after reduction of the chlorate with sodium
--....-. nr1fits
The sensitivity of the test for the nitrate& in Group VI is as follows, expressed in milligramsRer 10 cc.: NO8,0.410. SUMMARY
A systematic scheme for the separation and identification of the anions has been developed. This scheme uses as its basis the theories of oxidation and reduction, which stress the influence of the hydrogen-ion concentration. By control of the hydrogen-ion concentration of the test solutions, the interferences commonly met in the anion analysis have been eliminated to a very great extent, thus making it possible to separate and identify all of the common anions without undue interference. There are a few cases wherein interference has not been overcome completely. The applicability of this scheme has been shown by the numerous test analyses and by the determination of the sensitivity of the test for each ion, both alone and in the presence of large amounts of other ions.
mccan
~ n c r ~ , , , "~ ~ .~ ~ i l ~~ ~ f ~ deq ~ b 'basichen u n g lanrhnn-acerats' mit Jod. Eine hochrmpfindlirhe Kcnkrion auf Acetot-Ion," Rer., 62B,2770 (1929).
