N e w Applications of the Wohlers Effect in Spot Test Analysis FRITZ FEIGL, I. T. de SOUZA CAMPOS, and S. LADEIRA DALTO Laboratorio do ProdugGo Mineral, Ministhrio das Minas e Energia, Rio de laneiro, Brazil (Translated by RALPH E. OESPER, University of Cincinnati, Cincinnati 19, Ohio)
4 When barium sulfate is precipitated from a solution containing much potassium permanganate the product has a violet tint that is not discharged by reducing agents. This so-called Wohlers effect has hitherto been employed only for the detection of barium and sulfate ions. Additional analytical applications have been developed in which solutions containing potassium permanganate plus barium chloride are allowed to act on certain inorganic and organic compounds containing SUIfur. Such procedures reveal: oxidizable sulfur-bearing anions; lead, calcium, and strontium sulfate (also differentiation from barium sulfate); organic sulfonic and thioacids after conversion into their calcium salts; water-, and acetone-soluble compounds containing CS-groups. Benzidine sulfate and barium sulfate (free of other sulfates) can b e identified through the Wohlers effect. The tests can b e accomplished by the technique of spot test analysis. Examples are given of the identification of gypsum in mineral analyses and in the testing of building materials, fertilizers, etc. The behavior of alkali dithiooxalates supports the view that these salts either do not contain any CS-groups or that only the sulfhydryl form participates in the oxidation by permanganate.
I
1908, WOHLERS(8) reported the interesting observation that barium sulfate which is precipitated in the presence of potassium permanganate has a violet color that is stable against the action of reductants. Since barium sulfate suspended in water does not stabilize permanganate and since potassium ions are essential, this effect involves the inclusion of K M n 0 4 in the BaS04 lattice (9). The analytical utility of this Wohlers effect was not appreciated until 30 years later when Feigl and Aufricht (6) made it the basis of spot tests for Ba+2 and S04C2 ions. I t has now been found that violet barium sulfate is formed when concentrated solutions of potassium permanganate containing barium chloride are brouqht together with certain sulfurbearing inorganic and organic comN
pounds. This is true of alkali sulfides, sulfites, thiosulfates, polythionates, hydrosulfites, persulfates, and thiocyanates. Since brown hydrous manganese dioxide is produced during the oxidation of the foregoing compounds, it is necessary to remove this product, as well as excess permanganate, by means of redu’ctants, before the occurrence of the Wohlers effect can be seen. (Aidditionof hydroxylamine hydrochloride is recommended.) This behavior has served as the basis of a rapid preliminary test for sulfur-bearing inorganic anions. If calcium sulfate is warmed with a solution of potassium permanganate containing barium chloride, the following reaction occurs as might be expected in view of the great difference in the solubilities and solubility products: CaS04
+ Ba+2
+
BaS04
+ Ca+2
The barium sulfate produced in this way in its nascent state, may include potassium permanganate in its crystal lattice. Here again, the tinted barium sulfate is readily visible after hydroxylamine hydrochloride has been introduced. Strontium and lead sulfate behave in the same manner as calcium sulfate. Consequently these sulfates can be detected in the presence of barium sulfate. The detection of gypsum and anhydrite can be accomplished more easily in minerals and materials tested than by the methods previously prescribed for such purposes by spot testing ( 3 ) . When sodium, potassium, magnesium, strontium, and calcium salts of organic sulfonic acids are ignited, the residues contain the respective sulfates. These sulfates show the Wohlers effect on treatment with a solution of potassium permanganate containing barium chloride. Organic sulfonic acids can be detected in this way and distinguished from organic carboxylic (also arsonic) acids. The behavior of thiourea merits particular attention; in acid solution it is converted into the disulfide [-S-((NH) iYHzIB by permanganate and other oxidants ( I ) . Maly (7‘) found that in neutral solution it reacts with permanganate to yield urea and sulfuric acid :
CS(NH2)z
+ 202 + HzO
+
CO(NH2)z
+ HzS04
Accordingly, treatment of thiourea with potassium permanganate solution containing barium chloride yields violet barium sulfate. The latter is easily seen after hydroxylamine hydrochloride has been added to remove excess permanganate and manganese dioxide. Compounds that contain the >CSgroup were found to show this same behavior, permitting the specific detection by spot testing of thioketogroups in compounds that are soluble in water and acetone. DETECTION OF SULFUR-BEARING OXIDIZABLE INORGANIC ANIONS Procedure. Two drops of a solution of 7 grams of potassium permanganate and 2 grams of barium chloride per 100 ml. of water are placed in a micro test tube or micro crucible. A drop of t h e test solution is added; manganese dioxide precipitates; t h e suspension is kept in boiling water for 5 minutes. Care must be taken t h a t the reaction mixture contains excess permanganate; therefore the test solution may not be too concentrated. After cooling, the suspension is cleared and decolorized by adding a drop of a 20y0 solution of hydroxylamine hydrochloride. A violet precipitate indicates a positive response. When only small amounts of the colored precipitate are expected it is advisable to conduct the Oxidation in a micro test tube, and after the hydroxylamine hydrochloride has reacted, to shake the system with several drops of ether. The tinted sulfate collects in the water-ether interface (analytical flotation) and is thereby made visible ( 2 ) . The method was tested with O.lyo solutions of alkali sulfide, sulfite, hydrosulfite, thiosulfate, tetrathiomte, persulfate, and thiocyanate. X limit of identification (after flotation) of 5 to 8 pg. was obtained for thiosulfate and thiocyanate. If osidieable sulfur-bearing anion? are to be detected in the presence of sulfate ions, the latter can be removed in advance of the oxidation by adding barium chloride since after barium VOL. 36, NO. 8, JULY 1964
1657
sulfate is once formed it is no longer tinted by potassium permanganate. The method provides a rapid preliminary test for Oxidizable sulfurbearing anions. It has the advantage of eliminating the need for separate tests for S-2, SO3-*, etc. since they obviously are not present if the response is negative. DETECTION OF CALCIUM SULFATE (GYPSUM, ANHYDRITE)
Procedure. ,4 micro test tube or micro crucible is used. A tiny portion of the test material is treated with a drop of the reagent solution and kept in boiling water for 5 minutes. -1fter cooling, a drop of a 10% solution of hydroxylamine hydrochloride is introduced to destroy the excess permanganate. A positive response is signaled by a violet residue. The sensitivity of the method for detecting calcium sulfate was explored by means of gypsum solution. A single drop of various dilutions was taken for the individual tests. The limits of detection were in the neighborhood of 5 pg. of CaS04. The method was used to check precipitated calcium sulfate, ignited products, and also gypsite (pulverized and as crystals several millimeters long). The powdered mineral was tinted violet homogeneously, whereas a characteristic red-violet color appeared along the edges of the crystals. Superphosphate of lime can be easily distinguished in this way from fertilizers containing no calcium sulfate or soluble sulfates. Barite and pigments as well as building materials can be tested for gypsum. The adulteration of flour with gypsum can be revealed by testing the ash. Calcium sulfate is obtained when nonvolatile sulfur-bearing compounds are ignited with calcium oxide. Conbequently, the test given here can be u-ed for the detection of sulfur in organic compounds. However, the test bawd on the Wohlers effect is somewhat 1r-4 senyitive in general than those ordinarily recommended in texts dealing with organic
