THE ACTION OF AQUA REQIA ON GOLD-SILVER ALLOYS ETC. 99 XIV.-The Action of Aqua Regia on Gold-Silver Alloys in the Presence of Ammonium Salts. By WILLIAM BRANCH POLLARD. THE most practical method of dissolving gold and its alloys is undoubtedly by means of aqua regia. With gold alloys containing upwards of 15 per cent. of silver a coating of silver chloride forms on the surface of the metal and protects it from the further action of the acid. By rolling out these alloys until they are very thin solution can often be effected the coating on the metal never becoming sufficiently thick to stop the reaction; thicker pieces are only superficially attacked. With alloys which are very rich in silver “rolling out” is not sufficient to effect solution and a black coating forms and resists all further action of the acid.I n order to study the matter further the following alloys were prepared the composition being stated in parts per thousand: CU. 500 500 -333 667 -250 750 -453 277 270 -Au. Ag. 625 375 The alloys were rolled out to about a tenth of a millimetre in thickness and portions of them were used in the subsequent tests. It seemed probable that the best way t o effect solution of these alloys would be to try and increase the solubility of the silver chloride in the liquid; of the various chlorides which are known to dissolve silver chloride ammonium chloride appeared to be the most promising for this purpose. To test the point a gram of each of the above-mentioned alloys was placed into a bottle aqua regia (containing one part of con-centrated nitric acid and three parts of concentrated hydrochloric acid) added and the bottle placed on a hot asbestos board.The action of the acid started almost immediately but stopped very quickly the surface of the metal being covered with a black coat-ing. Ammonium chloride was now added to the hot liquid as long as it was dissolved. I n a very short time the black coating on the metal had completely dissolved leaving a bright metallic surface in each case. The acid now began to attack the meta 100 POLLARD THE ACTION OF AQUA REGIA ON GOLD-SILVER again and continued to do so until all had dissolved; no silver chloride separated and a clear yellow solution was obtained. On diluting the solutions with water copious precipitates of perfectly white silver chloride separated.Even pure silver was found to be soluble in a mixture of aqua regia and ammonium chloride, although the action was not very rapid and the volume of liquid required was large in comparison with the amount of metal dis-solved. It is to be noted that aqua fortis and sal-ammoniac were used by the alchemists for dissolving gold. It seems most prob-able that this mixture was used to dissolve native gold which sometimes contains enough silver t o prevent dissolution in aqua regia alone. The chief objection to the use of aqua regia and ammonium chloride seemed t o be that the action of the acid was very slow, and when much silver was present the volume of the solution became large. It was noticed that the alloy containing gold 453 silver 277, and copper 270 dissolved very easily in a mixture of aqua regia and ammonium nitrate the silver chloride remaining insoluble.Alloys of gold and silver only did not dissolve so easily in this mixture but when ammonium chloride was added as well a most marked improvement took place. For most alloys a mixture of 5 grams of ammonium chloride, 5 grams of ammonixun nitrate and 5 to 10 C.C. of aqua regia was found t o give quite satisfactory results. The amount of metal which can be dissolved depends on the amount of silver present. I f silver chloride separates this shows that enough ammonium chloride has not been added. I f on the other hand the action becomes slow more aqua regia may be required. With 5 grams of each salt and 5 to 10 C.C.of aqua regia 0.5 to 1 gram o l alloy could be dissolved easily and quickly the total volume of the solution being quite small. Considerably more gas is evolved when gold dissolves in the presence of ammonium salts than when i t is dissolved in aqua regia alone; on collecting the gas i t appeared to consist prin-cipally of nitrogen. The solution of the gold therefore does not take place in accordance with Priwoznik’s equation (Oesterr. Zeitsch. Berg.- u. Hiittenw. 1910 58 649) a secondary action occurring whereby the ammonium salts become oxidised with evolution of nitrogen. During the attlack of gold-silver alloys by aqua regia in the presence of ammonium chloride and nitrate it often happene ALLOYS IN THE PRESENCE OP AMMONIUM SALTS. 101 that purplish-brown crystals separated in the liquid.When water was added they disappeared and nothing remained but silver chloride. A specially good yield of these crystals was obtained when 2 grams of an alloy of the composition Au 625 Ag 375 were treated with a mixture consisting of 10 grams of ammonium chloride 3 grams of ammonium nitrate 5 C.C. of concentrated nitric acid and 15 C.C. of concentrated hydrochloric acid. The alloy in the form of thin sheet was heatTed gently in a beaker with the above mixture. When all the metal was dissolved the bottom of the beaker was found to be covered with very small, purplish-brown crystals. They were instantly decomposed by water with the formation of silver chloride and chloroauric acid, but were found to be fairly stable in conceiitrated hydrochloric acid.When prepared in this way it' was found difficult to prevent the crystals being contaminated with ammonium chloride. The following method was theref ore devised. Preparation of the Salt.-Twenty-five grams of gold were dis-solved in 50 c . ~ . of nitric acid and 150 c . ~ . of hydrochloric acid, the liquid being then saturated with about 30-35 grams of ammonium chloride. Three grams of silver nitrate dissolved in 10 C.C. of water were then added and a copious precipitate of silver chloride formed in the liquid. On adding a few crystals of ammonium chloride the silver chloride began a t once t o change into brown crystals. On heating the liquid the crystals were con-verted into silver chloride and on cooling again reappeared.This change could be repeated any number of times. The crystals were allowed t o remain in contact with the mother liquor for two or three days t o ensure all the silver chloride being converted into the brown salt. The mother liquor was then poured off saturated with ammonium chloride and a further amount of silver nitrate added to the solution. A second crop of crystals resulted and these were heated and allowed t o cool as before. The formation of the salt in a hot solution gives rise to larger and better formed crystals than when the preparation is made in a cold .solution. The process was then again repeated four preparations in all being obtained before the gold became exhausted in the mother liquor. The crystals were freed from the mother liquor in a centrifuge and afterwards dried at looo.They were then placed in a Soxhlet thimble and extracted with ether (distilled from sodium) until no more gold was removed. They were afterwards heated a t 1 5 5 O in an air-oven for some taime and the remainin 102 THE ACTION OF AQUA REQIA ON GOLD-SILVER ALLOYS ETC. traces of ammonium chloride sublimed away. A second treat-ment with ether followed after which the crystals were dried and analysed. Found Ag = 15.58 ; Au= 37.99 ; NH4= 6.88 ; C1= 39-44. 3AgC1,4AuC1,,8NH4C1 requires Ag = 15 - 62 ; Au = 38.06 ; NH4 = 6-97 ; C1= 39.35 per cent. Crystallographic E'xamination. The crystallographic examination was made by Miss I. E. Knaggs in the Miiieralogical Laboratory of the University of Cambridge under the direction of Dr.A. Hutchinson for whose help and assistance the author wishes t o express his gratitude. Crystal System.-Orthorhombic. Class.-Holohedral. Axial Ratio.-a b c=0-5376 1 0-3210. Forms Observed.-B(Ol O) M( 1 10) e( 101). No. of measure - Mean Calcu-Angle. ments. Limits. observed. lated. mB =(010) (110) ...... 4 61'43'-61'45' mm//'=(ii~) (iio) ...... 7 56'28'-56"35' me =(110) (101) ...... 5 63'10'-G3' 11' ee' =(101) ( i o l ) ...... 3 61'39'-6 1'45' 61 '44' 61 '44' 56'32' -61'42' 61"41' 63OlOi' -d a b i t .-The crystals available for measurement consisted of minute prisms sometimes modified by the brachypinakoid (OlO), and terminated a t either end by the macrodome (101). Some of the preparations consisted of larger crystals showing (110) an INTRAMOLECULAR REARRANGEMENT OF ALKYLARYLAMINES.103 (010) in approximately equal development. and were not suited for accurate measurement,. These had rough ends Cleavage .-None observed. Optical Characters.-The crystals were very dark red by reflected light and owing to their intense absorption it was difficult to make determinations of their optical characters. Some of the smallest and thinnest prisms when mounted so that light traversed them along the P axis we're found to be strongly pleochroic, absorption being almost complete for rays vibrating parallel to the axis of 2 whilst red light was t8ransmitted fairly freely by vibrations paralled to the axis of X. When the crystal was rotated so that light traversed it along the X axis strong absorption was observed of the ray vibrating parallel to P as well as of that vibrating parallel to 2. No characteristic interference figure could be observed in convergent light nor was it possible to determine the refractive indices either by the prism methods or by total reiflection. By the immersion method they were found 'to be high -greater than 1.74. GOVERNMENT LAB ORATORY, CAIRO. [Received October 17M 1919.