Am.  Jour.  Pharm. 
Dec,  1887. 
Gold  Sulphides. 
617 
GOLD  SULPHIDES.1 
By  L.  Hoffmann  and  G.  Kruss. 
The  statements  given  in  various  text-books  concerning  the  sulphides 
of  gold  are  very  conflicting,  compounds  Au2S,  Au2S2,  and  Au2S3,  be- 
ing variously  given.  The  sulphide  Au2S2  is  perhaps  usually  ac- 
cepted, the  existence  of  the  other  two  very  often  disputed.  These 
conclusions  are,  however,  by  no  means  justified.  Berzelius  believed 
he  obtained  Au2S  by  passing  hydrogen  sulphide  into  a  boiling  solu- 
tion of  auric  chloride.  Levol,  on  the  other  hand,  states  that  under 
these  conditions  free  gold  is  alone  deposited.  The  authors  find  that 
LevoPs  statement  is  correct,  if  care  is  taken  to  keep  the  temperature 
of  the  whole  solution  at  100°.  If  local  cooling  takes  place  the  pre- 
cipitate contains  varying  proportions  of  combined  sulphur,  but  no 
definite  compound  can  be  obtained.  In  all  their  experiments  the  au- 
thors washed  the  precipitated  sulphide  by  decantation  with  water,  al- 
cohol, ether,  and  carbon  bisulphide  successively.  They  found  that 
the  free  sulphur  was  retained  very  firmly,  and  could  not  be  com- 
pletely removed  by  washing  on  the  filter. 
When  hydrogen  sulphide  was  passed  through  a  solution  of  potas- 
sium aurocyanide,  no  apparent  change  took  place,  but  when  excess  of 
hydrochloric  acid  was  added,  and  the  whole  heated,  aurous  sulphide, 
Au2S,  was  precipitated  as  a  steel-gray  precipitate.  This  was  carefully 
washed  as  above,  and  obtained  in  a  dry  state  as  a  brownish-black 
powder  of  constant  .composition,  corresponding  with  the  above 
formula.  When  freshly  precipitated,  it  dissolves  in  water  to  a  brown 
solution.  It  is  therefore  necessary  in  purification  to  wash  it  with 
water  containing  hydrochloric  acid,  in  which  it  is  not  soluble.  When 
once  dried,  it  is  no  longer  soluble  in  water.  It  is  not  decomposed 
when  boiled  with  dilute  hydrochloric  or  sulphuric  acids.  Aqua  regia, 
chlorous  oxide,  and  other  oxidizing  agents,  oxidize  it  easily.  Bromine- 
water  slowly  dissolves  it,  with  formation  of  AuBr3  and  sulphuric 
acid.  Alkaline  monosulphides  dissolve  it  but  slowly  and  slightly, 
polysulphides  rapidly  and  completely,  with  the  formation  of  green 
solutions  of  sulpho-salts.  Caustic  potash  solution  does  not  attack  it 
even  at  100°,  whereas  the  compound  Au2S2  is,  under  like  conditions, 
decomposed  into  gold,  potassium  gold  sulphide,  and  potassium  gold 
1  Ber.  d.  D.  Ch.  Ges.,  1887,  p.  2369 ;  reprinted  from  Jour.  Chem.  Soc,  No- 
vember. 
