4 
THE VOYAGE OF H.M.S. CHALLENGER. 
Determination of the Chlorine. 
( Iil..; m. . in this section of the memoir, means total halogen calculated as chlorine.* 
1: , it ions might have been made by means of the old-established gravimetric 
, r ■ - : bv the “ titrimotric ” t process which was founded, many years ago, by Gay- 
I ii the same reaction. This latter method would naturally suggest itself to 
Dg thi process for the case in hand, if it were not for that beautiful 
■ a n, :i. 1 ..f silver-titration which was introduced, some years ago, by Volhard, and 
- r< irards elegance and ease of execution, is superior even to Gay-Lussac’s. 
1 j.r a- is known, consists essentially in this, that after precipitation of the halogen 
• ' f standard nitrate of silver, t he silver still dissolved is “ titrated ” by means of a 
standard snlphocyanate-eolution ill the presence' of iron-alum, the appearance of a perma- 
• r<*d colour (due to Fe(NCS) 3 ) marking the end of the reaction. Volhard himself 
v it< ' this titration without removing the precipitate of chloride. I found itimpossible 
• n jM-rf'-ctly sharp results in this manner, and therefore adopted it only for pre- 
rv d< t- rminations, the final titrations being executed in the following manner: — 
• ■ f -■ i- water were measured off into a tared phial of about 200 c.c. and weighed ; and 
: :■!■’! it ion of some pure water, there was added a slight measured excess of standard 
•• r 1 it ion, and the Wright of the silver solution ascertained and marked down. Enough 
• : - th<-n added to bring the total volume up to very nearly twice that of the standard 
• ■ r u-< d, the ingredients carefully mixed, then violently shaken together, and 
• hi.il put i'ide into a dark cupboard. After the lapse of half a day or a night, the 
: • h id - tth d so completely that the supernatant liquor could be decanted off 
’ • ■ 1>. ih. r without filtration, and so completely that a correction for the part adhering 
t 1 ■ .rid • w.n necessary only in the few cases where, by mistake, a somewhat large 
• f -tandard silver-solution had been added. The residual dissolved silver was 
n 1 ' ■/>imetriodfy t by titration with centesimal solutions of silver and sulpho- 
• 'dution- containing 1 *08 grms. of silver, and the equivalent weight of sul- 
• . p--r 1000 c.c. respectively), care being taken in all cases to determine the end- 
j" '■ dly by what we came into the habit of calling “zig-zag titration,” and to 
• ‘ it.- in of the last 3-4 end-point determinations (reduced individually to the 
1 f » ■ nt» simal silver added = 0) as a basis for the calculation. 
1 - v ' di *'it i<>n of the Solutions was effected as follows: — J 1 J 0 [KC1J = 7 , 459 grms. 
• of jiM.ta ium (prepared from purified chlorate by expelling the oxygen by 
'• ' the residue in water, adding hydrochloric acid, evaporating to dryness 
' ' in, and gently igniting until the weight remained constant) were 
’ ■■ 1 til'irine " to true chlorine, we the chapter* on “ Bromine” and on “ Alkalinity.” 
! ’ ■ f • • ;• t . • word n substitute for the custoinnry “volumetric,” which I could not well have 
«• I • a : » lotion* hy weight and not by volume. ( Gallic # . — Analyses It notations titrecs.) 
- . i - . , , ... ,i j n bracket* it means the weight of the respective element, radical or 
l r th* irtn'. !», when “O' 1 is taken ns representing 16 parts of oxygen. 
