ASSAY OP ARSENICAL DIPPING FLUIDS. 5 



eled any considerable distance the mischief probably has already been 

 done before its receipt. The growth of microorganisms, and conse- 

 quently the change in degree of oxidation, can be inhibited by the 

 addition of an appropriate antiseptic, such as formaldehyde, care- 

 fully added from a medicine dropper, in the proportion of 5 drops 

 of the commercial 37 per cent solution to each 100 c. c. of bath. 

 Carbolic acid and mercuric chlorid are wholly inappropriate for the 

 purpose. Also since oxidation obviously can not take place in the 

 absence of air, it is best to fill the bottle nearly full with the sample, 

 leaving only a little air space, cork, and cover the cork and lip of 

 bottle (which must be dry) completely with melted sealing wax, 

 paraffin, rosin, or some similar material. Whatever means are 

 employed to preserve the sample the operations ought to be executed 

 at the vat side immediately after taking the sample. A few matches 

 will furnish the heat necessary for melting the sealing material. 



No sample should, of course, be taken from a vat until the contents 

 of the latter have been thoroughly stirred up. Aside from general 

 reasons for this practice in all sampling it is entirely possible that the 

 oxidation of an arsenical bath standing at rest in the vat may proceed 

 from the surface downward. That is, the upper few inches in free 

 contact with air might have become almost entirely oxidized while 

 toward the bottom the bath might be still in its original condition, 

 or might even be undergoing reduction. 



The first step for the analyst, therefore, is to determine whether the 

 sample has been properly taken and properly preserved during 

 shipment. If the necessary preacutions have not been observed, 

 the analyst is justified in reporting only a "probable" or "provis- 

 ional" figure for "actual arsenious oxid." 



The final measurement of arsenic is made in all cases by titration 

 with standard iodin solution and starch indicator, much used by 

 chemists for a variety of purposes. The necessary solutions are the 

 following : 



(1) Starch solution. — Stir up about a gram of starch, best obtained 

 from the druggist, in about 20 c. c. of water and add the mixture 

 slowly to about 200 c. c. of boiling water. Continue gentle boiling 

 about 5 minutes. The solution should be freshly prepared every 

 day. 



(2) Diluted sulphuric acid (10 per cent). — Pour 100 grams (or 

 55 c. c.) of concentrated sulphuric acid slowly, in a small stream and 

 unth constant stirring, into 825 c. c. of water. 



(3) Standard solution of arsenious oxid. — Weigh out accurately 

 and exactly 2.5 grams of the purest obtainable white arsenic, and 

 about 10 grams of sodium bicarbonate, bring into a capacious beaker 

 or flask with about 200 c. c. of water, and boil gently until the arsenic 

 is all dissolved. Cool, and cautiously add dilute hydrochloric or 



