2 BULLETIN 163, TJ. S. DEPARTMENT OF AGRICULTURE. 



absence of a test one faces the alternatives of strengthening slightly 

 used baths by guesswork or of discarding them entirely. A field 

 test is therefore essential to the prosecution of dipping in a manner 

 which shall be at the same time effective and economical. 



METHOD OF EXECUTING THE TEST. 



The test here described employs the well-known reaction between 

 soluble sulphids and iodin * in neutral solution, whereby sulphur is 

 precipitated and a metallic iodid is formed. It therefore directly 

 estimates, not sulphur, but the metal — in this case calcium — com- 

 bined with sulphur in the form of sulphid or polysulphid. Only in 

 case that sulphur is combined with metal in unvarying proportion 

 can the method also estimate exactly the amount of sulphur present. 

 Theoretically this requirement is not met in the case of lime-sulphur 

 baths, the ratio of lime to sulphur in the mixture of calcium poly- 

 sulphids which may be present being susceptible to considerable 

 variation. As a matter of fact, however, practical experience of the 

 Bureau of Animal Industry with the test in the field indicates that 

 the ratio in baths prepared after the formulas specified for use in 

 official dipping is near enough to a fixed figure to render the test of 

 entirely adequate accuracy for practical purposes. The ratio 

 provisionally adopted is 4.6 atoms of sulphur to each atom of calcium, 

 or, by weight, 147.5 parts sulphur per 40.07 parts calcium. 



Briefly, the method of test involves the addition of standard iodin 

 solution to a measured quantity of bath until the resulting liquid no 

 longer gives color with a dilute alkaline solution of sodium nitro- 

 prussid, showing that calcium polysulphid has been entirely decom- 

 posed. The amount of iodin added to reach this point is then a 

 measure of the amount of "sulphid sulphur" in the bath. The 

 outfit is pictured in figure 1, and the parts composing it will be 

 described in detail. 



PREPARATION OF THE OUTFIT. 



I. The case. — The carrying case for the outfit is a rectangular box 

 with a hinged cover, made of ^-inch oak, of inside dimensions 7 \ 

 by 5| by If inches. The interior construction, of softer wood, is 

 sufficiently indicated in the diagram. The case must be strongly 

 mortised or nailed together, not simply glued, and should be var- 

 nished or painted. 2 



1 Titration with iodin for determining the " monosulphid equivalent" of lime-sulphur dips seems to 

 have been first seriously proposed by Harris (Michigan Agric. Coll. Exp. Sta. Techn. Bull. No. 6, Jan., 

 1911). There may be some question regarding the accuracy of the method for exact laboratory analysis, 

 but the uncertainty is not of sufficient seriousness to affect its usefulness for the present purpose. 



2 The cases used by the bureau are painted yellow (yellow being the color of sulphur) to avoid confusion 

 with the similar test case used for arsenical baths (see Department of Agriculture Bulletin 76) which is 

 merely varnished. 



