ANALYTICAL EDITION
lune, 1944
solid state. On the other hand, a wet fertilizer having high relative humidity over i t may be a good one, provided its moisture content can be reduced to a satisfactory value.
not quite coincide with the curve obtained by removing moisture from the wet mixture. Once equilibrium has been established, however, the values will become constant. In routine analysis, a single relative humidity measurement is enounh to classifv a verv wet fertilizer havine low relative humidity over i t as unsatisfactory, because even with such high mois(-ontent, this fertilizer still contains a concentrated solution of its hygroscopic components and some of them may still be in the
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~C,
369
L I T E R A T U R E CITED
(1) Adams, J. R., IXD. ENG.CHEM..21, 305 (1929). (2) Dunmore, F. B'., J . Resea?& S a t [ . &r. Standards, 23, 701 (1939). (3) Merz, A . R., Fry, R. H., Hardesty, J. O., and Adams, 6. R . , I N D . EWG.CHEM., 25, 136 (1933).
Stable Starch Solution for Dissolved Oxygen Determinations WESLEY S. PLATNER Water Quality Laboratories,
U. S.
Fish and Wildlife Service, University of Missouri, Columbia,
N
UMEIIOUS starch solutions for use in iodometric titrations including the IVinkler (15) determination of dissolved oxygen h a w h w n described (1, 4,8,9, 13, I d ) , but the
Mo.
Onts lot of this starcah solution wax used by tlie writer over a 12-nionth period without deterioration, mold growth, loss of potency, or production of reddish color in iodometric titrations for dissolved oxygen. The indicator properties of several starch solutions were compared photelometrically by titrating to final end-point partitioned samples of water prepared by the Kemmerer (T) met,hod for the iodomet'ric determination of dis\olvcd oxygen, in a Cenco Photelometer (Figures 1 and 2). Thi.; method of preparing starch solution lends i t d f to use in tilt: field, yields a solution which develops and maintains greater depth of color per unit, and has a sharper end point than ' any of tht, starches tested.
preparation of many of these invo1vt.s elaborate procedures recluiring cxact quantit,ies of reagent>. Various starch solutions have been criticized ( 2 ) because on aging they frequcmtly produce a reddish or violet color with iodine, which prevthnts charp end-point rmtlings. Thci method dewrit)cd hrre not only
NICHOLS' STARCH. FRESH ALK ALINE STARCH.FRESH
L I T E R A T U R E CITED
dlsberg, C . L.. and Griffing, E . P.. J . A m . Chem. S o c . , 53, 1401-2 (1931). Am. Puhlic Health Assoc., "Standard Methods for Examination of Water and Sewage", 8th ed., p. 231, 1936. Brooks, H. E.. Chem. A n a l y s t , 27, 9 (1918). Frericha, G., A p o t h . Ztg.. 43, 599-600 (1928). Jaiiihuser~vala.G. B., J . Textile Inst., 32, T201-8 (1941). Kano, N.,J . C'hem.Soc. (Japan). 42, 9745 (1921). Kemmerer, G., Bovard, J. F.. and Boorman, W.R., L-. S. Bur. Fish., Bull. 39, 51-140 (1923-24). Low, Ai.H., "Technical Methods of Ore .Inalysis", 8th ed., 0. 86, Sew York, John Wiley &: Sons, 1919. Nirhols, M. S., IXD.EKG.CHEM..AXAL.ED.,1, 215-16 (1929). Pollits, Z., 2. angezc. C'hem., 30, 1, 132 (1917). Reyrhler, A , , Bitll. soc. chim. h e i g . , 29, 118-222 (1920). Shapiro, C. S., J . Lab. Clin. .Wed., 20, 195-8 (1934). Spasskii, K.,Chem. Zentr., I, 3294 (1938). B.. -l,.ch. Pharm. C'hem., 41, 533-8 (19341. TT-inkler, L. TV.. R e i . , 21, 2848-54 (1888).
LOW'S STARCH.5YEARS
4
MlLLiLiTERS
Figure 1.
N ~ SODIUM m THiOSULPHATE
Photelometric Comparisons of Indicator Efficiency of Starches
Free iodine i6 similar samples was reduced b y thiosulfate titration in photolometer to a reading of 92, exactly 0.2 ml. of test starch added, and titration continued to disappearance of blue starch-iodine color.
eliminates the ncwl for laboratory cuiiveniences and weighed reagents but yield$ a ,tarch solution xhich rcniains unchanged a year or more. .\IE.iwoD. .Idvantage is taken of the property of ruiistic alkali to dissolve the coating on the starch griiins iyithout affecting the starch itself (3, 5 , 10, 11, M). .I 20% solution of $odium or potassium hydroxide, u r tlie solid caustic, is added with stirring t o a bii;.pcnsion of about 2.0 grams of powdered starch in 300 to 400 nil. of \niter until a thick, sirupy, almost clew solution is obtained. .\bout 30 ml. of 20% potassium hydroxid(: arc required to treat approximately 2 grams of potato starch. The solution is nllon-ed to stand for about 1 hour to enbure complete action by tlie alkali, and then made neutral or slightly acid nit11 concentratid hydrochloric acid ( G ) , using litmus papw as indicator. This product is designated :I- "alkaline, htarch". If acidity does not interfew x i t h the proposed titration (the final sample in dis,solved oxygen titration by the Winkler method is ml. of glacial :icetic acid is addrtl as a
100
w
z
- 804 W
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N I C H O L S ' STARCH LOW'S STARCH
Iy
'1 ;;
C O M M E R C I A L STARCH ALKALINE STARCH
J
1.66
MILLILITERS
Figure 2.
Nfloo SODIUM T H I O S U L P H A T E
Photelometric Comparisons of Indicator Efficiency of Starches in Quantities Producing Equal Color Intensities
In similar 50-ml. samples free iodine was reduced b y thiosdfate titration in photolometer to a reading of 82. Sufficient starch solution (1.51 ml. of commercial Lintnrr's 0.60 rnl. of Low's, 0.41 ml. of Nichols', 0.96 ml. of alkaline) was added to produce photeloheter reading of exactlv 0, and titration continued to clear end point.
