Determining Hygroscopicity
OF
Fertilizers
J. Y. YEE Bureau of Plant Industry, Soils and Agricultural Engineering,
U. S.
Department of Agriculture, Beltsville,
Md.
A rapid method is described for determining the hygroscopicity of fertilizers b y measuring the relative humidity of the air in equilibrium with the mixtures.
From curves showing these
values against the moisture contents, the storage quality of fertilizers can b e determined.
T
HE hygroscopicity of a fertilizer, or its tendency to absorb moisture, determines to a large extent whether it will remain
drillable under humid conditions and whether it will cake on storage. A rapid and accurate method for measuring the hygroscopicity of a fertilizer would, therefore, be a means of predicting the behavior of the fertilizer under practical conditions.
e
x
"1
1E
80-
84
-
82
-
,
I
I
however, vi11 riot always give the exact value6 because it ib impractical to prepare humidity chambers to cover the entire humidity range in narrow intervals. The method described in this paper, however, permits accurate determinations of the hygroscopicity of fertilizers in about 30 minutes with no weighings and without the humidity chambers required for the usual method. Over each fertilizer mixture, there must exist a partial aqueous vapor pressure corresponding to the vapor pressure of the complex solution contained in that particular fertilizer mixture. If, therefore, a fertilizer mixture is kept in a closed container until equilibrium is established a t a desired temperature, and a method is found t o determine the relative humidity over the sample, a measure of the hygroscopicity of the sample will have been obtained. For measuring the relative humidity in a small ellclosed space, the electric hygrometer as developed by Dunmore ( 8 ) of the National Bureau of Standards, was found to be most satisfactory. The electric hygrometer unit consists of a moisture-sensitive film containing lithium chloride on a bifilar coil of palladium wire wound on a thin-walled polystyrene tube. The resistance between the two terminals of the electric hygrometer varies with the relative humidity to which the unit is exposed. The humidity-resistance calibration curve for one such unit is shown in Figure 1. I t requires five units with moisture-sensitive films containing various amounts of lithium chloride to cover the whole humidity range.
0
.e
0
0 70a L
0 C
2 p"
76-
74 -
APPARATUS 72b
I
I
I
I
2
3
Capacity Meter Reading,Figure 1
.
The arrangement of the apparatus used for this deterrnination is very simple, as shown in Figure 2. A is a 16-ounce bottle about 1/3 full of a fertilizer mixture, B. C is a n e l e c t r i c hygrometer unit. D and D' are terminals of the unit coming through the two holes in the rubber stopper. These terminals are soldered to copper tips F and F' on top of the glass rods,, G and G ' , coming through the same openings in the rubber stopper to hold the unit in place.
1000
Relative Humidity-Resistance Curve
A fertilizer material will absorb moisture from the surrounding atmosphere if the relative humidity of that atmosphere is greater than that corresponding to the vapor pressure of a saturated holution of the fertilizer material a t the same temperature, and will lose moisture at relative humidities below that value. The aqueous vapor pressure of a saturated solution is thus a measure of the hygroscopicity of the solid in question. Adams and Merz ( 1 ) have determined the hygroscopicity of a large number of pure fertilizer compounds and their mixtures by measuring the relative humidity over their saturated solutions by means of an isoteniscope. This method, however, requires the evaporation of a large quantity of water from the liquid phase and is, therefore, not applicable to mixed fertilizers which generally contain only a small amount of moisture. The withdrawal of much water would disturb the equilibria in such systems. Ordinarily, when it is desired to determine a t what relative humidity a fertilizer mixture begins to take u p moisture, a number of tared samples are exposed to various known relative humidities in controlled-humidity chambers. The relative humidity a t which the sample just begins t o gain weight is noted. This is the threshold value above which the fertilizer mixture will absorb moisture and below which it will not. This method,
F i g u r e 4. E l e c t r i c Hygrometer Unit Assembly
367
A=
INDUSTRIAL AND ENGINEERING CHEMISTRY
368
~~
Table Fertilizer Jlirture NO.
I.
Relative Humidity over Fertilizer Mixtures at 30" C. Relative Humidin;,< Moisture Absorbed a t Equiover Sample by Electric H~~~~~~~~~ librium a t Relative Humidity of Method 39.4Cj, 65.2% 72.57,
1 2 3
60.2 59.7 59.1 65.6 65.7 72.6 55.6 < 51.4 64.3 61.9 61.4 67.6 71.6 73.2
4
5
t
;? 9
10 11
.
12 13
11
5.57 11.66 7.26 0.40 -0.18 -1.21 17.08 12.99 2.18 5.12 3.20 -1.38 -1.82 -1.77
-0.27 0.01 -0.34 -1.31 -1.59 -1.67 10.23 0.04 - 1.20 -0.89 -2,J.I -2.23 -2.31 -1.99
14.36 23.32 14.32 14.96 9.70 -0.55
Vol. 16, No. 6
the fertilizer will take up a t a given relative humidity and a t what relative humidity the absorption of moisture will begin with a given moisture content in the fertilizer. Figure 3 s h o w three curves, somewhat idealized, that rep1.e$ent relative humidity conditions over three t'ypes of fertilizers designakd as A, B, and C. .4n interpretation of these curves will rerve to illustrate the points mentioned above.
26.07
22.34 15.65 12.95 11.17 10.44 5.43 -1.28
PROCEDURE
The procedure for making a determination is also very simple. -4n electric hygrometer of the proper humidity range is inserted in the bottle containing the fertilizer sample to be tested, as shown in Figure 2, and the whole assembly is allowed to stand for about 20 minutes to come to equilibrium. The resistance between the electric hygrometer terminals is then measured by connecting F and F' to a resistance meter (notShown), or a Weston Model 764 capacity meter. The capacity meter may be used as an ohmmeter, since it is measuring only ohmic resistance in a nonreactive circuit ( 2 ) . 9 steady reading of the meter signifies that equilibrium has been established. The relative humidity over the fertilizer sample is then read off from the calibration curve for the unit used, like the one shown in Figure 1. These measurements can be conveniently made in a constant-temperature room, or carried out elsewhere, provided a thermometer is inserted through the rubber stopper to record the 'temperature of the sample a t the time of the measurement. Enough time should be given for the samples to come to equilibrium. E X P E R I M E N T A L RESULTS
In order to test the validity of this method, the relative humidities over a number of well-cured fertilizer mixtures Fere measured a t 30" C. by the electric hygrometer method. The equilibrium moisture absorption values of the!e fertili7w at 59.4, 65.2, and 72.57; relative humidities a t the same tempei,. ire had previously been determined. The results obtained for t,he*e fertilizer mixtures are tabulated in Table I. These data show that this method gives c.on>istent results agreeing closely with those obtained by the moisture-Rbsorption method. Results showing the effect of temperature on the relative humidit,y over a fertilizer mixture are tabulated in Table 11. They reveal that the relative humidit) over a fertilizer increwes with increase in temperature. Well-cured fertilizer sample.: only should be used in making these determinations, because tlic relative humidity over ii raw fertilizer mixture changes as the reactions progress between the various component,s in the mixture to form more stable salt8 (3).
Table 11.
Effect of Temperature on Relative Humidity over a Fertilizer Mixture Relative Humidity over Fertilizer Temperature
c.
in moisture content. Fertilizer C, intermediate between .4 and B , is a fairly good mixture because in an atmosphere of 70'% relative humidity its moisture content cannot be more than 6%; otherwise the solution present in the fertilizer will have a relative humidity higher than 70Y0, in ivhich case the fertilizer will lobe moisture. DISCUSSIONS
INTERPRETATION
OF
RESULTS
By means of a plot showing the relative humidity over a fertilizer against its moisture content, it is possible to judge ( a ) ahet,her or not the fertilizer contains a large amount of hygroscopic components and whether these components are all in solut,ion or largely in the solid st'at,e, and ( b ) the amount of moisture
I n actual practice, this type of curves may not alway5 come :I.: iegular as these. It, is ~vellknown that when a fertilizer once gets wet and then dries again, it becomes a little more hygroscopic than the original mixture. This shows up more with mixtures containing soluble components in large granular forms than t,hose having such components in the fine state. On account of this, the curve obtained by introducing moisture to t,he mixturc may
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
-
~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 Water Quality Laboratories,
WESLEY S. PLATNER U. S. Fish and Wildlife Service, University of Missouri, Columbia, Mo.
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 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
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.
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", 8 t h 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).
LOW'S STARCH.5YEARS
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", 8 t h 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.
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,
TT-inkler, L. TV.. R e i . , 21, 2848-54 (1888).
100
w
z
- 804 W
-
---
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
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
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.
