This paper tlcscrihes equipment (Figure 1) designed to permit simultaneous rollection of ga!: generated during thermal decomposition n ith its registration as a peak on the thermogram obtained on differential thermal analysis. The differential thermal analysis cquipnient (Figure 1) v, as very similar to that cievcloped and distributed by the R. L. Stone Co., Austin. Tex. A quartz "lead-out" tube was positioned over the sample holder, and led through the top of the furnace, through a stopper in the top of the furnace, and subsequently, through a stopper in the top of the bell jar. A thrre-n-ay stopcock located above thc bell jar permitted evacuation of the sampling manifold m d iample b o t t k and collection of the gas sample. d GE therinocouple vacuum gage tube was incorporated t o determiiic the degree of vacuum obtained. This system \ tlifferciitial thermal analysis of finely pon dered poly(viny1 chloride) received from Lucoflex C'orp. oi .\merica. Poly(viny1 chloride) WLS subjected to differential thermal analysis with an air atmosphere a t a p p r o h i a t e l y 3 inm. over the sample. The thermogram reproduced as Figure 2 was obtained, similar to that reported by Morita and Rice ( I ) . The gas evolved during the rcgiitration of the 300"
1 i 1
aC
1
' I
7L
I
1
4-
l
'
1 2C
* I r-
-I
L
100
endotherm \+a i collected and analyzed n i t h a GE aiialjtical mass spectronieter. The inass ipectrum shonrtl the principal tleromposition product to be hydrogen chloride. K h e n a similar analysis \\-as caondurted at approximately 400°, along the flat-topped exotherm, no hvdrogen chloride n R detected. The terhnique tlcscribed permits simultaneous deterinination of differential thermal analysis and collection of samples of gas evolved during the thermal reactions associated n it11 peaks on the thermograms. The technique is ideallj wited for experimmts conducted in vacuum. However. the results presented, conducted a t lo\\ oxygen pres-ures, show a greater extension of application. This modification to the usual differential thermal analysis equipment can render it a mol e valuable tool for dudies of the mechanism of the degradation of polymerq and complex inoi ganic materials.
V 300
400
500
TEMPERATURE,
Figure 2.
LITERATURE CITED
__
1
200
60C
700
'C.
Differential thermal analy-
sis of poly(viny1 chloride)
(1) Morita, H Rice. H 11 , AK'AL. CHEM. 27, 336 (1955 1 ( 2 ) xotz, K. J Jnffe, H. H., J . .im. Ceram. Soc., in press. (3) Powell A , J Sci. Instr. 34, 225 (1957)
Saturated Salt Solutions for Static Control of Relative Humidity between 5 " and 40" C. Louis B. Rockland, Fruit and Vegetable Chemistry Laboratory, 263 South Chester Ave., Pasadena, Calif.
70x1H O L L ~ I )Iiumidit>
[ liunihcrs are eiiiploycci 111 the laboratory for studics on the stability of foods and feeds, the corrosion of metals, the physical proprrties of textiles, wood, and plastics, and in the preparation of moisture sorption isotherms. Sulfuric acid, glycerol, or saturated salt solutions me employed most frequently (8) for humidity adjustment. The two-phase (vapor-liquid) sulfuric acid or glj cero1 systems are relatively teinperatureinqensitive and can be used over a broad relative humidity range. However, the utility of two-phase systems is limited to conditions under n-hicli the test suhstances within the chamber do not have a significant independent water vapor pressure. Teqt suhstances which absorb or evolve u nter vapor modify the composition of the twophase systems and alter the ambient humidity. Saturated salt solutions are generally the m w t uqeful, as the three-phase (vapor-liquid-solid) systems are independent of changes in their total moisture content. Salt solutions suitable for maintaining a wide range of specific relative humidities a t miscellaneoiis
C
temperatures have been reported and tabulated (3-5, 7 , 9 ) . However, small temperature variations may cause significant changes in their equilibrium relative humidities. It is often difficult to find a series of stable salt solutions which have low temperature coefficients and nliicli yield the desired range of individual relative humidities a t two or more temperatures. Relative humidity variations with temperature have been determined for a number of saturated salt solutions (1, 8, 7 , 9). Several of these solutions have low temperature coefficients and give essentially invariable humidities a t ordinary laboratory temperatures. Additional stable salt solutions having low temperature coefficients were sought to supplement the list of available solutions, especially a t the lower relative humidities. Reagent or chemically pure glades of salts were employed without further purification. The salt solutions were prepared in 1-gallon, wide-mouthed glass jars using purified water (distilled followed by percolation through a mixed-bed ion exchange resin) and were waled with rubber-gasketed screw caps.
Table I. Effect of Mechanical Convection on Equilibration Rates of Electric Hygrometer with Atmosphere over Saturated Salt Solutions
Relative H?' !d'Time for Relative Humidit\ 11y Inrremetit, Minutes ' Increment, Lithi~inichloi.itlv Sodium hroniide ...
0.1
15
:i( 1
20
5 5
0 I 0 2 0 5
0.2 0.j 1.0 2
. .
25 !I,O 30 11 0.2 35 1!1 I.,? 4 0 -1 -10 40 4 10 0.7 45 .. 30 1 3 a Change from ambient relative humidity at 22.5' C. Sensing element was adjusted to an init,ial relative humidity of 11 and 597; at' zero time for trials x i t h sodium bromide arid lithium chloride solutions, respectively. Static values obt'ained without mechanical convection; dynamic values obtained with continuous mechanical agitation of atmosphere.
Relative liumitiitj. estimations n cre conductctl with 311 arrangcment of an electric hygi omc+r tlcvrihed previouiljVOL. 32, NO. 10, SEPTEMBER 1960
1375
(6). Mechanical convection was provided within the sealed jars b y a fan blade mounted between the sensing element and the salt solution, and coupled magnetically t o an external motor. The complete apparatus was operated at a constant temperature. I n addition, each chamber was equilibrated for several days at each temperature and immersed in a water bath during the relative humidity estimation. Previous studies on the hygroscopic equilibria of large particles, such as walnuts (6) , demonstrated the importance of providing mechanical convection within the chamber during the estimation of relative humidity. Simi-
Table
II.
Reproducibility of Relative Humidity Estimations
s o . of
Temperature, c. Range Average
Estimationsa
Salt Lithium chloride Sodium bromide
larly, the data presented in Table I indicate that mechanical convection of the supernatant atmosphere decreased the time to achieve equilibration from 10 to 50 times and facilitated rapid estimations of relative humidities. Continuous agitation (dynamic values) over extended periods caused a small undesirable temperature rise within the chamber. Values were most reliable and reproducible when mechanical convection was employed intermittently. Therefore, the fan was operated in a cycle of 20 seconds on, 600 seconds off, except a t 5’ C. when the cycle was changed to 30 seconds on, 300 seronds
10 11
22.5 to 24.1 22.4 to 23.3
22.3 22.9
Relative Humidity, Range Average 11.3 to 12.0 56.0 to 5 6 . 5
11.5 56.1
70 Std. dev. 0.26 0.18
-4lternate estimations of each salt solution during 60-day period.
Table 111.
Relative Humidity Variations with Temperature of Saturated Salt Solutions
Salt GROUPA Lithium chloride Potassium aretate Magnesium bromide nIannrsium chlorile Potassium carbonate Magnesium nitrate Sodium bromide Cupric chloride Lithium acetate Strontium chloride Sodium chloride Ammonium sulfate Cadmium chloride Potassium bromide Lithium sulfate Potassium chloride Potassium chromate Sodium benzoate Barium chloridc Potassium nitrate Potassium wlfate Disodium phosphatf! Lead nitrate
GROUPB Zinc nitrate Lithium nitrate Calcium nitrate Cobalt chloride Zinc sulfate a
__
-
5 O
IO”
16 26 32
24
31
33 .. 54 59 65 72 77 76
40”
11
31
23 31
11 23 30
11
23 31
23
:30
23 30
33
33
33
33
3%
3%
31
47 53
44 52
42 52 57 67 69 75 79 82 82 85 84
41 51 57 67 65 68
84 88
73 75 79 82 84 85 86
43 52 57 67 68 71 75 79 82 83 85 86
40
58 68 72 77 75 80 83 86 84 87
45 53 58 68 71 75 75 79 83 85 84 87
79 79 81 85 84
68 75 79 75 80 81 83
89 88 93 96 98
89 88 93 95 97
88 88 92 95 97
88 88 91 94 97
87 88 90 93 97
86 88 89 92 97
84 86 88 91 96
82 83 87 89 96
98 99
98 99
98 98
98 98
97
96 96
93 96
91b 95
43 61
43
41 55
38 49 56 67 90
31 41 54
24 31 51 62 86
21 19 48 59 85
19 11 46 57 84
81
83
..
14
..
59 66
, .
, .
95
93
A = author. 38” C. 18’ C.
1376
Relative Humidity, 5 at ’ C. r -0 15” 20’ 25” 30’ .3 i)
ANALYTICAL CHEMISTRY
13 24
60
73c 92
12
57
68
io
11
97
64
88
6G
-_ 10
51 57 67 64
Ref.
off, as heat effects were iiiinimized and equilibration rates were slower a t the loi\er temperature. CoiiPtant values were generally observed within 2 to 3 hours at 40’ C. and mithin 3 to 6 hours a t 5’ C. However, final readings were not taken until at least 8 hours, and generally not until after 12 hours of equilibration. The reproducibility of relative humidity estimations conducted in this manner is indicated in Table 11. tabulation of the variations with trrnperature of 15 salt solutions is prc5mted in Table 111, with a summary of data reported previously. The humidity data presented in Table I11 were interpolated from smoothed curves drawn through points observed a t 5’, 20°, 30’, and 40’ C., or from similar data obtained from the literature. Values interpolated from the curves are in good agreement with individual literature values a t the same temperature (3-5, 7 , 9). The salts solutions listed in Group -\, arranged in order of their relativt humidities a t 25’ C., vary less than 107, between 5’ and 40’ C., although variations of less than 5% may be seen for a number of solutions. Within the temperature range 20’ to 30’ C., essentially invariable (* 1%) relative humidities can be obtained for humidity increments of 1OYc or less TTithin the range 11 to !17% relative humidity. The salts listed i n Group 13 yield saturated solutions which have more extreme temperature roefficients. Although precise temperature control would be required for the salt solutions listed in this group, a wide range of relative humidities may be obtained a t specified temperatures with relativelv felv salts.
LITERATURE
CITED
(1) Carr, D. S., Harris, B. I,, Ind. Eng. Chmn. 41, 2014 (1949). (2) Griffin, R. C., Terhnical Association of the Pulp and Paper Industry, Yew Tork, T a p p i Data Sheet 109-1090 (Ikcember 1944). (3) Hodgman, C. D., “Handbook of Chemistry and Physics,” 36th ed., pp. 2309-10, Chemical Rubber Publ., Cleveland, Ohio, 1954-55. (4) Lange, N. -4., “Handbook of Chemistry,” 9th ed., pp. 1420-3, Handbook Publ., Saiidusky, Ohio, l95G. (5) Richardson, G. M., llalthus, R. S., J. .lppl. Chem. (London) 5 , 557 (1965) (6) Rockland, L. R., Food Research 22, 604 (1957). ( 7 ) Spencer, €1. M., “Internationa Critical Tables of Sumeriral Data. Physics, Chemistry and Technology,” Vol. 1, pp. 67-8, 1st ed., McGraw-Hill, New York, 1926. (8) Wexler, 9.,Brombacher, W. U., Natl. Bur. Standards (U. S.), Circ. 512 (1951). (9) Windsor, W. E., Sobel, F., Morris, V. B., Jr., Hooper, 11. V., Reo. Sci. Instr. 24,334 (1953).
