Use of Karl Fischer Reagent for the Determination of Water in Condensates of Sodium Tetraborate and Diols SIR: Karl Fischer reagent provides a rapid and quantitative procedure for determining water in most organic materials and many inorganic materials (1). Side reactions in inorganic procedures are more frequent. Therefore, knowledge of the stoichiometry of these reactions is desirable. Mitchell et al. presented the results of comprehensive and exacting studies of numerous reactions involving the use of Karl Fischer reagent with inorganic oxides and Mitchell and related materials (3). Smith also published a text in which they described t h e various applications and reactions of the Karl Fischer reagent (2). Equations published in (2) and (3) show that sodium tetraborate in the presence of methyl alcohol reacts with 7 moles of 1 2 and SO2 (reactive ingredients of Karl Fischer reagent). During a study of condensates of sodium tetraborate and diols, i t was discovered that there is little reaction between Karl Fischer reagent and the condensates. This shows t h a t the reaction of sodium tetraborate with diols precludes the reaction described by Mitchell et al. and allows determination of water in the condensates.
c
0
20
40
60
20
40
Fischer reagent with the standard water in methanol. The number of milliliters of standard water in methanol required to titrate the sample is subtracted from the number of milliliters of standard water in methanol required t o titrate the blank. This value is equal to the number of milligrams of water or “water equivalent” in the sample. The per cent water or “water equivalent” is calculated by dividing the milligrams of water by the weight of sample (in milligrams) and multiplying by 100.
A 0.1-gram sample is placed in a 125-ml. Erlenmeyer flask; 5.0 ml. of Karl Fischer reagent is added. The sample is stirred and immediately titrated at room temperature with standard water in methanol (1 ml. = 1 mg. of water) until the end point, characterized by the disappearance of the brown iodine color, is reached. A blank titration is similarly performed by titrating a 5-ml. portion of the Karl ~~
Water Equivalent Determinations on Condensates of Sodium Tetraborate and Diols
Condensate Sodium tetraborate decahydrate/l,2propylene glycol Sodium tetraborate pentahydrate /1,2propylene glycol Sodium tetraborate decahydratelethylene glycol
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1 part diol, 0.2 parts tetraborate (hydrates) Apparent water ex ected Water.of From hydration NalBdOr calcd., 7’ calcd., Yo
ANALYTICAL CHEMISTRY
BO
100
120
140
160
180
Figure 1. Water equivalent vs. time of heating of a solution of sodium tetraborate decahydrate dissolved in 1,2-propyIene glycol
EXPERIMENTAL
Table 1.
60
TIME IN MINUTES
Water equivalent determined, % Before Before After condensa- evapora- evaporation tion tion
7.9
5.5
13.2
8.0
0.4
5.2
7.2
12.1
5.6
0.4
7.9
5.5
13.3
10.2
1.9
Direct titration of the sample with Karl Fischer reagent to a brown iodine end point is possible. However, the end point received by the “back-titration” procedure, described above, is much sharper and more reproducible. A 0.1-gram sample of C.P. sodium tetraborate decahydrate mas dissolved a t room temperature in absolute methanol. Karl Fischer reagent was added and the sample was immediately titrated with standard water in methanol by the described procedure. The calculated water equivalent was 77.5%. A 40-gram sample of C.P. sodium tetraborate decahydrate was dissolved in 200 grams of C.P. 1,2-propylene glycol in an open beaker a t room temperature. A water equivalent determination was made by the described procedure on a 0.1-gram sample removed from the reaction beaker. The beaker was heated at 85’ C. for 1 hour and then a t 105’ C. for 3 additional hours, a t which time solids began to form. Water equivalent determinations were made a t 20-minute intervals throughout the heating period. Figure 1 represents the curve of water equivalent versus time of heating for this experiment. Water equivalents varied from 13.2% after solution of the sodium tetraborate a t room temperature down to 0.4% a t the time when solids began to form.
RESULTS A N D CllSCUSSlON
The first three columns of Table I show that sodium tetraborate reacts with I h l Fkcher reagent as previously reported Table 1 arid Figure 1 show t h a t hen \odium tetrabrorate ii cheniically bound in a diol conden-ate the Karl Fi-cher reagent reacts with free water only. Similar experimentq. haT e been conducted in which soilium tetraborate
pentahydrate was substituted for the decahydrate. Experiments h a r e also been conducted where ethylene glycol was sub4ituted for 1,2-propylene glycol. Rewlts are included in the table. The experiments indicate that the Karl Fischer reagent can be utilized in the determination of water in condensates of >odium tetraborate and diols. I n these condensates, the degree of condensation can be predicted by the Karl Fischer reagent.
LITERATURE CITED
(1) Fischer, K . , Angew. Chem. 48, 394-6 (1935). ( 2 ) Mitchell, J., Smith, D. M., “hquametry,” Interecience, Kew Tork, 1948. (3) Mitchell, J., Smith, 11. hI., Ashley, E. C., Bryant, W. M . I)., J . A m . Chem. SOC.6 3 , 2927-30 (1941).
CHARLES B. JORDAR. U. S.Army Coating and Chemical
Laboratory Aberdeen Proving Ground, Md.
Spectrophotometric Determination of 2-Phenylindole with p-Dimethyla minobenzaldehyde SIR: A violet,-colored compound is formed when 2-pheny1:ndole reacts with p-dimethylaminobenzaldehyde (Ehrlich’s reagent) in acid alcoholic solution. The color intensity is proportional to the amount of 2-phenylindole present. It is known (2-6. 8) that p-dimethylarnino1)enzaldehyde can be used as analytical reagent for the detection of pyrrolc derivatives ,vhich have an intact CH-group in thmf 01 or /3 position relative to the cyclic XH-group. The conditions for a quanti1,ative application of the reaction between the reagent and 2-phenylindole have b.en investigated. EXPERIMENTAL
Apparatus. .I I k c k m a n DU spectrophotomrtrr with 1-em. cells is used for all ahsorbance me:tsurements. Reagents. Reagent grade chemicals are used throughout’. ~ - P H E N I L I N D ODissolve LE. 20 mg. of sublimated 2-phenylindole (Bayer), twice recrystallized from methyl alcohol, (m.1). 192’ C.) in l liter of methyl alcohol. Use dilutions of this solution t o make standards. p
-
dole are shown in Figure 1. The P D A B ’ H C l does not absorb in the range betriveen450 mp and 700 mpc,while the reaction product with 2-phenylindole exhibits a maximum absorbance a t 560 nip and a weak absorbance a t 365 mp. The minimum detectable amount of 2-phenylindole by this method is about, 0.2 gram per liter X 10-3. The color development can be obtained only in acid solution. KO Pignificant differences could be detected by increasing the hydrochloric acid concentration in the methyl alcohol solution. J17hen the reaction is performed varying the PDAI3 concentration, no significant differences can be detected working with a large molar amount of PDAB. At lower concentration less color develops.
After the addition of PDAR the color develops immediately and its intensity increases slowly with time. ilfter 30 minutes the color attains its maximum intensity. This remains stable for one hour, then decreases slowly. the decrease
Table I.
Absorbance Data
g.11. x 10-3
A
1 2 3 4 6 8 10
0.085 0.160 0.245 0 325 0,480 0 635 0.801
r)Ihl~~THTLAJIINOBI.:NZALDEHYUE.
Dissolve 2.50 grams of p-dimethylaminohenzaldehyde (l’D.lB)] reagent grade (C. Erha), in 1 lit,er of methyl alcohol containing 25 i d . of 37% hydrorhloric acid. Use’ the solutions immediatcly aftcr th’3ir preparation. Procedure. To 5 ml.. of a n alcoholic solution of 2-phenylindole. add 1 ml. of :I 0.257c I’D113 solution in methyl alcohol ; the final concentration of 2-phenylindole ranges from 1 to 10 grams lwr liter X 1 0 - 8 . Shake the mixturc and measure the color develol)r~iaftrr 50 minutes on a Beckman 1)L sl)ectroi,hotometer at 560 mp vs, intthyl alcohol using 1-cm. cell.. WAVELENGTH,
RESULTS A N D DISCUSSION
T h e nbw-ption spectra of the alcoholic wlutions of I’DBU. HC1 and the reaction product v;ith 2-phenylin-
Figure 1. __--
__
mp
Absorption spectra
PDAB. HCI (2.5 grarnr/liter) Reaction product ( 1 4 grams/liter X 1 0-3 2-phenylindole) VOL. 36, NO. 2, FEBRUARY 1 9 6 4
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