(2) Coldstein, G., Manning, D. R., Menis,
ACKNOWLEDGMENT
Financial support from the Rescarch Corp. and the National Science Fomdation i R gratefully acknowledged. LITERATURE CITED
(1) Brown, W.B.,Steinbach, J. F., ANAL, CHEW31, 1805-6 (1959).
(7) Welcher F. J., "Organic Analytical Reagents," Vol. 3,p, 332,Van Nostrand,
O., Ibid., 31, 192-5 (1959). (3) Jacobs, W.D.,Yoe, J. H., Anal. Chim. A c h 20, 43543 (1959). (4) Paulin Linu? "The Nature of the Chemic&? Bond, p. 149, Cornell University Press, Ithaca, N. Y., 1980. (5) Pearse, G. A., Pflaum, R. R., ANAL. CHEM.32,213-16 (1960). (6) Vosburgh, W. C., Coo er, R. U., J. An.Chem. Soc. 63,437 rl941).
Princeton, N. J.
8 ) Ibid.,
fI'
[9) Yoe, iof'Barton, C. J., IND.ENQ. CHEM.,ANAL.ED. 12, 405-9 (1940). (10) Yoe, J. H.,Jonee, A. L., Ibid., 16, 111 (1944).
RECEIVED for review September 12, 1900. Accepted March 10, 1961.
Stability of the Curcumin Complex in Boron Determination D. EMERTON WILLIAMS and JAMES VLAMIS Deparimenf o f Soils and Plant Nutrition, Universiiy o f California, Berkeley, Calif.
b The determination o f , boron using curcumin-oxalic acid reagent i s somewhat limited b y a rather rapid hydrolysis of the complex at room temperature, necessitating reading of samples within a 2-hour period. The stability of the dry complex or of alcoholic solutions of the complex when stored at room temperature and at 0 ' C. was investigated. Samples stored in akohol were stable for 5 days at 0" C. but for only a few hours at room temperature. Dry samples stored up to 3 days at. room temperature and at 0' C. gave the same readings after 2 hours in alcohol as freshly dried samples. Samples stored dry at room temperature for 1 to 4 weeks had a 2-hour lag period before maximum color was regained. Because of this lag, dry storage samples should not be read sooner than 2 hours following alcohol addition.
ethyl alcohol, the stabilit was compared a t room temperature a n i a t 0" C. The sam les mere removed from storage, r e a x o n a Nett-Summerson colorimeter (540 mcc) at 2, 24, 48, 72, and 96 hours following alcohol addition, and returned to storage between readings. All solutions stored at 0" C. were allowed to reach room temperature just prior to reading on the colorimeter. A second group of samples was also separated into two sets, which were stored dry at room temperature and at 0" C. Complex samples containing boron concentrations of 1.0 and 2.0 p.p.m. were removed from storage at varying periods of time, dissolved in alcohol, and read on the colorimeter at 0, 1, 2, and 3 days. Following each determination the samples were returned to storage to be reread the following day. The initial reading for each sample was made 2 hours after alcohol addition. .4 third group of samples was stored dry a t room temperature. Samples were removed from storage a t 0 and 24
alcohol prior to reading on a colorimeter. After addition of alcohol, the complex must be read within 2 hours to prevent loss of color from a slow hydrolysis of the complex to curcumin and boron compounds. A larger number of boron samples could be prepared in 1 day if the boroncurcumin color were more stable and allowed for more flexibility in the scheduling of analyses. An examination of the stability of the boron-curcumin complex was undertaken t o determine how long samples could be kept after formation of the complex. EXPERIMENTAL
Boron-curcumin samples were pre%red using 0.2-, 1.0-, and 2.0-p.p.m. oron concentrations. Following development of the rcd complex, on a 55" C . water bath, the solutions were examined for color stability in a variety of ways. After the residues were dissolved in
E
T
HE determination of boron, using curcumin dye t o develop a red boron-curcumin complex, was described by Dible, Truog, and Berger (1). The complcx is taken up in 95% ethyl
400c m c
.-
Alcohol S o l u t i o n s Room Temperoture
Stored
.-
I
I
I
2
I n RFM.
Figure 1. Decrease in color values with time at room temperature ANALYTICAL CHEMISTRY
r o o m temperature
100 90 80
50
-
Stored at O'C.
4P .P-
20
1098
at
'\XQtored
/' b
Boron
.O'C. 2 PPM. Boron
m
Figure 2. Changes in boron-curcumin readings as a function of time of storage in alcohol solution
0
at
I
I
IO
20
I
I
30 4 0
I
I
I
1
50
60
70
80
T i m e in H o u r s
1
1
90 100
400
-
served in graph 1. The 1.5-minute reading gives the lowest value with a time lag of approximately 2 hours for development of maximum color. The wide variation in color observed for 2-p.p.m. buron samples did not appear in the 1.0- and 0.2-p.p.m. samples. Another set of boron samplcs containing 1 and 2 p. .m. of boron was stored dry after tge curcumin-boron complex was developed on the water bath. Duplicate samples were stored at room temperature and a t 0" C. over a period of 3 days. On successive days duplicate samples were removed from stora e and alcohol was added. After stan&g 2 hours, the samples were read on the colorimeter, returned to their original place of storage, and reread on successive days over a 5- to 11-day period. Readings were stopped a t 5 days on the room temperature samplcs because of lack of color stability. At 0" C., readings were continued for 11 days. Data are presented in Figure 4. In Figure 4 the initial 2-hour readings of all samples are plotted as starting a t 0 days regardless of whether alcohol was added a t 0, 1, 2, or 3 days. Little difference is observed in the initial 2hour readings on the samples, whether dry storage was 0 or 3 days, a t room temperature or a t 0" C. Addition of alcohol to samples stored a t room temperature resulted in a decrease in the color reading with time, as was shown in Figure 2. However, storage of the complex in the dry condition over a 3-day period a t room temperatiire did not result in loss of color. Samples stored a t 0" C. to which alcohol had been added showed little loss in color for the first 5 days, with a slow decrease in color reading with further storage in alcohol. All room temperature data are plotted on the same curve, all 0" C. data are plotted on a second curve. Separate plots are made for the 1- and 2-p.p.m. samples.
400 Stored
m
0 and 2 4 h r s
E
2 300-
dry
I week
P
Q 300
V
a
U
; 200u
;200
.-E
.-:
V
0
:loo-
30' I hr. + 3 hrr. 0 6 hrr. X
L
L
0
1
I
I
2
0
# 3
-
400 S t o r e d dry 82 weeks
rn C
I
I
2
Boron In P P M .
Boron In P P M .
400
I
+ 4
-
rn
2 300 -
S t o r e d dry 4 weeks
C
300-
01
a
:
* 200-
;100-
X
t 2 8 3 hrs U 6 hrs
0
1
Ihr
0 15'
+ 3 hrs. 0 6 hrs.
0
m 0 Boron
Figure 3.
in
1
1
I
2 P.PM
0 Boron
I
I
I
2
i n P.RM.
Boron-curcumin stability as a function of time stered dry at room temperature
Time intervoli refer to time in alcohol following dry storage period
hours and a t 1, 2, and 4 weeks and dissolved in alcohol. Following the addition of alcohol, readings were taken a t 15 and 30 minutes, and a t 1, 2, 3, and 6 hours. RESULTS
The stability of the complex stored in alcohol a t room temperature is shown in Figure 1. The readings obtained a t a given boron concentration decreased with time. However, the standard curves for each day were straight lines, showing that the rate of hydrolysis was the same a t all boron concentrations. I n 72 hours the reading on the 2-p.p.m. sample had fallen from 430 to a value of 210. The samples kept refrigerated at 0" C. dwreased only slightly, if a t all, in $2 hours (Figure 2). In Figure 2, data are replotted on a semilog scale, showing the decrease in color reading when samples were stored a t room temperature as compared to the constant values obtained when samples were stored a t 0" C. The stability of the complex in the dry state was also studied and the data are plotted in Figures 3 and 4. Samples, prepared as before, w r e covered and stored dry a t room tcmperature following their removal from the water bath. hldohol WILS sddrd to the dry complex samples a t 0, 1, i , 14, and 28 days (Figure 3). After addition of alcohol, the samples were kept a t
room temperature and read a t 15 and 30 minutes, and 1, 2, 3, and 6 hours. Graph 1, Figure 3, presents data obtained for samples after 0- and 24hour dry storage periods. These curves show, after alcohol addition, the normal decrease in color reading with time. Graphs 2, 3, and 4 present data for samples which had been stored dry for 1, 2, and 4 weeks. In these graphs a reverse pattern is noted from that ob-
-
,8
0
0
B\
'88 I80 70 60
Stored a t room temperature
-
1
1
1
1
1
,
1
1
1
1
1
,
VOL. 33, NO. 8, JULY 1961
1099
DISCUSSION AND CONCLUSIONS
From t l i r data of Figures I , 2, and 4, it appears that hydrolysis of the complex occurs rapidly in alcoholic solution at, room temperature. A decrease from 430 to 165 takes place over a 06hour pcriod at the 2-p.p.m. boron concentration, whereas no difference was observed in readings taken betwecn 16 minutcs and 2 hours. The rate of decrense is the same a t all concentrations used, as shown by straightline ciirveq in Figurr 1 and the slopes of the ciirvcs for thc room tcmpcrature readings plottcd on semilog paper for 0.2-, 1.0-, and 2.0-p.p.m. boron concentrations. T h a t thc hydrolysis is affccteri by tcmpcraturc is wcn hy the
ncgligiblc decrease in color of samples stored at 0" C. (Figure 2). Dry storage of the boron-curcumin complex showed the noimal hydrolysis of the complex at room temperature following addition of alcohol to the samples, provided samples were not stored longer than 24 hours. If the samples were stored dry for 1 to 4 weeks, the normal hydrolysis was replaced by a 2-hour lag in maximal color development. The development of a lag pcriod for the 2-p.p.m. sample at 2week dry storage resultfd in a decrease in wading from 440 to 270, with full rccovery a t 2 hours. Alcohol addition to dry storage mmplcs a t 0, 1, 2 , or 3 days (Figure 4) resulted in thc same amount of hydrol-
ysis regardless of the length of time of storage. Again samples, stored in alcohol R t 0" C. showed only a small decrease in color reading u p to 5 days as compared to the rapid decrease a t room temperature. The curcumin-oxalic acid reagent is stable when refrigerated. A reagent sample stored for 1 ycar a t 0" C. gave readings when used in boron detcrmination identical with those obtained the previous year with the same reagent. LITERATURE CITED
(I) Dible, W. T., Truog, E., Berger, I
