applied to the determination of sugars with p-anisidine hydrochloride (6) and amino acids with ninhydrin ( 2 1 , l a ) . It is rapid, and a useful procedure for the quantitative study of mixtures of compounds of biochemical interest. This is well illustrated in Figure 3, based on a study of the rate of formation of (p-hydroxyphenyl)-p-gentiobioside and the simultaneous liberation of glucose when arbutin is incubated with a p-glucosidase preparation from the cambial region of Populus grandidentata.
3.0-
-
E 2.0-
.I” 0
HOURS
Figure 3. Formation of (p-hydroxyphenyl)-P-gentiobioside and liberation of glucose
LITERATURE CITED
Arbutin incubated with f?-glucoridare preparation from Populus grandidenfafa
(1) Beper, D. L., Pearl, I. A,, Inetitute of
are orange-colored under acidic conditions. If the p H is increased, however, a color change results, which often aids in the identification of certain phenols. These dyes in alkaline solution usually exhibit maximum absorption peaks at a higher n a v e length than when they are in acid solution and therefore, if neressary, the absorbance of the former can be readily measured in a simple colorimeter. For t1~e.e reasons the quantitative relationship between the absorbance of the azo dyes and the weights of phenols \T as examined in alkaline rather than in acid solution. Attempts to bring about the color change by spraying the chromatograms with alkali were abandoned, as this invariably resulted in the colors streaking down the paper. The alkali IT as therefore incorporated into the eluting reagent. I n the case of catechol, some difficulty was experienced with the elution of the azo dye from the paper. By increasing t h e water content of the eluting reagent, Iiowever, this was overcome.
Standard curves for arbutin. saligenin, and catechol are shown in Figure 1. The relationship between weight of phenol and absorbance was linear from 0 to 100 y for catechol and arbutin and 0 to 70 y for saligenin, and the errors were within 147,. The stability of the azo dyes formed with the above phenols appeared to be high. The absorbance of the blanks, ho\\-ever, gradually increased over a period of 2 to 4 hours and then remained steady. This gave an apparent decrease, with time, in the intensity of the dye solutions, the standard curves remaining parallel but below the original curves, and no longer passing through the origin (Figure 2). This, however, is of little importance as far as the accuracy of the method is concerned, as the determination of standard phenols together with the unknowns on the same paper chromatogram alleviates any error which might arise from this phenomenon. The general technique has also been
Paper Chemistry A4ppleton,Wis., unpublished results, 1956. (2) Bray, H. G., Humphr~s,B. G., Thorpe, W. V., White, K., Kood, 1’. B., Biochem. J . 52, 416 (1952). ( 3 ) Bray, H. G., Thorpe, K. V., in Glick’s “Methods of Biochemical AGlysis,’’ ~ o l .I , p. 27, Interscience, New York, 19.54. (4) Bray, H. G., Thorpe, IT. V., White, K., Biochetn. J . 46,271 (1950). ( 5 ) Clarke, D. D., S o r d , F. F., in Paech and Tracey’s ”AIodern Methods of Plant Analysis,” Vol. 3, p. 332, Springer-Vwlag, Berlin, 1955. (6) Pridham, J. B., A s . 4 ~ . CHEX 28,
1967 (1956). ( 7 ) Pridham, J. B., unpublished results, 1955. (8) Sevag, 11. G., Lackman, D. B., Smolens, J., J . Riol. Chem. 124,425. (1938). (9) Stone, J. E., Blundell, 11,J., h s a ~ . CHEX 23, 771 (1951). (IO) Swain, T., Riochem. J . 53,200 (1953). (11) Thompson, J. F., Stewart, F. c., Plant Physiol. 26, 421 (1951). (12) Thompson, J. F., Zacharius, R. 11., Stewart, F. C., Ibid.,26,375(1951). (13) Trim, A . R., in Paech and Tracey’s “Modern llethods of Plant hnslysis,” Tol, 2, p. 295, SpringerT‘erlag, Berlin, 1955.
RECEIVED for revie17 December 1, 1956. i2ccepted l l n r c h 21, 1957.
Gravimetric Determination of Boron Precipitation as Nitron Tetrafluoborate CLAUDE A. LUCCHESI’ and DONALD D. DeFORD Northwesfern University, Evanston, 111.
b A simple gravimetric method for the determination of boric acid in aqueous solution is based upon the conversion of boric acid to tetrafluoboric acid and the precipitation of the latter with the organic reagent, nitron. Fluoride ion and weak acids and bases do not interfere. Over the range in which the method has been tested, from 125
to 250 mg. of boric acid, the average absolute accuracy for 24 determinations was f 1.1
yo.
T
of almost all boroncontaining materials requires a preliminary treatment which ultimately results in a n aqueous boric acid sample. A t present the most precise, reliable, HE ANALYSIS
and generally useful methods for the quantitative determination of aqueous boric acid are based upon the unique property of boric acid to form with polyols acidic chelates which can be titrated with strong base. These meth1 Present address, Analytical Research Department, The Sherwin-Williams Co., Chicago 28, Ill.
VOL. 2 9 , NO. 8, AUGUST 1957
1169
ods, however, are inherently subject to interference from all acids and bases. Slthough the interference from strong acids and bases can be eliminated by preliminary neutralization, similar interference from acids having a pK, between 5 and 10 cannot be eliminated in this way (11). Furthermore. fluoride ion interferes in the poly01 methods by forming a relatively strong complex with boric acid. R h e n interferences are present, they must be separated from the boric acid before the latter can be titrated. Of the variety of methods which have been proposed for the isolation of boric acid, the Chapin distillation method (IO) is perhaps the most reliable and the most versatile, but even this method is excessively time-consuming and not always accurate ( 3 ) . When large amounts of fluoride are present, even a double distillation will not permit a n accuracy of better than d = l % ( I ) . Recently, an ion exchange procedure utilizing a mixed bed anionic-cationic resin has been reported for the separation of boron from intcrfering substances ( I d ) ; this procedure was found to remove all common interferences except fluoride ion. After completion of the work described here, an extraction procedure for separating a reproducible fraction of boron from fluoride salts was reported by Ross, Meyer, and White (6). The method described here enables boric acid to be determined in the presence of fluoride ion and weak acids and bases with a minimum of equipment and operator time. The method is based upon the conversion of boric acid to tetrafluoboric acid and the precipitation of the latter with the organic reagent, nitron. I n the development of this method a number of substances known to form complexes with horic acid in aqueous solution were studied. Of the several reactions investigated. only that of boric acid with fluoride ion could be utilized in a quantitative method ( 5 ) . I n 1926 Lange (4)used nitron acetate for the precipitation of tetrafluoboric acid, but he did not indicate the reliability, accuracy, or precision of the method. He made no attempt to adapt the precipitation to serve a' a method for the determination of boric wid. I n a, kinetic study of the reaction which takes place b e h e e n boric acid and hydrofluoric acid in aqucous solution, Wamser ( 8 ) used Lange's procedure for the determination of the tctrafluoboric acid formed. Wamqer ( X ) has shown that Tvhen hydrofluoric acid is added to an aqueous solution of boric acid. tetrafluoboric acid is formed slonly. By also placing nitron acetate in the aqueous solution with hydrofluoric acid, it is possible to precipitate the tetrafluoboric acid
1170
ANALYTICAL CHEMISTRY
Table I.
Typical Results Obtained with Recommended Procedure
Boric Acid, 3fg.
Taken
Found
250 I 250.1 244.4 244.4 125.4 125 4 124 3 124 3
252 1.253 9 . 2 5 2 . 4 248.8:2 5 1 , 2 , 2 5 0 , 8 245.4,248.7,246,6' 247.3,243,3,248.4 126.6,127.2,126.2 126 8,126 8, 126 6 121 5,123 7,123 1 126 1,126 4,126 7 I
as it is formed, thereby approaching the conditions of a precipitation from homogeneous solution. Further, by employing excesses of hydrofluoric acid and nitron, the precipitation can be made quantitative. The reactions involved are as follows: HaBOa
HBF,
+ 4HF + CzoHiJa
HBFa
+ 3H20
e C?oHisS*.HBFa
(1) (2)
The precipitate so formed is filtered, diicti, and weighed. The weight of boric acid in the sample is then calculated by use of the gravimetric factor. EXPERIMENTAL
Reagents. The stock solution of nitron (1,4 - diphenyl - 3,,5 - endanilohydrotriazol, C20H16N1, Eastnian Organic Chemicals KO. 1077) was prepared by dissolving 37.50 grams of nitron in approximately 250 ml. of 5% (by volume) acetic acid. The final solution was stored in a dark bottle to prevent possible decomposition of the iiitron because of photochemical action. The saturated solution of nitron tetrafluoborate used as a wash solution \vas prepared by adding an excess of the qolid reagent to 1 liter of distilled water. This mixture was agitated for 2 hours before use. The boric acid used in the development of this method \!as standardized hy titration with standard carbonatefree sodium hydroxide in the presence of mannitol to a phenolphthalein end point. The boric acid, glacial acetic acid, and 48% hydrofluoric acid were all reagent grade. Procedure. Place t h e aqueous boric acid sample, which should contain fioni 123 t o 250 nig. of boric acid, in a 250-ml. polyethylene beaker. Dilute t o about 60 ml. with distilled \rater, and add 15.0 ml. of t h e nitron solution and 1.0 to 1.3 grams of 48% hydrofluoric acid. Allow the solution t o stand a t room temperature for 10 t o 20 hours, and then place in a n ire b a t h for 2 hours. Filter through a Selas or Coors porous porcelain crucible, and wash with five 10-nil. portions of the saturated nitron tetrafluoborate solution. Drain the precipitate dry after each iiashing. Dry a t 105" t o 110' C. for 2 hours and weigh. The gravimetric factor for boric acid is 0.1545 and that for boron is 0.02704.
~
Error, yo i n 8. ~,+ I 5. +o I
-
~
9
-0 5, +0.4; i O . 3 +0.4, $ 1 . 7 , $ 0 . 9 t 1 . 2 , -0.5, $1.6 + 1 . 0 . $ 1 . 4 . +1.0 + l 1 , + 1 1,+1 0 - 2 2, -0 5, -I 0 +l 5, $1 7 , $ 1 . 9
DISCUSSION
Typical results obtained with the recommended procedure are given in Table I. ,411 determinations were carried out in triplicate. The results of the 24 determinations average 0.7y0 d= 1.0% high. The precision and accuracy realized are comparable to those obtained in other nitron precipitation methods and are better than those usually obtained with the distillationtitration method when applied to samples containing fluoride ion. In general the agreement among replicate determinations made at the same time and under identical conditions is better than the agreement between different samples. This fact suggests the presence of some undetermined variable which was not adequately controlled. Because a method for the detcrmination of relatively large quantities of boric acid was being sought. samples containing less than 125 mg. of boric acid w r e not tested. However. there is no reason to believe that the above procedure cannot be adapted for determinations in the concentration range helon- 125 nig. of boric acid. Effect of Excess Reagent. A considerable ewess of nitron is ncetied to ensure t h e quantitative conversion of the tetiafluoboric acid t o nitron tetrafluoboiate. When a n excess of only 2070 u a s used, results averaged about 27, lon on samples containing 250 mg. of boric acid. I n t h e iceommmdcd procedure t h e excess nitron Yaries between 80 and 250%, depending upon the weight of boric acid in t h e sample. Effect of Wash Solution. A saturated solution of nitron tetrafluoborate must be used for washing the precipitatc t o avoid loss due t o thc rather high solubility of t h e precipitate. Thc use of 5y0 acetic acid as a .i\-ash solution gave results which n e r e 2 t o 37, loiv \?hen t h e sample contained 250 mg. of horic a d The volunic of n a s h solution u s d is not critical: identical results erp obtained n h e n t h e volunir of n a s h solution n a s varied b e t n e m 23 and 55 ml Effect of Time of Standing at Room Temperature. The time of standing a t room temperature a i t r r all the
wagcmts were added t o the sample was varied from 10 t o 20 hours. X o significant differences in t’he results n-ere noted. Effect of Hydrofluoric Acid on Crucibles. There n-as a slight apparent loss in weight of the precipit a t ? due to hydrofluoric acid attack on the porcelain filter crucibles. D a t a froni 12 determinations showed t,hat on the average the crucibles lost 1.4 rng. for each determination. This loss in \%-eight appears as a small positive error of t,he order of 0.1% n h c n samples containing about 250 nig. of boric acid are being analyzed. I3ecause the precision of the recomnicntlctl procedure is approsimately i1 yo,this error is not significant. Interferences. Nitrate, perchlorxtt,, iodide, thiocyanate, chromate, (~Iiloi,atr,nitrate, and bromide form more or less insoluble salts with nit r o l l . and these substanws Ii-ould be c3sl)ected to interfere ( 3 ) . In the determination of 1iitr:ite with
nitron, Grant ( 2 ) removed the bromide interference by decomposing hydrobromic acid with chlorine water added dropwise to the boiling solution until the yellow color of bromine disappeared. Hydriodic acid lvas removed by adding an excess of potassium iodate to the neutral solution and boiling off the iodine. X t r o u s acid and chromic acid were rrmoved by reduction with hydrazine sulfate. I n the determinntion of perchlorate, Vurtheim ( 7 ) reduced nitrate and chlorate by treating the alkaline sample with Devarda’s alloy (59 parts of aluminum, 39 parts of copper, and 2 parts of zinc). These procedures for the removal of all of the commonly encountered interferences should be applicable also for the removal of interferences in borate samples prior to precipitation of the boron as nitron tetrafluolmrate.
(2) Grant, J., I n d . Chemist 8 , 160 (1932) (3) Hillehrand, W. F., Lundrll, G. E. F., Bright, H. A., Hoffman, J. I., “AppliCd>organic Analysis;’! 2nd ed., p. 157, Wiley, New l-ork, 1953. Lange, W.,Ber. 59B, 2107 (1926). Lucchesi, C. A , , Unio. Microjlrtis Pikbl. (Ann Arbor, Mich.) 13,109; Dissertation Abstr. 15, 2007 (1955). ROBS,W.J., Bleyer, A. S., Jr., White, J. C., ANAL.CHEM.29,810 (1957). Vurtheim, rl., Bee. trao. chirn. 46, 97 11927). Wimw;. ._ .-. .. C . A , , J . A m . Chem. Soc. 70, 1209 (1948). Welcher, F.,;T., “Organic Analytical Reagents, Vol. 111, Van S o s trand, Ben, York, 1948. JTherrr. E. T., ChaDin, W. H., .I. Am.”Chem. soc. 30.-16E47 ( -1908). Wberg, E., in “Handhbch der Analytischen Chemie,” 1-01. 111, p. 24, R. Freseriius, G. Jander, eds., Springer-Vrilag, Berlin, 1942. Wolszon, J. D., Hayes, J. R., Hill, W.H., h . 4 1 . . Cmni. 29, 829 (1957).
LITERATURE CITED
(1) Allen, E. T., Zies, E. G., J . An?. Crmru. SOC. 1, 739 (1918).
RECEIVED for review Xovemher 21, 1956. Accepted April 17, 195i.
Neutron Activation Cross-Section Graphs W. WAYNE MEINKE and R. S. MADDOCK Department of Chernisfry, University o f Michigan, Ann Arbor, Mich.
Revised values of all known thermal neutron activation cross sections have been plotted against the half life of the radioisotope produced. The standard error of each value is included.
A
cross-section values for thermal neutrons are important primary data for the analytical chemist becmse they indicate the relative probability with which a giren radioisotope \vi11 be formed when a substance is exposed to a source of neutrons. Thus, the detection sensitivity for an element in activation analysis as well as the yield of tracers in an irradiation depend upon these values. Unfortunately, absolute values for cross sections are not very well known. Hughes and his coworkers at Brookhaven National Laboratory hare for a nuniber of years been compiling thermal neutron cross-section values, 3s well as c.nluc.s for higher energy neutrons and first made their conipilation available in 1952 ( 2 ) . Meinke and Anderson (4) .bowed that these values for thermal energies could be summarized readily by plotting them against the half life oi the daughter radioisotope produced. This type of graph has proven useful in (iiscussions of the use of low-level CTIVAIIOP~
portable neutron sources in activation analysis (4,6 ) , as well as in considerations of the sensitivity of thiq method (3).
Recently Hughes and coworkers have revised their cross-section values ( I ) . Figures 1 and 2 give plots of new crosssection graphs using these new LLbest” values and also including the standard error quoted by Hughes for each value. Where only masimum or minimum values have been given for the cross section they are indicated by arrows on the graph. As in the previous plot the cross-section values are “atomic cross sections”-Le., the isotopic cross section (quoted by Hughes) multiplied by the natural abundance of the isotope. Figure 1 includes cross sections for all stable isotopes for which natural abundances are known. Figure 2 includes all cross sections given by Hughes for isotopes which are in themselves radioactive and which do not occur in nature. The atomic and isotopic cross sections are identical in Figure 2. An isotope that is underlined on the graph indicates that neutron activation produces a metastable daughter activity. A wavy line, on the other hand. indicates activation to the ground state of an isomeric pair. (Antimony-123 gives tivo metastable antimony-124
states and therefore has two underlines.) In most cases the metastable state decays independently by beta emission or contributes only a small amount to the activity of the ground state. I n some cases, however, the metastable state decays completely into the ground state. thus augmenting its activity. The points with an underline and a wavy line are examples of this latter case and represent the total cross-section values foy the formation of the ground state both directly and by decay of the shortlived metastable state. 4s an example, stable cobalt-59 is activated to 10.4-minute cobaltr6Om with a 16 =t3-barn cross section and a t the same time to the 5.28-year cobalt-60 ground state with a 20 f 3-barn cross section. Since all the 10.4-minute metastable state of cobalt-60 decays directly to the ground state, the 5.28-year actikity is produced with an effective cross section of 36.0 f 1.5barns. ‘These graphs contain all the ( n , y) cross-section values given by Hughes for thermal neutrons except for a few reactions where the cross sections are too small or the half lives too large to be plotted on the graphs. These values are given in Tables I and 11. From these graphs it is possible to tell a t a glance which elements will be VOL. 29, NO. 8, AUGUST 1957
1171
