INDUSTRIAL AND ENGINEERING CHEMISTRY
January 15, 1932
39
of boric acid was prepared, containing 0.1077 mg. of elemental boron per cc. Aliquots of this solution were titrated by the above described method with the results shown in Table I.
The results of the boron determinations of a series of natural waters are reported in Table 11. The figures represent duplicate determinations on separate aliquots. To a third aliquot of each sample 1.21 mg. of boron as boric acid were added. Column 4 shows the recovery of this added boron TABLEI. TITRATION OF SOLUTION OF PURE BORICACID expressed in milligrams of boron. BY ELECTROMETRIC METHOD The method here described has been used on 300 samples BORON PER cc. BORIC ACID EQUIVALBNT 0.046 N SODIUM HYDROXIDEOF SODIUM of irrigation water in comparison with the modified Chapin Total Less blank HYDROXIDE SOLN. BORON method (3) with satisfactory results. The boron content cc . Mg. Cc . cc . Mg. of such waters as determined by direct titration is consistently 0 (blank) 0 0 0 (blank) 0.07 0.06used as blank slightly higher than that obtained by the distillation method, 0 (blank) 0 0.08 O'O4/ 2.5 0.269 0.60 0.54 0.498 but since it is known that by the latter method only 90 to 95 2.5 0.209 0.59 0.53 0.508 per cent of the boron is recovered, it is believed that the results 2.5 0.269 0.81 0.55 0.489 5.0 0.638 1.13 1.07 0.503 by the direct-titration method are nearer the truth, 5.0 0.538 1.13 1.07 0.503 The direct-titration method may not be dependable for 1.11 1.05 5.0 0.538 0.512 1.077 2.25 10.0 2.19 0.492 use with solutions containing appreciable quantities of phos1.077 2.15 2.09 10.0 0.515 phates, such as the nutrient solutions used in the plant cul10.0 1.077 2.20 2.14 0.503 tures, or with solutions containing relatively high concentrations of silicates. It is recommended for use with irrigation ON IRRIGATION TABLE11. BORONDETERMINATIOXS and drainage waters in which the range of hydrogen-ion conWATERS BY ELECTROMETRIC-TITRATION METHOD BORON,DUPLICATE DETNS. BORON RECOVERED centration is usually between p H 7 and 8, and in which the LAB.No. content of phosphates and silicates is low. P.p . m. P. p . m. IlilO. I
4519 4522 4525 4528 4531 4534 4537 4540 4543 4546 4549 4552 4555 455s 4564 4567 4570 4573
Av.
0.21 0.02 0.04 0.10 0.06
0.17 0.04 0.05 0.05 0.04
1.19 1.23 1.24 1.23 1.22
0.60 0.22 0.07 0.11 1.22 0.35 0.22 0.64 0.31
0.63 0.15 0.05 0.09 1.25 0.29 0.22 0.65 0.30
1.20 1.23 1.20 1.20 1.21 1.15 1.21 1.24 1.214
ACKNOWLEDGMENT The author wishes to acknowledge his indebtedness to Francis Scofield, who first pointed out the possibilities of the Cavanagh procedure in the titration of boric acid, and also to Albert P. Vanselow and Frank M. Eaton for suggestions during the early part of this work. LITERATURE CITED (1) Cavanagh, B., J. Chem. Soc., 1927, 2207-16. (2) Foote, F. J., IND. ENG.CHEM., Anal. Ed., 4, 39 (1932). (3) Wilcox, L. V., Ibid., 2, 358-61 (1931). RECEIVED September 5, 1931.
Determination of Boron in Waters Method for Direct Titration of Boric Acid FRED J. FOOTE, Limoneira Co., Santa Paula, Calif. OST procedures for the determination of boron necessitate the absence from the solution of all other substances titrating between a p H of about 5 and a pH of about 8.4. To eliminate these substances from a sample containing less than one part per million of boron without the loss of some boron or the addition of a varying amount with the reagents used has been found to be a troublesome and time-consuming process. In the past year a method has been developed and used for titrating boron in irrigation waters and water extracts of soils which overcomes this difficulty, and determinations made by it are rapid and capable of great accuracy. This method should be applicable to any aqueous solution of boron, although modification may be necessary in some instances. Results by the direct method have been compared with results by the distillation method formerly used in this laboratory, and the new method found to be more reliable (1). Hundreds of determinations have been made on waters and soil extracts without encountering any trouble. The principle of the determination is that with no mannitol present, boric acid is so weak that it is only partially neutralized at a pH of 7.6, but in the presence of sufficient man-
nitol it is completely neutralized a t this pH. No other acids or bases encountered or tested are affected by mannitol, so that the boric acid can be accurately titrated by bringing the solution to a pH of 7.6, adding mannitol, and titrating back to the same pH. The alkali used for titrating is standardized against a known amount of boric acid in the same manner, so that the small amount neutralized up to pH 7.6 is taken care of in the standardization factor. The amount so neutralized is about 12 per cent of the total boric acid present. Since there is no pH change involved in the two end points, there is no interference from other substances in the solution. It has been found, however, that the solution must be practically free of carbon dioxide, or equilibrium is difficult to attain and low results are obtained. Curves illustrating the titration of boric acid are shown in Figure 1. The initial point for the direct titration is A , and the end point is R. DETERMINATION OF PH For the determination of pH, either colorimetric or electrometric apparatus may be used. The colorimetric method has been used for most of the determinations and found en-
40
ANALYTICAL EDITION
tirely satisfactory where a practically water-white solution, not highly buffered at pH 7.6, is titrated. The details of the procedure used for determining boron in water samples, using the colorimetric pH indicator, are as follows: Put 100 to 500 cc. of sample into a 250- or 500-cc.
CC. o f NaOH. 1 c c . e .545 m g . 0 . FIGURE 1. TITRATION OF BORIC ACID
wide-mouth Pyrex Erlenmeyer flask. Make distinctly acid to methyl red with hydrochloric acid, using only one drop of a 1 per cent solution of methyl red so that it will not interfere with the pH determination later. Boil gently 5 minutes with two or three stirrings to drive off carbon dioxide. Cool to room temperature. Add 5 drops of 0.4 per cent phenol red indicator for every 100 CC. of sample. This must be regulated so as to have the same color concentration as is used with the standard pH indicator. Stopper the flask with the stopper described below and adjust the pH to 7.6 with carbon dioxidefree, approximately 1.0 N , sodium hydroxide. Equilibrium is more quickly established if enough sodium hydroxide is added to give a pH of 8.0 to 8.4, the solution vigorously shaken for 10 or 15 seconds, and then adjusted to pH 7.6 with approximately 0.1 N hydrochloric acid and 0.015 N sodium hydroxide. Equilibrium can be assumed when there is no perceptible change of color with 15 or 20 seconds of vigorous shaking. One should not proceed until the pH is constant a t 7.6 while shaking for this length of time. This is the initial point and the buret should be read. Add 3 grams of mannitol for every 100 cc. of solution, and immediately titrate back to 7.6 with standardized sodium hydroxide, taking care that exhaled carbon dioxide does not enter the flask. This is the end point of the titration and the pH should remain constant while shaking vigorously for 15 or 20 seconds as before. Read the buret. The sodium hydroxide used is 0.015 to 0.02 N . (1 cc. equivalent to 0.160 mg. of boron has been found satisfactory.) Standardize this with a known amount (1 to 5 mg.) of boric acid in 100 or 200 cc. of distilled water by the same procedure as the determination, It is convenient to use 10 cc. of a solution containing 0.7145 gram of boric acid in 500 cc. for standardization (1 cc. contains 0.25 mg. of boron). Subtract a blank titration, using the same amount of mannitol in distilled. water, from all determinations. I n order to get the most accurate colorimetric results, it is necessary to match the color of a LaMotte ampul pH indicator. The solution cannot be poured through the air into
Vol. 4,No. 1
a test tube for comparison as the carbon dioxide absorbed might appreciably affect the results. To overcome this, place a test tube in the stopper of the flask so that it can be filled by inverting the flask. Then compare it with the standard tube against a white background. The arrangement used is shown in Figure 2. The rubber connection should be heavy enough so that the tube will not flop around while shaking. If two tubes are used side by side, the ampul standard is not absolutely necessary. One tube can be left filled and clamped after equilibrium is established, and the titration completed on the remainder of the solution by matching the second tube with this. The second tube is then filled and clamped, the first tube drained, and the titration completed on the remainder of the solution by matching the solution in the second tube. This procedure can be used for off-colored solutions, but it is not generally so satisfactory as the separate ampul indicator, as the clamp might allow some solution containing mannitol to pass and the color in the tube would then fade. Also, some rubber tubing affects the pH in a short time. The initial and end point of the titration need not be at pH 7.6. With sufficient mannitol present, the neutralization is complete at pH 6.0. More mannitol is required the lower the pH of the end point. The standardization factor for a solution titrated a t pH 6.0 is not the same as at pH 7.6, because less boric acid is neutralized before the mannitol is added. However, Figure 1 shows that the curve without mannitol and the curve with mannitol are almost parallel between pH 6.5 and 7.6. Thus the results are practically the same over this range without changing the factor for the alkali. This is true only-when sufficient mannitol is present
FIGURE 2. STOPPER USEDIN COLORIMETRIC DETERMINATION
to cause all the boric acid to react at the lower pH. A pH higher than 7.6 can be used for the titration, but the amount of boric acid neutralized with no mannitol present is increased. AMOUNT OF MANNITOL The amount of mannitol needed varies with the volume of solution, the concentration of boron, and the pH used for the end point of the titration. I n the usual procedure for titrating boric acid, successive additions of mannitol are made until the pH is not affected thereby, Such a procedure, with the large volume of sample titrated by the direct method, requires the use of a great deal of mannitol. If this reagent is not neutral, an appreciable error may be introduced by using varying amounts of it. The titration can be satisfactorily and advantageously performed by using a constant concentration of mannitol, but the concentration of boron must not exceed the maximum that the mannitol added will cause to react.
January 15,1932
41
INDUSTRIAL AND ENGINEERING CHEMISTRY
If pH 7.6 is used for the initial point, 3 grams of mannitol for each 100 cc. of solution are satisfactory for any concentration of boron up to 5 mg. per 100 cc. No ordinary water approachea thia concentration. With more concentrated solutions of boron, more mannitol must be uaed, or the standardization factor is changed. All determinations.and recoveries reported in this paper were titrated by using 2 grami' df mannitol for every 100 cc, of solution. The alkali used was standardized with boric acid using the same concentration of mannitol. The addition of more mannitol would have increased all titration figures slightly. The amount of boron found would not be changed, however, 88 the decrease in
Boron determinations by diroct titration were made on each
filtrate. Results are shown in TabIe I. TABLE I. BORON RETAINED BY PRECIPITATES TOTAL 1X SOLN. -130J?ON IN ORIOINAL 1st SAYFL. WATBII~ filtrate BoLloa
P. p . m. 433 434
1170 102s
2nd filtrate
Boltopr :I 500 cc. WITEOW C0stC.M-
RETAINFD--3rd
TIIATIN0
filtrate
Total P. p. rn- P. p . m. P. p . m. P. p . m. 0.380 0.050 0.027 OAOII 0.841 0.000 0.018 0,417
P. p a m. 0.w 0.4~6
There was evidently a small amount of boron retained after dissolving the precipitate twice. It has since been found that if sodium carbonate is used for the precipitation, 1888 boron is retained than when sodium hydroxide is used. Sodium carbonate was used in precipitating the alkaliinsoluble material from acid-soil extracts which contained a0 much precipitate that neither colorimetric nor electrometric determinations of boron could be made. The procedure uscd was to heat 250 cc. of the 2-1 extract to boiling, add saturated sodium carbonate solution until just alkaline to phenolphthalein, and boil 5 minutes. Filter through a Biichner funnel with suction, and wash dvith 50 cc. of hot water. Redissolve the precipitate in 50 cc. of dilute acid, boil, make alkaline, filter, and wash as before. Repeat the redissolving and precipitating. All or an aliquot part of the combined filtrates can be used for the determination, as described for water samples. Determinations on the seyaate filtrates representing 150 grams of soil gave the results shown in Table 11.
TABLE 11. BORON RETAINED BY PRECIPITATES WITH ACIDEXTRACTS OF SOIL MADEALKALINE RETAINED-
-BOBON
Cc. of NaOH. lcc.=^y.l62 m $ . B . FIGURE 3. TITRATION OF 2.5 BG I . OF BORIC ACID BY DIRECT METHODWITH DIFFERENT CONCENTRATIONS OF MANNITOL the standardization factor would offset the increase in the amount of alkali used. Figure 3 shows the path of the titration with 2.50 mg. of boron in 280 cc. in the presence of 4 and 8 grama (2 and 4 grams per 100 cc.) of mannitol. These curves show that increasing the concentration of mannitol 2 grams per 100 cc. does not greatly affect the amount of alkali used at pH 7.6. Therefore, the quantity of mannitol added need not be precisely measured. Three grams of i n a d t o 1for each 100 cc. are recommended for water samples, 88 the pH change near the end point M greater for a given amount of alkali than when 2 grams are used. RESULTS
OBTAINED UNNQ METHOD
The direat-titration method of determining boron giva accurate reeulta with small amounts of boron provided the solution is not too highly buffered. It has been used to determine the boron 88 impurity in such aalta aa calcium chloride, aodium carbonate, and sodium sulfate. If the solution M highly buffered, the mum of the buffering can be removed. I n all cades where a precipitate is extracted, it should be tested for boron by dissolving or hesting in dilute hydrochloric acid (pH 3 or below), and repreoipitating. The necmity for this is shown in the following dehnhationa: h p l a of 2800 oc. of water were evaporated to about 76 00. in alkaline solution. The midue WM 5lted and washed with 100 00. of hot water. The precipitate WM dissolved by heating in 100 maof water and acid, made just alkaline to phenolphthalein witb rodium,hydroxide, filtered, and wrshed with 100 00. of hot water, The redissolving and preoipitating wero repeated.
ALKALI
U B ~TO D
1st filtrate
2nd filtrate
3rd filtrate
Ma.
Ma.
Ma.
LLLRS. NeOH 0.67 1.00 LLLRS. NarCOi LLLRa. N~YCOJ 0.87
0.il 0.08 0.20
0.00
0.06 0.00
1.08 1.07
SAYPI~E PPT.
5-66
V-8-8
NaOH NrCO;
1.20-0.33
1.10
0.01
0.06
By distrllrtion method
Total
Ma.
Ma.
1.03
1.61
1.67 1.15
1-45
It is necessary to run a blank using approximately the same amount of reagents, as e. P. chemicals often contain appreciable &mountsof boron. Table I11 compares results of determinations on waters by the direct-titration method, using 10 g r a m of mannitol in 800 cc., with results by the distillation method. TABLE111. COMPARISON OF DETERMINATIONS ON WATER SAMPLES sr DISTILLATION AND DIRECT-TITRATION Mmom SAYPL~
433 434 4S6 436 487
DXUTILLATION M~TROD Amt. of sample Boron CC.
P. p . m.
P. p . m.
2600 2600
0.43 0.40 0.40 0.99 0.88
0.48 0.42 0.41 0.41 0.41
2600 2500
2600 2600 lo00
0.45
43s 489 440 441
lo00 lo00 1000
0.73
443 444 446
loo0 loo0 lo00
0.46
Ma
DIIIECT Trirrnon (600cc. or SAYPL.) Boron
0.46
0.4s 0.43
0.40
0.46
0.111
0.41 0.43 0.M 0.17 0.48 0.46
0.41
As a rule the results of direct titration arb alightly h i g k than the resulta of the distillation method. Some boma h Io& by the distillation method, M it haa been ut3ed h m , (b peoially when the aample M high in total solids. A rnw of 95 per cant in mmmon when boron is added to dhtibd
‘42
writer. and dctermincd by the distillation method. It is tliought, therefore, that tlie results of the direct-titration method arc more nearly correct. Table IV shows results of duplicate boron determinations and results ‘on diflercnt‘sized aliquots of the same water snniplc by the direcetitration method, using 2 grams of mannitol for each 100 cc. of sample. ‘rAnLE
SANPLE
400
11
2731 2831 3931
Iv.
used that is subdividcd into 0.08 cc. and readings estimated to 0.01 cc. Table V shows typical recoveriea of boron when added to a water. All determinations were made on 600 cc. of,ssmple using 10 grams of mannitol.
TABLE v.
IhPLICATE DETERMINATIONS OF BORON IN WATlrn SAMPLE8
SAMIV.E
(Comparison of reeulte on diRarerlt ~ N I O I I R ~ofS sample) AM+. OF AMT. OF AUT. OF SAMPLE IlORON SAYPLlE BORON SAMPLE BORON Cc. P. p . m. Cc. P . P. m. Cc. P . P. m, 600
... 600 500 600
600 600
0.83
0.75 1.05
600 600
500
0.84 0.72 0.79 0.74 1.07
3.06 3.05 3.34 1.21
250 250 600 225 225
3.10 3.06 3.30 1.17
... 0.78
200 200
4231 4731 4831 4931 6031
500
0.06
6431 6331 91310
!::2
2’110” :::;”0
600
4.26
ZOO
0
Vol. 4, No. 1
ANALYTICAL EDITION
600
0.68
4.27
100 100
1831 1891 433 A 433 A 433 A 433 A 433 A
0.82 0.72
225
1.16
:!:g 100
4.27
Pacific Ocean at Ventura, Cdit.
The smaller samples are about as accurate as the larger ones. This is mainly due to the @eater change Of pH which a given amount of alkali will cause in a sntall sample as compared with a large sample. For small samples, a buret should be
BORONWHENDETEHYINBD DIIIECT-TITRATION METHOD
RECOVERIES OF
IW
---BORON Added
ADDED Found
Rsoovsred
Mo.
Mo.
Mo
0
0.465
0 0.498 2.49 4.98
0.270 0 773 Y 767 6,303 10.270
0.600
0.080
ibik
0:bi)b
I I
0.06
.
I%ORON
RECOVERED 96 .ii:2
O:i64 2 488 6.034
09.0 100.9 100.3
I
o.uo1
This report is not intended to be s complete study of the method for determining boron here dcscribed. The procedure has been developed primarily for determining boron in waters, and for waters in this region it is entirely satiefactory. Lack of time prevents further investigation into its possibilities, but no reason for failure on more concentrated aqueous solutions is known. LITERATURE CITED (1) Wilcox, L. V., IND.ENQ.CHBM.,Anal. Ed., 2, 358-01 (1930).
R E C ~ ~ VJUIY E D 28,1931.
Note on Micro-Dumas Method for Determination of Nitrogen RALPHT . K. CORN WELL,^ National Institute of Health, U.S. Public Health Service, Washinglon,D. C.
I
N ADAPTING the classical Dumas method for the determination of nitrogeu in organic substances to a scale suitable for microanalysis, Pregl originally obtained too large volumes of gas in the micronitrometer (8). This error he showed was caused by the formation of carbon monoxide, “due to a permanent disturbance of the equilibrium between carbon dioxide on the one hand and carbon monoxide and oxygen on the other, caused by the red-hot copper spiral.” Later Pregl (8) improved his method to overcome this error. Experience in teaching Pregl’s method at the University of Pittsburgh, inquiries which have come to this laboratory, and a search of the literature (1, 2, 4-7) all show that high results may still be obtained by this method. Since by using the following very slight modification of Pregl’s method practically theoretical figures have always been obtained, even by beginners, it seemed desirable to publish this note. Furthermore, the time required for the determination is appreciably shortened and no difficulty has been noted in getting “micro-bubbles” a t the end of the ’analysis, The directions given by Pregl (9) are followed exactly through the burning of the sample. The carbon dioxide is then turned on and the stopcock of the micronitrometer adjusted so that “one bubble rises per two seconds.” At the end of 5 minutes (during which time the portion of the tube containing the sample is reheatod), the flame of the long burner is tiirned down slightly (8). During the next 5 minutes the gas is alowIy turned off, 80 that a t the end of 10 minutes from the time of, starting the carbon dioxide, the 1
Prcwnt addreu, &rlrilr Induitriil Corp., Frmlrddnburl, VI.
gas for both burners is turned off completely. After 2 or 3 minutes the stopcock of the micronitrometer can be opened further and the bubbles allowed to rise a8 fast as possible. Here the only precaution necessary is to regulate the speed 80 that the bubbles do not go together and stop at the bottom of the graduated portion of the micronitrometer. In this manner various types of organic substances containing nitrogen have been analyzed successfully in this laboratory. The following are a few illustrations: NITROGEN DETERMINATIONS No.
SUBPRESSTANCE NITROQEH T E U P . luna
Mu. 1 2 3
4
6.734 6.244 3.466 8.b60
Cc. 0.284 0.261 0.255 0.266
C. 27 28
28 27
Mm. 760 750 780 757
FonYuLA OF
PER.CENTA0. O F NXTROaBN
S U D U T A N CTheory ~ Found
CsHir01N 6.40 CIHIIOIN 6.40 CitHnOtNt
CitHaOtNi
8.08 8.09
11.44 6.43 8.02 8-14
LITERATURE CITED (1) Bock, F., and Beaucourt, K., Mikrochsmie, 6, 69 (1920). (2) Dulmlcy, J. V., Be?., 50, 1710 (1917). (3) Dubnky, J. V., “Dic Methoden der organieohen Chomie,” 2nd ed., Vol. I, p. 14’4, Houben-Weyl. (4) Flsrrchontr&ger,B., Mikroahamie, 8, 1 (1930). (6) Halls, F., Zhid., 7, 202 (1029). (6) Hernler, F.,i%fikrochrmhPregl Faahhr., 151 (1929). (7) Lauer, W. M., and Sundls, C. S., Zbid., a35 (192Q). (8) Prod, F., “Quantitative Organic Mimndydr,” PP. 73-8, BlaWaton, 1924. (D) Prod, B.,Ibid., pp. 72-92,
R ~ C ~ I Y Jinurry BD 81, 1Q31. Publlrhed by permidun vf bbr aenrral of the U.8. Publlo H r l t h &rrioa,
l r m
