Simple Method for Microdetermination of Boron, Alone or Simultaneously with Nitrogen and/or Phosphorus, in Organoboranes ROBERT C. RITTNER and ROBERT CULM0 Olin Mathieson Chemical Corp., New Haven 4, Conn.
b A simple method for the microdetermination of boron in organoboranes b y a Kjeldahl-type procedure has been developed. The sample i s digested for approximately 30 minutes with concentrated sulfuric acid in the presence of selenium and a 3:l mixture of cupric sulfate-potassium sulfate. The contents of the digestion flask are then quantitatively transferred to a 400-ml. beaker. The solution is diluted to approximately 275 ml. with carbon dioxide-free distilled water and the boric acid i s titrated with 0.01N sodium hydroxide b y the fixed pH method, using 7.10 as the critical pH. If nitrogen, phosphorus, or both are to be determined simultaneously with the boron, 30% hydrogen peroxide i s added to the above-mentioned digestion reagents. After digestion, the contents are quantitatively transferred to a 50-ml. volumetric flask, the solution i s made up to volume with distilled water, and suitable aliquots are withdrawn for each determination. The nitrogen i s determined b y the standard microKjeldahl distillation and titration, the phosphorus b y a spectrophotometric method, and the boron as described a bovs.
D
the past year, our microanalytical laboratory received an excessively large number of alkylamino decaborane and alkylaminophosphoro decaborane derivatives for . I n many instances, the amount of sample subniit'ted was quite limit'ed. I t was advisable, therefore, to develop simple and rapid micromethods for the determination of the elements involved (C, H. B! S , P) and, if possible, t o develop method> in which several elements could be determined . -1 method for the siniultaneoiis determination of carbon, hydrogen. and boron has been described in a previous publication (?). Prior to the present publication, the nitrogen in these compounds n-ai determined siiccessfully by the standard micro-Kjeldahl procedure (5), and the phosphorus with equal reliability by the URISG
1268
ANALYTICAL CHEMISTRY
method of Ala and McKinley ( 6 ) . In both these methods, a sulfuric acid digestion was used to convert the nitrogen to ammonium bisulfate or the phosphorus to orthophosphate. It occurred to us that if, during t,he sulfuric acid digestion, the boron \\-ere also converted quantitatively t o boric acid, we should be able t o develop a simple and rapid method for the microdetermination of boron, alone or siinultaneously with nitrogen and /or phosphorus. This paper describes such a method. Some of the other workers who have developed successful net-digestion procedures for boron are l b r a i n j o n and Kahane ( I ) , who digested with a mixture of nitric, perchloric, and sulfuric acids; Strahrn and Hanthorne ( 9 ) . 11-ho used trifluoro peroxy acetic acid; Dunstan and Griffiths (a),who used alkaline potassium persulfate; and Snyder, Kuck, and Johnson (S), who employed alkaline hydrogen peroxide. EXPERIMENTAL
Apparatus. 'Yhe pH m r t e r used for t h e titrations is a Heckman Model H-2 equipped with t h r standard glass a n d calomel electrodes. The qpectrophotometer uqed for t h e 1,hosohorus determination ic t h e Heck;nan'llodr>l K. T h e micro-Kjeldahl diitillation unit was uurchased from .I H. Thomas Co., Philadelphia, Pa. The digestion unit \vas obtained from the .hierican Instrument Co. ( . i l I I X C O ) , Silver Spring, Md. I t is circular and can acconimodate 12 Kjeldahl flasks a t a t>irne. The quartz Kjeldahl flask:: which m r e used in the digestions n-ere pre1)ared by t,he glass blower.; a t Olin IIathieson and ha\-? thr samC dimensions as the 10-nil. ca1)acity horodicate Reagents. -111 wagriit\ ivhich were used K C I Y t h e I,(-t g r ~ i d coxn~ IIy ami1:iblt~ :iiid did not rcquire a n y f urt h c r puri fi c a ti n11. Procedure. 1. f- t o :i 10-nil. capacity quartz
Kjcldahl fla&. X few mg. each of a 3 : l CuSOt-K,S04 mixture and selenium powder are added and also 1 ml. of concentrated H?S04. -4 frncarborundum or alundum boiling stones are a1.o included. The flask is placed on the digestion unit and heated for approximately 30 minutes. It is then removed and cooled, and the content.; are diluted with C02-free distilled H 2 0 and transferred quantitatively to a 400ml. beaker. The volume of solution is diluted to approximately 275 nil. with CO?-free distilled H20 and the pH i,s adjusted roughly t'o 4 or 5 with - 6 s SaOH and then more carefully to exactly i . l O , using -0.01.\9aOH (need not be standardized) . Excess mannitol (7 to 9 grams) is nolv added until the pH remains relatively constant. The tip of a 10.0-ml. microburet (graduated in 0.05-ml. intervals) is placed below the surface of the solution, and standardized 0.01-V XaOH is added until the pH is brought back to 7.10. .ill t,he XaOH used in the procediw niurt be carbonate-free and should he stored in polyethylene bottles to eliminate the absorption of boron from glass. S o blank determinations are necessary, since those run did not indicate tht. presence of any boron. 2. BORONAND SITROGI,:S. In determining boron and nitrogen simultaneously, the procedure is almost identical to that describrd above for boron alone. After t)he 30-minute digestion period, the sample is cooled. diluted, and transferred quantitatively to a 25-m1. volumet,ric flask, which i+ made up to volume. -4 10-ml. aliquot is n-ithdrawn for the boron detwmination (as described previously) and a 10-ml. aliquot is placed in the daiidard micro-Kjeldahl distillation unit. .Ukali (15 nil. of 30YGSaOH) is addcd and the S H 3 is collected for a 3 - m i n u t ~period in a 50-ml. Erlenmcyer flask containing 5 ml. of 270 boric acid arid 1 d r o p of bronicresol green--methyl rcd indicator. The contents of the flask are then titrat,cd with standard 0.Ot.Y " 2 1 in the usual manner. 3. BOROS, SITROGLS, AID PHOSPHORUS. The procedure for the siniiiltaiieoiis determination of boron, nitrogen, and phosphorus is again quitr similar to procedures 1 and 2. In addition t o the reagents mentioned above, 5 drops of 30% H202are also added 3 or 4 time. during the cournr of a 45-minute digestion period. lkfore
rach addition of 307, H202,the fl are cooled quickly ( i o danger of breakage because they are made of quartz). .lfter digestion, the contents of the flask are cooled, diluted, and transferred quantitatively to a 50-ml. volumetric flask, which is made uii to volume. Suitable aliquots zre nitlidran-n for the boron and nitrogen determinations ( m n e as describec previously), and a filtered aliquot is withdrawn for the 1)liosphorua determination, since the method used is a sl)ectrol)hotometric Iirocedure ( 5 ) . The phosphorus aliquot i- transferred to a 100-ml. volumetric flask; t h e contents are diluted to ail~)roximately65 ml.; 1.5 ml. of concentrated H 2 S 0 4 , 10 ml. of ammonium vanadate containing 0.20yc vanadate, and 10 ml. of aminoniuni molybdate containing 10% 1100~ are added; and the flask is made up to volume with distilled H20. After standing 30 minutes, the absorbanlie is read a t 410 n i ~I W . a blank. The per cent 1' is det'erniined by reference to a calibration ciirve which has been previously prepared. Boron and Ilhosphorus or nitrogen and phosphorus can also be determined simultarwxisly by this method.
Table 1.
Experimental Results for Microdetermination of Boron and/or Nitrogen and Phosphorus
Boron, C:tlcd. Found
3.5 30 20 70
18 63
19 65 33 50
34 07 4 67 9 41
03 33 80 86
40 40 80
. . .
5.30 ...
17 65 o-I'hr.nvlme dinieth1 lamine borate, 6 6 1 CsHi B0:S P-47 1
21 11 27 71 26 31 30 49 39 33 32 70 1X 72 16 02
"
5.21
11.s5
5.22
5 .OO 5,l- wet digestion procedure. ( 1 , 2, 8, 9). For horon, the s t m d a r d deviation iq +0.15, relative error &0.05%. For nitrogen, the standard deviation i.. =tO.OT, relative error i1.347,. For phosphorus, the standard deviation i y i 0 . 1 6 , relativ: error +0.307,. ics are based on 5 determinations of three representative comliounds analyzed for boron, nitrogen, and pliosphorus by this procedure. The method for boron alone which we have descsribed in thct Iirocediire is quite simple. .\lthough y;e rrcomniend the use of CuSOi-K&04 and selenium catalyhts: we have found t h a t simply K2P04and HZSO4 are enough to convert tlie boron quantitatively to boric acid. HYSO4alone has b e m found to be effective in some casc's, but with more difficult compounds, it is not quantitat'i1.e. Evidently, the use of K&04 raise.; the boiling point of the sulfuric w i d and is essential for complete con\-cr.;ion of tlie tilor? difficult organoboranes to boric w i d . With this modification, the proccdure is simplified even more. T h e fixed-pH metliod of titration, firJt introduced by F'oote ( 3 ) .is advantageori-. Other i ~ o i k e r s [llartin and Haycs ( 6 ) , Strahm and Hawthorne ( 9 ) ] ,as well as the authors in a previou,5 pulilicatioii (?), hsw had great surcess with it. Seither the presence of phos~ ) l i o r o uin i the saini)le nor interferences
35 35 20 20 18 18 19 19
Sitrogen, c/c Calcd.
22 .8d
8.60
l2,lG 12.13
..
..
22.87
.
,
. .
. .
, . .
22.60 8.81 8 .73 ..
, . .
1" 12
11.94 12.06 ...
... ...
39 70 39 50
511
...
39 39 32 32
30
...
...
4
