Determination of Sodium Borohydride and Sodium Sulfite in the Presence of Each Other in Pulping Liquors Lucio Sion Nahum Cartiera Vita Mayer & C., Research Division, Cairate (Varese), Italy
RECENTLY a high yield pulping method to obtain high brightness pulp for papermaking was developed in Vita Mayer laboratories ( I ) . Essentially, the process consists of the impregnation of wood chips with SOz solution, followed by impregnation with a solution of NaBH4 in 1-2N NaOH, draining, conversion of the alkali in the chips into NaHS03 by SOzgas, and cooking. The operation of the process requires the determination of NaBH4 content in the alkaline liquor before, during and after the impregnation in order to follow the NaBH4 consumption and to establish the amount of NaBH4 which has to be added to restore the suitable concentration before recycling. Since the chips which are treated with the alkaline solution of NaBH4 are still imbued with SOz solution, Na2S03 will be present also. Therefore it was necessary to develop a simple, quick, and accurate method for the separate determination of NaBH, and NaZSO3. For this purpose, the sum of both was determined on a separate sample, and the amount of NazS03was determined on another sample after destruction of NaBH4with a ketone. The best results in the determination of total amount of NaBH4 and Na2SO3 were obtained by oxidation with a measured excess of iodine and back-titration with thiosulfate, as previously reported for the determination of NaBH4 (2). Hypochlorite was not suitable for the oxidation of NaBH4 and NazSOa,because a large amount is consumed by the organic substances extracted from wood during impregnation. Several ketones were tried to destroy NaBH4; very good results were obtained with D-fructose. It would not be possible to destroy NaBH4 by acidification of the sample, because, before reaching a suitable pH, side reactions between NaBH, and Na2S03 cause inaccurate and not reproducible results (3). EXPERIMENTAL
All reagents used were reagent grade. Fructose was added as a solid or as an aqueous solution. Samples were prepared by mixing in different ratios measured volumes of standard solutions of NaBH4 (in 1N NaOH) and N a S 0 3 . Also samples of impregnating liquors from pilot plant or laboratory impregnations were tested. Procedure. To one sample, 50 ml of 0.1 N Iz were added. After acidification with 10 ml of 4N HzSOa, excess IZ was titrated using x ml of 0.1N Na2S203in the presence of starch. A blank was run, using y ml of 0.1N Na2SzO3. To another sample, a strong excess of fructose (6-10 grams) was added. After 5 minutes, 10 ml of 4N followed immediately by 50 ml of 0.1N Iz were added. Alternatively, 50 ml of an acidic 0.1N IZ solution, prepared by mixing 200 ml of 1N Iz with 400 ml of 4N HzS04 and diluting (1) C . E. Kriiger, G. Ruffini, G. Gandini, and M. Ghislandi,
patent pending. (2) R. F. Skoblionok, K. N. Mochalov, and B. G. Berner, Zh. Anal. Khim., 23, 1518 (1968); Chem. Abstr., 70, 16832d (1969). (3) Hiicker, Bayer, Leverkusen, Germany, private communication 1971.
to 2000 ml with water, were added. Excess with z ml of 0.1NNa2S203. Na2SO3(meq)
=
12
was titrated
( y - r ) 0.1
NaBH4 (meq) = ( z
- x ) 0.1
The blank is unnecessary if only NaBH4 content is to be determined. RESULTS AND DISCUSSION
In order to be suitable for the purpose of this study, a ketone must destroy NaBH4 quantitatively and should not react with Na2S03; moreover, the alcohol originating from its reduction by NaBH4should not react with 12. Only 95-98 % of NaBH, was destroyed using 2-butanone or 2,4-pentanedione. 6-Methyl-2,4-heptanedione gave quantitative results, but the final point was very poor, since the insoluble ketone extracts iodine from the aqueous layer, and the reaction with thiosulfate is slow. Dimedone (5,5-dimethyl1,3-~yclohexanedione)destroys NaBH4, quantitatively, but, in the presence of NaZSO3,the results were higher. Poor results were obtained with acetone, cyclohexanone, benzophenone, 2-pentanone, 3-pentanone, 4-methyl-2-pentanone, and p-aminoacetophenone. Table I shows some examples of determinations with samples containing only NaBH4or NazS03or both in the absence and in the presence of fructose. Examples 3 and 14 demonstrate the complete destruction of NaBHr by fructose; examples 4 and 16 show no interference between Na2S03and fructose. Examples 8, 10, 11, and 12 show the effectiveness of this analytical method with different ratios of NaBH4 and NazS03, provided a sufficient excess of fructose was added, while examples 6 and 9 gave higher results because fructose was not enough to destroy quantitatively NaBH4 (compare respectively with example 8 and 10). Before the determination, the sample is strongly alkaline, generally containing about 40 g/l. NaOH or more; actually NaBH, is stable only in alkaline solution; moreover, in the presence of bisulfite, under slightly acidic conditions, NaBH, is known to react to give hydrosulfite ( 4 5 ) . It is also known (6)that iodine reacts with alkali; therefore the titration with thiosulfate must be carried out after acidification of the solution. According to the procedure described in the experimental part, the acidification of the solution does not affect the results at all, because, when the acid is added, NaBH, has already quantitatively reacted with either iodine or fructose. Thus, in the absence of fructose, the acid was added after the iodine; otherwise a relevant portion of NaBH4 would be destroyed by the acid before it could react with iodine. If NaBH4 has been destroyed by fructose, it is necessary either t o acidify before adding iodine or to use an acidic solu(4) P. Luner, R. La Plaine, and R. C . Wade, Pulp Pap. Mag. Can. 65 (c), T-101 (1964). ( 5 ) C . A. Richardson, D. C. Johnson, and T. G. Goodart, Tappi, 53, 2275 (1970). (6) F. P. Treadwell, “Trattato di Chimica Analytica,” Vol. 11, F. Vallardi, Milano, Italy, 1954, p 679.
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Table I. Determination of Mixtures of NaBH4 and NszSOa Standard solutions taken (ml) Oxidant NaBH4 NanSOa D-Fructose, g Expected, rneq Found, rneq 1 Iodine 5 0 0 1.930" 2 0 20 0 Iodine 1.660" 3 Iodine 5 0 6 O.Oo0 O.Oo0 Iodine 6 4 0 20 1.660 1.665 5 Iodine 3.590 3,570 5 0 20 Iodine 6 5 20 5 1 660 (1.700) 7 Iodine 20 1.660 5 6 1,675 Iodine 20 1.660 8 5 10 1.660 Iodine 9 20 1.660 (2.410) 10 5 Iodine 1.660 10 20 1.660 10 10 Iodine 11 6 0.830 5 10 0.825 12 Iodine 5 6 0.415 5 0.415 13 5 Iodate 1.930 0 0 1.925 14 Iodate 5 6 0 O.Oo0 0.005 Iodate 15 0 20 0 1.660 1.655 16 Iodate 20 1.660 0 6 1.650 17 Iodate 5 20 3.590 0 (3.530) Iodate 18 5 6 20 1.660 1.655 a These two values, obtained by the conventional iodine method, were taken as the expected values in the other examples. Each determination was repeated at least on two samples. Reproducibility of the method was within =tO.OlOrneq. Example
I
tion of iodine; adding iodine in the alkaline solution and then acidifying, leads to higher wrong values. The difference is due to fructose: actually it was verified that 7 grams of fructose in alkaline solution consumed about 0.2 meq 1 2 , and this reaction was not reversible after acidification. Examples 13-17 refer to the use of iodate instead of iodine. They confirm that NaBH4 was completely destroyed by fructose (example 14), so that iodate can be used to determine residual Na2S03(example IS). However example 17 shows lower results were obtained in the determination of the sum of NaBH4 and Na2S03; then iodate is not suitable for accurate determination of NaBH4in the presence of Na2S03. The reaction between NaBH4 and fructose leads to the sodium salt of the strong acid which is known to form as a complex between boric acid and mannitol or sorbitol. The consequent decrease of titratable alkali cannot be exploited for the determination of NaBH4 because the very small ratio between the latter and total alkali would cause high relative errors.
A series of 10-ml samples of liquors, which had been used for the impregnation of poplar chips, were analyzed by the iodine method. The samples contained 0.05O-O.187 meq/ml NaBH4 and 0.0154.025 meq/ml NazSOa. Each determination was repeated with 7 and 10 grams of fructose obtaining the same results within =tO.OOl meq/ml. During the impregnation of poplar chips with alkaline solutions of NaBH4, up to 5 of wood is dissolved in the impregnating liquor. Samples of alkaline solutions of NaBHa, which had been used to impregnate poplar chips and did not contain any sulfite, were treated with fructose and added to solutions of known Na2S03content. The resulting solutions were acidified, treated with iodine, and titrated with thiosulfate. The amount of consumed I2corresponded to the Na2SO3 present and was not affected by the presence of organic extractives. RECEIVED for review June 15, 1971. Accepted September 13, 1971.
Titrimetric Determination of Uranium with Electrogenerated Vanadium(V) Clara Gale Goldbeck and Morris W. Lerner United States Atomic Energy Commission, P.O.Box 150, New Brunswick, N.J. 08903 THEDAVIESAND GRAY( I ) method of determining uranium involves reduction to U(1V) by Fe(I1) in a H3P04 medium, oxidation of the excess Fe(I1) with "03 and molybdate catalyst, and titration of the diluted solution with potassium dichromate to a barium diphenylamine sulfonate end point. This method is applicable to the analysis of many types of samples since few elements interfere, but its potentially enormous utility has been hindered by a poor titration end point. (1) W. Davies and W. Gray, Talanta, 11, 1203 (1964).
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However, this laboratory recently reported (2) a modified method in which V(1V) is added to the solution before titration to dramatically sharpen the end point and to enable potentiometric titration to be used. The function of the.V(IV) is not certain, although it has been postulated that it reacts with the U(1V) to give V(II1) and U(VI), a reaction that has been shown to occur in the presence of Fe(I1I) (2), and that (2) A. R. Eberle, M. W. Lerner, C.G. Goldbeck, and C. J. Rodden, US. At. Energy Comm. Rep. NBL-252 (July 1970).
