Colorimetric Microdetermination of Nitrogen in Cellulose Nitrates by the Phenoldisulfonic Acid Method J. L. GARDON' and BENGT LEOPOLD Industrial Cellulose Research, Lfd., Hawkesbury, Ont., Canada
b The determination of nitrogen in cellulose nitrate, using the phenoldisulfonic acid method, has been studied. The reaction of cellulose nitrate and a sulfuric acid solution of phenoldisulfonic acid and subsequent neutralization produce an intensely yellow color, proportional to the nitrogen content at a constant pH. The color intensity per unit weight of nitrogen is higher for cellulose nitrates than for inorganic nitrates if the pH of the solution used for colorimetry is higher than 6. The color intensity changes with the pH. Above pH 8 the color intensity i s affected by atmospheric oxygen. When these factors are taken into account, nitrogen contents can be determined with accuracy to 2%, using 0.2- to 0.8-mg. samples in the pH range 7.1 to 7.5.
I
determination of molecular weight distribution of cellulose samples, the general procedure is to nitrnte the sample, subject the cellulose nitrate to a fractional separation, and determine the molecular weight of each fraction from its intrinsic viscosity. The relationship between the molecular weight and intrinsic viscosity depends largely upon the nitrogen content of the cellulose nitrate (6,10,11) and if the original sample is not fully nitrated, the nitrogen content varies from fraction to fraction. It is usually impractical to determine the nitrogen content of each fraction in routine work, because conventional methods are time-consuming and the minimum amount of material needed is relatively large, 30 to 50 mg. for duplicate experiments, which is often more than the total amount present in one fraction. A rapid micromethod for the determination of nitrogen in cellulose nitrates is therefore needed. A colorimetric method for the determination of inorganic nitrates with phenoldisulfonic acid is discussed by Snell and Snell (12). The dry nitrate is dissolved in a sulfuric acid solution containing phenoldisulfonic acid. The nitrophenol disulfonic acid thus obtained is intensely N THE
Present address, Textile Research Laboratories, Rohm & Haas Co., Philadelphia, Pa.
yellow when diluted with water and rendered alkaline with potassium or ammonium hydroxide. This method was adopted for cellulose nitrates by Brooks and Badger (3). They did not attempt to determine the nitrogen content, but, by assuming that it was constant, determined the actual amounts of nitrocellulose without essentially modifying the experimental method of Snell and Snell. The method as modified in this laboratory for the determination of nitrogen in cellulose nitrates requires only 0.2 mg. of material, is rapid, and can be easily carried out by untrained personnel. It can also be used for the determination of the amount of cellulose nitrates when there is no variation in nitrogen content between fractions as when the sample has been fully nitrated or has a nitrogen content of a t least 13.8% ( 2 ) . This eliminates not only the weighing and complete drying of various portions, but also a potential source of error, because it is very difficult to dry cellulose nitrate samples completely without degradation and subsequent difficulties in the viscosity determination. Reproducible results with cellulose nitrates can be obtained only when the p H of the final solution for colorimetric determination does not exceed 7.8. Below 7.8, the color is p H dependent and appropriate corrections must be made. Above p H 6, cellulose nitrate gives a higher color for a given amount of nitrogen than an inorganic nitrate. Thus it is necessary to calibrate in comparison to a conventional nitrogen determination. The applicability of the present method has been tested for cellulose nitrates containing over 10.5y0 nitrogen. Cellulose nitrates containing less than 10.5% nitrogen are generally insoluble in acetone. EXPERIMENTAL
Phenoldisulfonic
Acid
Solution.
Thirty-one grams of colorless phenol were dissolved in a mixture of 200 ml. of concentrated sulfuric acid and 120 ml. of fuming sulfuric acid containing 30% free sulfur trioxide, and placed in a steam bath for 2 hours. T h e solution is stable for months and small variations in the formula have no effect on the results.
Recommended Procedure f o r Cellu-
lose Nitrates of Unknown Nitrogen Content. Two milliliters of an acetone solution containing 0.2 to 0.8 mg. (preferably 0.5 to 0.7 mg.) of cellulose nitrate are pipetted into a 50-ml. beaker and evaporated a t 60" C. One milliliter of phenoldisulfonic acid solution is added from a pipet with a broken off tip. The solution is heated on a steam bath for 20 minutes and occasionally shaken to dissolve all solids. It is diluted with 5 ml. of water, and about 5 ml. of 40y0potassium hydroxide are added with stirring. The p H after the addition of potassium hydroxide should be 2 to 3, and is determined by the color. It is faintly yellow but the full color has not yet developed. The solution is transferred to a 50-ml. volumetric flask and filled to the mark with a 2M phosphate buffer solution of p H between 7 and 8 (preferably between 7.2 and 7.6). The solution is allowed to stand for a t least 4 hours, and its absorbance is determined a t 400 mp in a cell of 1-cm. length against an identically prepared blank solution using a Beckman Model DU spectrophotometer. The solution usually has a p H 0.1 to 0.2 lower than that of the buffer. The nitrogen content, N , can be calculated by Equation 1 from the absorbance reading, T , the weight of the material in milligrams, w , and the pHdependent constant, f , obtainable from Equation 4. lOOr/(w&) = N%
(1)
Additional Experiments. T o cover a wider p H range, 2M phosphate buffers were used below, and 2 M glycine buffers above p H 8. Experiments were carried out as described above with aqueous potassium nitrate solutions. Cellulose Nitrate Samples. In Table I, samples I to I V were nitrated according to Alexander and Mitchell (1) and sample V (low nitrogen content) according to Lindsley and Frank (6). The nitrogen contents reported in the third column of Table I are averages of four determinations. RESULTS AND DISCUSSIONS
Dependence of Color on pH. For calibration, g, the absorbance of a buffered nitrophenol disulfonic acid solution of a concentration correspondVOL. 30, NO. 12, DECEMBER 1958
* 2057
Table I.
Results of Nitrogen Determinations
Sample IV was used for calibrating the present method. All determinations n c r e carried out in the pH range 7 to 7.5. = nitrogen content by the conventional method (reference in fourth column). = average value of nitrogen contents determined by present method. C.L. = 95% confidence limits of iV,% taking into account the error in $ and the stnndard error of the individual N*yovalues. S, = standard error of individual iVz% values. n = number of determinations. Sample Source of 50. Material hil% Ref. N,% C.L. S, I Acetategradecottoil 13.41 4 ~ 0 . 0 7 ( I S ) 13.52 3 ~ 0 . 1 5 0.11 17 I1 A4cetategradecotton 13 7 3 =!C 0 . 0 1 (IS) 13 69 =to. 13 0.05 13 111 Ramie 1 3 . 5 4 k 0 . 0 4 ( I S ) 13.53 zk0 14 0.10 16 11Cotton linters 13 65 k 0 . 0 4 ( 5 , 7 ) v Cotton linters 10.55 =!COo.10 (5, 7 ) 10.62 zk0.12 0.09 10
ing to 1 mg. ,of nitrogen per 50 ml. is defined. If the nitrogen content is known, y can be calculated from Equation 1 by replacing 6 with y. The variation of y v i t h pH for solutions prepared from cellulose nitrate sample IV and from potassium nitrate are shown in Figure 1. Results for potassium and cellulose nitrate are identical beloa pH 6. Above p H 6, cellulose nitrate shows higher y values than potassium nitrate. Below p H 8, y increases with pH for both potassidm and cellulose nitrate, and above p H 8, the results are scattered and do not show any systematic variation Kith pH. Some of the values for potassium nitrate actually lie on a straight line corresponding t o y = 9.075. The average value of y above pH 8 is 9.8 for cellulose nitrate. To explain the shape of these curves, it was assumed as a first approsiniation that the color producing ionization follows the equilibrium laws of monobasic acids
1
=
+
(31)
IH-1)
1
FiKi
To determine the constants in Equation 3g it can first be assumed t h a t KzIlO-6. When pH
