p-Phenylenediamine Derivatives as Reagents for Ultraviolet Absorptiometric Determination of Witrite Ion DONALD F. KUEMMEL with M. G. MELLON Purdue University,
f afayette, Ind.
The ultraviolet spectra of the diazonium salts formed from p-phenylenediamine and many of its derivatives can be used for the absorptiometric determination of the nitrite ion. These salts absorb strongly in the 320to 400-mp region, w-here the reagents themselves do not absorb significantly. The sensitivity of these reagents tow-ard nitrite ion is comparable to that of existing colorimetric methods, the molar absorbancy indices of the diazonium salts being in the 33,000 to 40,000 range. Chloro-p-phenylenediamine was singIed out for thorough investigation of reproducibility, reagent stability, and interference effects.
The ultraviolet method for nitrite ion offers the distinct advantage of eliminating the coupling step, with its attendant problems in p H adjustment, timing, and limited solubility of the azo dye. I n an attempt to find a better reagent than sulfanilic acid for the ultraviolet determination of nitrite ion, particularly in regard to sensitivity, more than 100 aromatic amines were surveyed as t o the effect of nitrous acid on their ultraviolet absorption spectra. This survey showed p-phenylenediamine derivatives to be exceedingly sensitive reagents toward nitrite ion (U,M 33,000 to 40,000). The work reported herein is a summary of a detailed investigation of p-phenylenediamine derivatives which followed the original survey of aromatic amines. The chloro-p-phenylenediamine system was singled out for investigation of reproducibility, reagent stability, and interference effects, and is discussed in some detail. To reduce the amount of repetition, the designation “para” is usually omitted and is t o be assumed in all subsequent discussion of the p-phenylenediamines.
T
HE reaction of aromatic amines with nitrous acid, sometimes referred to as the diazo reaction, has been used extensively in the colorimetric determination of nitrite ion. I n these methods, the diazotization of the amine is followed by a coupling step with a suitable reagent to give a colored azo dye, the intensity of which is related to the original nitrite ion concentration. The most widely used colorimetric method for nitrite ion is that employing sulfanilic acid as the diazotee, and 1-naphthylamine as the coupling agent. Rider and AIellon (6) have made a critical study of this method. These reagents i\-ere proposed by Griess ( 9 ) in 1870, and remain today the basis of the standard method for nitrite ion sanctioned by the American Public Health Association and the American Water JTorks Association ( 1 1. The spectrophotometric determination of nitrite ion utilizing the diazo reaction alone, without subsequent coupling of the diazonium salt, has received little attention. The primary reason for this is probably the fact that the absorption maxima of a majority of the diazonium salts lie in the ultraviolet region, necessitating special instrumentation (quartz optics, etc. ) if these maxima are t o be used in absorptiometric methods. The only reported method for nitrite ion utilizing the ultraviolet absorption of diazonium compounds is that of Pappenhagen and LIcllon ( 4 ) . This method is based upon the changes caused by the nitrite ion (via nitrous acid) in the absorption spectrum of sulfanilic acid at 270 m p , 1Xeaunrements n-ere made cs. a reagent blank, because sulfanilic acid itself absorbs significantly a t this wave length, The major disadvantage of the method is its lorn sensitivity (UJI = 15,300j compared to accepted colorimetric methods.
Table I.
APPARATUS AND REAGENTS
Apparatus. Absorption spectral data were obtained on three different instruments. A Cary Model 10-11M recording spectrophotometer mas used in all the preliminary work nhere the entire ultraviolet spectrum mas wanted. A11 calibration curves, interference effects, and stability and precision studies were obtained using the Beckman Model B and Model DU spectrophotometers. The sensitivity control on the Beckman Model B was kept a t 2 for this work. hlatched 1-cm. quartz absorption cells were used with all the instruments. Glass-stoppered volumetric flasks of 50-ml. capacity were used in the preparation of all of the solutions run on the instrument. All pH measurements were made on a Beckman Model H-2 glass electrode pH meter. Reagents. The sodium nitrite, potassium permanganate, sodium thiosulfate, and potassium iodide used in the preparation and standardization of the stock nitrite solution were Baker’s analyzed reagents. All the p-phenylenediamine derivatives investigated were the best grade obtainable of Eastman organic, chemicals. The amine hydrochlorides were used whenever available because of the greater stability of the acid salts compared to free amines. The Eastman catalog number of the various reagents is included in the data of Table I. Stock Solutions. Stock solutions of the various amines investigated were prepared in 0.005- or 0.01X concentration by dissolving an accurately weighed portion of the reagent in distilled v-ater acidified with 1 to 4 hydrochloric acid, followed by dilution to the desired volume. The final acid concentration of the stock solutions used in the studies of the effect of p H and reagent con-
Sensitivity of p-Phenylenedcamine Derivatives toward Nitrite Ion Eastman Catalog
?io. Compound 394 H2S-CaHa-SHz 1330 HzN-CEHI-XHCHE HzN-CsHa-N (CHa)2 192 HzN-CsHa-S(Et)z 1374 H~N-C~HI-NHCOCHE‘ 13 HzZ-CeHa-N(CH3) COC Hs T1773 H i h -CsHs(CHa)-NHz 1206 36.54 H2N-C6Ha(Cl)-NH% H*N-CsI-I3(CH3)-S(Et)’ 3478 a D a t a given apply t o system a t p H 0.3.
X of M a x . Change,
a ’ ~
at
llp
XU*X.
353 365 376 378 335 338 353 354 378
39,400 22,300 33,500 37,700 23,500 12,200 36,300 37,900 39,400
1674
Time a f t e r lIixing, Min. 10-15 10-20 13-20 3-23 20-130 7-15 10-20 5-100 3- 10
Reagent Absorption, Aa = 0.01 at Concn., M p IM pH 280 0 0003 14 315 0 001 1 1 350 0 0004 1 2 0 001 1 1 315 290 0 0001 0 3 0 0008 1 1 305 0 001 1 1 300 340 0 001 12 0 0004 1 3 300
V O L U M E 2 8 , NO. 11, N O V E M B E R 1 9 5 6
1675
250
275
325
-00
AAVE
Figure 1.
Effect of nitrous acid on absorption spectrum af p-phenylenediamine at pH 1.4
_ _ _ 0.0002A4 solution of
reagent 0.0002.~ splution of reagent 10 t o 15 minutes after addition of 0.059 mg. of nitrite ion per 50 ml.
centration was approximately 1 t o 100 or 1 to 50 hydrochloric acid ( 5 or 10 ml. of 1 to 4 hydrochloric acid per 100-ml. volume). Stock solutions of the amines prepared for the calibration work were usually relatively acidic (1 : 12 to 1:4 hydrochloric acid) solutions. The acid content of each stock solution was so adjusted that the aliquot most frequently takeii would contain sufficient acid t o give the desired p H (1.0 to 1.5). This eliminated the need for a separate addition of acid in preparing the test solutions to be run on the instruments. Stability studies on the reagents themselves were carried out on these acidified stock solutions, as they were generally more stable than those of the same concentration of reagent prepared in more dilute acid. The standard nitrite solutions used throughout this work \yere approximately 0.00025M (0.0115 mg. of nitrite ion per ml.), and were prepared by successive dilution of 0.05- and 0.01X solutions of sodium nitrite. The 0.0531 nitrite solution was standardized titrimetrically, by adding an excess of potassium permanganate to oxidize the nitrite, followed by a thiosulfate titration of the iodine liberated by the action of the excess permanganate on potassium iodide. Details of this standardization procedure are givrn bv Kolthoff and Sandell ( 3 ) . The exact concentration of the 0.00025J1 nitrite solution was calculated from the standard value of the 0.0551 solution, using the necessary dilution factors.
LEliSrH
353 (Vrl
Figure 2. Effect of nitrous acid on absorption spectrum of p-amino-N-methylacetanilide at pH 1.1
- 0.0008.M solution of reagent - - - 0.0008-Wsolution of reagent 7 to of nitrite ion per 50 ml.
:1 minutes after addition of 0.178 mg.
I ?
W"E
LEI~GTH
\mu1
Figure 3. Effect of nitrous acid on absorption spectrum of iV,N-diethyl-p-phenylenediamineat pH 1.1 - 0.OOl.M solution of reagent
_ _ _ of0.001.M solution of reagent 3 to 23 minutes after addition of 0.059 mg. nitrite ion per 50 ml.
SURVEY OF PHENYLEKEDIAMINES
Effect of Nitrous Acid on Absorption Spectra. Figures 1 to 3 illustrate the type of absorption changes which takes place upon the addition of nitrite ion to phenylenediamine derivatives in arid solution. rlll of the compounds investigated give very similar changes. The principal absorption maxima of all the diazoilium salts occur in the 330- to 380-nip region, whereas the reagents themselves shon- no significant absorption. Table I summarizes the ultraviolet spectral data for the various reagents. The data apply t o systems having pH's of 1.0 to 1.4, unless otherwise stated. The sensitivities of the various reagents toim,rd nitrite i o n are expressed as molar absorbancy indices ( a 1 1values) These values xere calculated according to the equation, a.11 = A.4.(2300)/c, where A A , is the absorbancy change of the system a t a specific wave length upon the addition of c nig. of nitrite ion per 50 ml. and 2300 is a factor converting the nitrite concentration to moles per liter. The ultraviolet cutoffs of the reagents themselves are also listed in Table I. The n a v e length a t which the reagent starts to absorb significantly (A48= 0.01) is given, together with the reagent concentration aiid p H of the solution to rrhich the cutoff applieq. Stability of the Systems. The diazonium salts formed from the p-phenylenediamines are light-sensitive. If no precautions are taken to shield the solutions from light after mixing, the diazonium salts decompose a few minutes after they are formed, as is el-idenced by the rapid decrease in the intensity of the
absorption maxima in the 330- to 380-mp region. The absorbancy obtained depends upon how long the solutions m r e exposed to the light before they rrere placed in the spectrophotometer. If the solution is transferred to the absorption cell and placed in the cell compartment immediately after mixing, an extremely stable system results. Under these circumstances, the absorbancy quickly rises to a maximum and remains constant if the solution is not exposed to the light. All the work reported herein pertains to solutions placed in the instrument immediately after mixing. The time intervals specified in Table I pertain to the time after mixing during which the systems n-ere found to give a constant absorbancy reading, if kept in the darkness of the spectrophotometer cell compartment. S o formal stability studies of the various systems n-ere undertaken for periods longer than those specified. However, it was apparent during the course of the investigation that the majority of the systems are stable for hours if light is excluded. Effect of pH and Reagent Concentration, The effect of p H and reagent concentration of the system was determined in some instances. Varying the pH in the range 1.0 t o 2.4 usually changed the spectrum of the reagent considerably, but had little or no effect on the intensity of the absorption maximum of the diazonium salt in the 330- to 380-mp region. I n general, it was observed that both the formation and light-catalyzed decomposition of the diazonium salt proceed a t slower rates as the p H is raised. Perchloric and sulfuric acid were substituted for hy-
1676
ANALYTICAL CHEMISTRY
drochloric acid in some of the tests involving phenylenediamine and 2,8toluenediamine. The use of these acids resulted in s y s terns which gave less intense absorption maxima, and xhich reached these maxima more slowly than the corresponding systems in hydrochloric acid. The concentration of several of the reagents was varied from 0.0002- to 0.001- or 0.002M, with negligible effects upon the intensity of the absorption maxima of the diazonium salts.
Calibration Curves. The Model D U spectrophotometer was calibrated for the determination of nitrite ion using several of the reagents. Details of the calibration procedure will be found in the following section. The phenylenediamine, chlorophenylenediamine, 2,5-toluenediamine, and N,N-diethylphenylenediamine systems all followed Beer's law a t the wave lengths given in Table I up to the calibration limit of 0.06 mg. of nitrite ion per 50 ml. Slit widths of 0.08 to 0.11 mm. m r e used in this work on the Model DU. The Beckman Model B spectrophotometer was also calibrated using a few of the reagents. The aminoacetanilide system follows Beer's law over the range 0.00 to 0.08 mg. of nitrite ion per 50 ml. when this instrument is used. The N,N-diethylphenylenediamine system s h o w a slight deviation from the usual straightline plot above 0.04 mg. of nitrite ion per 50 ml., in contrast to its behavior using the Model DU. CHLOROPHENY LENEDIAMINE SYSTEM
250
275
300
325 WAVELENGTH
350
375
(mpl
Figure 4. Effect of n i t r o u s acid on absorption s p e c t r u m of chloro-p-phenylenediamine a t pH 1.2 - 0.001-Wsolution of reagent _--
0.001M solution of reagent 3 t o 11 minutes after addition of 0.038 nig. of nitrite ion per 50 ml.
Stability of the Reagents. Aromatic amines in general are sensitive to air and light, and should be protected therefrom as much as possible. The amine hydrochlorides were chosen for this work, when available, so as to minimize the decomposition n-hich often occurs in amino compounds upon prolonged standing. Stability studies were carried out on hydrochloric acid solutions of several of the reagents. A 0.0075M solution of phenylenediamine in 1 to 4 acid and a 0.05M solution in 1 to 50 acid were found to be stable for 48 hours. The less acidic solution showed a more rapid rate of decomposition after this period. A 0.01M stock solution of S,N-diethylphenylenediamine in 1 to 12 acid was found to be stable for 7 days after its preparation. I t gave reproducible absorbancy readings upon the addition of nitrite ion during this period, despite the development of a slight pink hue in the solution after 3 or 4 days. A 0.001M solution of aminoacetanilide in 1 to 50 acid gives reproducible absorbancy readings for 5 days after its preparation. Stock solutions 0.OlM in 2,5-toluenediamine, in both 1 to 100 and 1 to 4 hydrochloric acid, showed signs of decomposition 2 to 3 days after their preparation. Mole Ratio Studies. The similarity in absorption characteristics among the various systems investigated is apparent from Table I. Compounds having N-substituted amino groups give the same type of absorption maximum, in the same region and of comparable intensity t o those having two free amino groups. This would indicate that only one of the free amino groups is involved in the reaction with nitrite ion, and that diazotization and not tetrazotization is the predominant reaction. To confirm this belief, mole ratio studies were carried out on the phenylenediamine, chlorophenylenediamine, and 2,5,-toluenediamine systems. The data in Figure 5 pertaining to chlorophenylenediamine are indicative of the type of plot obtained in these studies, and the range of concentrations employed. Values of 0.92 and 0.87 mole of nitrite per mole of reagent were obtained for the phenylenediamine system. 2,%Toluenediamine gave 0.90 and 0.82 mole of nitrite per mole of reagent. Results of the mole ratio studies using chlorophenylenediamine are discussed i n the following section.
Variables Influencing Absorption Spectrum. The effect of nitrous acid on the absorption spectrum of chlorophenylenediamine is shown in Figure 4. The addition of nitrite ion causes absorption changes similar to the phenylenediamine derivatives discussed in the previous section. Varying the pH from 1.2 to 2.4 causes a decrease of 0.06 absorbancy unit in the 354-mp maximum at the absorbancy level shown i n Figure 4. A change of 0.02 absorbancy unit in this maximum is also observed when the reagent concentration is varied from 0.0002- to 0.00131, The changes noted above indicate that the system is not entirely insensitive to pH and reagent concentration. However, these variables cannot be considered critical, since slight deviations from the specified conditions in any proposed method cause negligible error as far as p H and reagent concentration are concerned. All the data presented in this section refer to the behavior of the chlorophenylenediamine system in the absence of light, as this system exhibits the same light instability as other phenylenediamine derivatives. Stability of the Reagent. The reagent, a white free-flowing powder, was obtained as the dihydrochloride. The presence of the electronegative chlorine atom on the ring was expected to inerease the stability of this compound compared to phenylenediamine itself, or its derivatives containing alkyl groups on the nitrogen or benzene ring. A 0.005M solution of the reagent in 1 to 100 hydrochloric acid is colorless, but develops a faint
"
2.0
1.0
MOLES
NO;/MOLE
REAGENT
Figure 5 . Mole ratio plot for chloro-p-phenylenediamine system a t 354 mp Instrument. Beckman Model B. slit width approximately 0.8 mm. Reagent concentration. 2 X 10-j.W Nitrite concentration. 0.51 X 10-5 t o 4.08 X 1O-C.V Acidity. p H 1.2
1677
V O L U M E 28, N O . 11, N O V E M B E R 1 9 5 6 pinkish brown tint after standing for 15 days. Qualitatively, this is indicative of greater stability, since stock solutions of many of the other diamines in this series developed pink huee after 2 or 3 days. No quantitative stability studies m r e undertaken on this weakly acidic solution.
Table 11. Calibration Curve Data and Results of Precision Studies for Chloro-p-phenylenediamine System Reagent ConCn. pH. 0.9 t o 1.1 Kitrite Ion Concn., hIg./5@ MI.
0.001i%f
.4s a t 354 h1p Range Average 0 192 0 0117 0 190-0 195 0 378 0 0234 0 369-0 388 0 578 0 03.51 0 552-0 594 0 774 0 0468 0 752-0 785 0 972 0 0585 0 939-0 996 0 n = number of test solutions used to obtain ion concentration.
11.5
5 19 42 11 21 data
Standard Deviation of A, a t 354 R I p b 0 0021 0 0054 0 0093 0 0105 0 0127 a t each level of nittite
-/
b u =
\-.
Stability studies were carried out on several acidified 0.01W stock solutions of the reagent. These solutions \?-ere prepared h ~ dissolving 0.2155-gram portions of the reagent in 40 ml. of water and 60 ml. of 1 to 4 hydrochloric acid, giving a final acid concentration equivalent to approximately 1 to 7 hydrochloric acid. The stability of these solutions was indicated by the constancy of absorbancy readings obtained wit'h the same quantities of nitrit,e ion over the period studied. The stability of three different stock solutions investigated a t different times during a 3-month period varied from 5 to 8 days. KO color was observed in any of the stock solut,ions during these studies. The reason for the variation in stability is not definitely known. The stock solutions were kept in the dark when not in use. I t is possible that they were exposed to unequal amounts of light during the first few days after their preparation, and hence showed different rates of deterioration. I t is recommended that 5 days be taken as the maximum period during which any given 0.01JI reagent stock solution can be used. Mole Ratio Studies. On the basis of previous work with other phenylenediamine derivatives, the species accounting for the absorption maximum a t 354 mp was again believed to be the diazonium salt rather than the tetrazonium compound. T o support this contention, two mole ratio studies were carried out on this system. One study involved varying the nitrite ion concentration while keeping the concentration of reagent constant, and gave a value of 0.81 mole of nitrite ion per mole of reagent. The plot of the data obtained in this study is given in Figure 5 . I n the second study, the nitrite ion concentration was kept a t a constant level while the reagent concentration u'as varied. ,4value of 1.28 moles of reagent, per mole of nitrite ion was obtained. This value agrees well with the value of 1.23, calculated by taking the reciprocal of the 0.81 ratio of nitrite ion to reagent obtained in the first study. .411 solutions m r e kept in the darkness of the cell compartment imtil the absorbancy reached a maximum and remained constant. This took 2 to 3 hours for some of the test solutions. Calibration Curve. Both the Beckman Model B and Model DIT spectrophotometers were calibrat,ed for the determination of nitrite ion up to 0.06 mg. per 50 ml., using the chlorophenylenediamine system a t 354 mp. .4 0.11-mm. slit width was used on the Model DU, and an approximately 0.8-mm. slit on the Model B. The system followed Beer's law when either instrument was used, and practically identical absorbancy readings were obtained on both instruments for each of the five calibration solutions. These solutions were prepared by adding 5 ml. of the
0.0111.1 acidified stock solution of the reagent to various aliquots (1 to 5 ml.) of the stock nitrite solution, followed by dilution t o 50 ml. The solutions were transferred to the absorption cell and placed in the spectrophotometer immediately after mixing. Absorbancy readings \\-ere taken 5 to 10 minutes after the solutions had been placed in the instrument. hIolar absorbancy indices of the absorbing species a t 354 mp varied from 37,400 to 38,200 for these solutions, with an average value of 37,900. Calibration curve data are combined in Table I1 with the results of the precision studies. Precision Studies. iifter the calibration curve had been set up, the calibration was checked repeatedly over a 3-month period using the RIodel B spectrophotometer. The variation in the absorbancy readings obtained for a given amount of nitrite during this period gives an idea of the reproducibility of the system, assuming negligible errors in reproducing instrument variables. Kine different 0.01M reagent stock solutions and four diffeient 0.0002531 stock nitrite solutions, prepared as described earlier, were used over the 3-month period. The same aliquots fl to 3 ml.) of the stock nitrite solution used in the calibration work were taken for the precision studiep. Table I1 summarizes the sbiorbancy readings obtained for the five different nitrite ion concentrations used. The stock nitrite solutions m r e carefully prepared from the same bottle of sodium nitrite, and upon standardization were found to have the same strength. However, it must be eniphasized that the chlorophenylenediamine system is more sensitive to small changes in nitrite concentration than the titrimetric method used to standardize the nitrite stock solutions. Thus, small differences in nitrite ion concentration among the four stock solutions, not detected by the method of standardization, contributed to the variation in absorbancy readings noted in Table 11. The limits of reproducibility of the Model B spectrophotometer itself for absorbanries above 0.50 unit must also be kept in mind when the precipion of the chlorophenylenediamine system is evaluated.
Table 111. Effects of Suspected Interfering Ions
Ion
+
Ba Be+? Cat+ CdfL: cu +
+-
A:+;
Fe+- + +
K
Xg+* >In++ SHaT+ pii
+
VO; Zn
a b
Reagent Concn. 0.001.M Nitrite Concn. 0.0351 mg./50 ml. 1.0 t o 1.1 ?:;runlent. Beckinan Model B, 354 mp Amount Relative hiax. Amount of Ion, Error, for
