the problem. However, working of high temperature alloys is a very expensive process; and, therefore, economics dictate that the chemistry of an alloy be ascertained as soon as possiblethat is, in the as-cast state. To obtain an accurate analysis of an as-cast ingot by any method it would be necessary to average the values obtained by making analyses of all portions of the material between the surface and the center of the ingot. The average of all of these determinations would be representative of the titanium content of the cross section. This procedure would have to be followed regardless of the size of the ingot, since this work has shown that titanium segregation is substantially independent of ingot size. Fortunately, a simplified sampling procedure can be utilized which greatly facilitates x-ray determination. Just before a heat of an alloy is poured into the ingot mold, a small tapered ingot is cast. This is cut a t the place where the crosssectional area is such that one half of it tan be irradiated a t one time for
x-ray fluorescent determination. Thus, the average titanium content is obtained with a single determination. Results obtained this way have been consistently in good agreement with wet chemistry determinations of finished materials. In more recent nickel base high temperature alloys such as Udimet 500 and Ren6 41, elements other than titanium were found to segregate. Sometimes these variations in composition were from top to bottom of the ingot instead of across the ingot. Therefore, in establishing any analysis procedure for high temperature alloys, proper selection of sample is especially important. The sampling method described above has proved satisfactory for analysis of these high temperature alloys provided that due consideration is given to the type and direction of segregation. For example, it may be necessary to make analyses of both vertical and horizontal sections of the sample. Surveys of as-cast ingots of an alloy must be made to determine what sampling procedures are necessary.
ACKNOWLEDGMENT
The authors are grateful to the hIetallurgica1 Products Department of the General Electric Co. for permission t o publish this information. Eugene W. Thiele was most helpful in providing the wet chemistry analyses and advice. LITERATURE CITED
(1) Brissey, R. M., ANAL. CHEM. 25,
190-2 (1953).
( 2 ) Fornwalt, D. E., Komisarek, J., Anal. Chem. in Nuclear Reactor Tech.
TID-7568 (Pt. l), 25&64 ($pril 1959).
(3) Laurila, E. A , , Saari, L., Castern, O., SOC.Mining Engineers A I M E Preprint 60B19. (4) Mitchell, B. J., ANAL. CHEM. 30,
1894-1900 (Dec. 1958).
( 5 ) Peterson, L., Jernkontorets Ann. 142, 203-8 (1958). ( 6 ) Reith, A. M., Weisert, E. D., Metal Progr. 70, 83-7 (July 1956).
RECEIVED for review August 15, 1960. Accepted January 4, 1961. Presented in part, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., May 1958.
Spectrophotometric Determination of Ammonia as Indophenol W.
T. BOLLETER, C. J.
BUSHMAN, and
P. W.
TIDWELL
Monsanto Chemical Co., Texas City, Tex.
b A sensitive photometric determination of ammonia is based on the reaction with phenol and hypochlorite, to give an indophenol which is intensely blue in an alkaline medium. The reaction variables for color development were studied b y a designed experiment and a procedure was developed which is more sensitive and reproducible than previously reported methods. The optimum concentration range is from 0.3 to 3.0 p.p.m. of ammonia in the color developed solution and the standard deviation is 0.03 p.p.m. at the 1-p.p.m. level. The specific absorptivity is 0.34 per p.p.m.-cm. The reaction of substituted anilines with phenol in an alkaline medium was also investigated. The specific absorptivity was determined for several para-substituted anilines which react to form blue indophenols. Ortho- and meta-substituted anilines do not give a blue reaction product which permits distinction of the para derivative.
T
BLUE COLOR of indophenol formed by ammonia, hypochlorite, and phenol in an alkaline medium was HE
592
ANALYTICAL CHEMISTRY
first reported by Berthelot (1). A number of workers (5, 6) have modified the method for the determination of ammonia from various sources. Although sensitive, the method generally gives poor reproducibility. Crowther and Large (3) reported a modification in which color development was obtained without heating. Riley and Sinhaseni (7) applied this procedure to the determination of ammonia in natural waters. Although sensitive and reproducible, this modification requires a lengthy color development period and yields solutions which are stable for about an hour. I n this investigation, the reaction variables were studied and a relatively simple procedure was developed which is rapid and sensitive, and gives reproducible colored solutions that are stable for a minimum of 12 hours.
C.P. ammonium chloride in 1 liter of ammonia-free 11-ater.
COLORIMETRIC PROCEDURE
I d d to a 50-ml. volumetric flask, 25 ml. of saturated boric acid solution containing from 10 to 300 pg. of ammonia. While swirling the flask t o ensure good mixing, add in order 5 ml. of chlorine water and 5 ml. of 8% phenol solution. Place the flask in a steam bath for 3 minutes. Remove the flask from the bath and cool it rapidly. Add 5 ml. of 3M sodium hydroxide and dilute to volume. After about 5 minutes, read the absorbance of the blue solution in a 1-em. cell a t 625 m,u against a reagent blank. Calibration data were obtained by adding aliquots of dilutions of the stock ammonia solution to 25 ml. of boricacid solution and carrying out the color developing procedure. The calibration data are given in Table I.
REAGENTS RESULTS
Chlorine water, saturated solution (prepare fresh every few days). Standard ammonia solution for calibration. Prepare a stock solution containing 1000 p.p.m. of ammonia (NH,) by dissolving 3.145 grams of dry
The colorimetric procedure described above differs from those previously reported in two major respects, vis., the chlorine water (hypochlorite) is added before the phenol and the solu-
tion is not made alkaline until after heating and cooling. This latter change eliminates the possible loss of ammonia which could occur when heating an alkaline solution. As a consequence of these changes, better reproducibility and greater sensitivity are obtained over previously reported methods in which heating was necessary. Effect of Volume of Reagents. The intensity of the blue color maximizes for a given amount of each reagent. I n this investigation the volumes of chlorine mater, phenol, and sodium hydroxide required to give maximum absorbance were determined by a statistically designed experiment as described by Box ( 2 ) . Although the time of heating and the volume of boric acid solution are significant variables, they were held constant a t 3 minutes and 25 ml. to minimize the complexity of the experiment. The experimental work was done in two parts; part one represented the basic central composite experimental design and part two was a set of confirmatory runs made to corroborate the results in part one. The results indicated that maximum color was developed by using 6.4 ml. of chlorine water, 5.9 ml. of phenol solution, and 5.00 ml. of 3M sodium hydroxide. Further work showed that some deviation from the volumes of chlorine n atcr and phenol could be tolerated TI ithout seriously affecting the intensity of the color formed. More convenient volumes of these were used in subsequent studies. This was justified since analysis of the data showed the system to be most sensitive to the volume of sodium hydroxide solution, somewhat less sensitive to changes in the volume of chlorine water, and relatively insensitive to changes in the volume of phenol solution. Consequently, rather wide variations in phenol solution volumes can be tolerated without greatly influencing the absorbance. The volume of sodium hydroxide must be closely controlled as the intensity of the color is p H dependent. The pH of solutions having maximurn absorbance is 9.9 to 10.0. The nature of the effect of volume of chlorine water on absorbance depends on whether an insufficient or an excessive amount is used. If too little chlorine water is added, color development is incomplete. If too much chlorine water is added, maximum absorbance is not obtained because the solution becomes turbid, presumably due to the formation of insoluble chlorinated phenols. If the order of adding the chlorine water and phenol solution is reversed, the color is not developed. Effect of Heating Time. To obtain full development of the blue color i t is necessary t o heat the solution con-
taining the ammonia, chlorine water, and phenol before adding the sodium hydroxide. The intensity of the color is dependent on the time the solution is held a t an elevated temperature. Therefore, the total heating time is dependent on the temperature of the heating bath and the volume of solution which must be raised to the elevated temperature. This effect is shown in Table 11. The absorbance data in column A are for reagent volumes given in the colorimetric procedure. The data in column B are for twice these volumes, i.e., 50 ml. of boric acid, 10 ml. of chlorine water, etc., diluted to 100 ml. The concentration of ammonia was 1 p.p.m. in each of the color developed solutions. Since prolonged heating reduces the intensity of the color, it is important to cool the solution rapidly after removing the flask from the steam bath. This can be done conveniently by cooling the flask briefly in tap water and then in an ice bath for 2 to 3 minutes. Interference Studies. The effect of substances which might interfere in the analysis for ammonia was determined by adding l ml. of a 1% solution of the substance to a solution containing 1 p.p.m. of ammonia. Compounds found not to interfere were aliphatic amines, sodium chloride, potassium nitrate, sodium sulfate, and barium chloride. Increased absorbance resulted from the presence of copper, zinc, and iron salts. Iron(II1) showed the greatest effect by giving a 30% increase in absorbance when present in 100 times the ammonia concentration. Bromide ions interfered as the result of being oxidized to bromine which reacted immediately with phenol to form the insoluble tribromophenol. I n the presence of hydroxylamine, little or no blue color was developed upon addition of the sodium hydroxide. However, by adding oxidizing agents such as hydrogen peroxide color development was obtained with a resultant positive interference. Serious interference results from the presence of aromatic amines which are oxidized by chlorine to highly colored products, and from compounds such as p-aminophenol which react with phenol in an alkaline medium to give an indophenol. p-Hydroquinone and related compounds do not interfere. Sensitivity and Reproducibility. The optimum concentration range of the method is 0.3 t o 3.0 p.p.m. of ammonia in the color developed solution. The sensitivity can be increased by using larger absorption cells. The reproducibility of the method was determined from replicate analyses on a given ammonia solution by two analysts. The standard deviation was 0.03 p.p.m. a t a level of 1 p.p.m. of
Table 1.
Calibration Data
Absorptivity Absorbance Per P.P.M. NHa at 625 mp P.P.M.-Cm. 0.3 0.101 0.337 0.6 0.200 0.333 0.9 0.302 0.335 1.0 0.338 0.338 1.5 0.498 0.332 2.0 0.680 0.340 3.0 1.01 0.337 5.0 1.65 0.330 6.0 1.94 0.323 Table II.
Time, Min. 0 1
2
4
6
8 10
12 14
16
Effect of Heating Time
Absorbance at A 0.125 0.310 0.337 0.337 0.326 0.312 0.307 0.302 0.286
...
625 mp B
... ...
0.228 0.280 0.301 0.332 0.337 0.316 0.295 0.280
ammonia in the color developed solution. The sensitivity of the method described in this paper was compared with those reported by Riley (6) and by Noble ( 5 ) . The absorptivity-per p.p.m. of NH3-per em. of the three methods is 0.335, 0.210, and 0.114, respectively. DISCUSSION
Applications of Method. The method is applicable to the determination of ammonia from Kjeldahl digestions and in solutions from which the ammonia can be distilled into a boric acid solution. The method can also be applied to nearly neutral solutions containing as little as 1 pap.m. of ammonia by carrying a 10-ml. sample aliquot through the colorimetric procedure. The ammonia solution should be nearly neutral since the color developing reactions are p H dependent and maximum sensitivity and reproducibility are not obtained when the pH of the solution a t the time of heating differs significantly from that of saturated boric acid solution. For the same reason, acid solutions other than boric acid are not recommended for trapping distilled ammonia. Color Reactions. The production of the blue indophenol from ammonia, hypochlorite, phenol, and sodium hydroxide proceeds through several steps. The first is probably the reaction between ammonia and hypochlorite to give chloroamine (4). The chloroamine then reacts with phenol to give quinonechloroamine which couples with another mole of phenol VOL. 33, NO. 4, APRIL 1961
593
and forms the yellow associated indophenol (8). The indophenol dissociates in an alkaline medium t o give a blue color. The color change associated with a change in pH is reversible. The reactions are summarized by the following equations:
absence of hypochlorite to yield colored products. However, only parasubstituted anilines capable of giving a quinoid structure yield products having the blue indophenol color. o-Aminophenol gives a green reaction product while the color from the re-
only for the quantitative determination of certain para-substituted anilines but also for detecting the para isomer in the presence of the corresponding ortho and meta compounds. ACKNOWLEDGMENT
The authors express their appreciation to Monsanto Chemical Co. for permission t o publish this work. LITERATURE CITED
The required order of addition of reagents is explicable in terms of the proposed reaction scheme. The hypochlorite (chlorine water) must be added to the ammonia before the phenol to get color development. If phenol is added first, the chlorine adds to the phenol, thereby reducing the hypochlorite concentration and blocking the reaction of chloroamine with the phenol. The series of reactions is supported by the fact that phenol reacts with paminophenol in an alkaline medium yielding a blue compound having a visible spectrum the same as that of the indophenol from ammonia. Other substituted anilines react with phenol in an alkaline medium in the
action of m-phenylenediamine is yellow. Known amounts of several substituted anilines were carried through the ammonia procedure omitting the chlorine water and adding 8.5 ml. of 3M sodium hydroxide instead of 5 ml. The larger volume of sodium hydroxide was used since it was observed that maximum absorbance was obtained when the pH of the color developed solutions was 10.8 t o 11. The absorptivity-per p.p.m.-per em. a t 625 mp of several substituted anilines are as follows : p-aminophenol, 0.25; p-N,N-dimethylphenylenediamine, 0.15; p-phenylenediamine, 0.12; o-aminophenol, 0.02; and m-phenylenediamine, 0.0. This color reaction can be used not
(1) Berthelot, M. P. E., Rdpert. de chzm. appl., 1859, 282. ( 2 ) Box, G. E. P., Biometrzcs 10, 16-61 f~ 19fi4L - . -
~ ,
(3) Crowther, A. B., Large, R. S., dnalyst 81, 64 (1956). (4) Mellor, J. W., “A Comprehensive
Treatise on Inorganic and Theoretical Chemistry,” Vol. VIII, pp. 598-604, Longmans, Green and Co., London, .
-
,I
1Y4i.
(5) Koble, E. D., ANAL.CHEM.27, 1413 (1955). (6) Riley, J. P., Anal. Chim. Acta 9, 575 ( 1953). ( 7 ) Riley, J. P., Sinhaseni, P., J . Marine
Biol. Assoc. United Kingdom 36, 161
(1957).
(8) Rodd, E. H., “Chemistry of Carbon Compounds,” Vol. 111, pp. 721-2, Part B, Elsevier, Amsterdam, 1956.
RECEIVEDfor review August 29, 1960. Accepted January 6, 1961. Division of Analytical Chemistry, 138th Meeting, ACS, New York, N. Y., September 1960.
The Theoretical Sensitivity and Linearity of Photoelectric Systems for Pola rimetry AUGUSTE 1. ROUY and BENJAMIN CARROLL Chemistry Deparfment, Rvtgers, The Sfafc Universify, Newark, N. 1. tb The theoretical sensitivity and the limit of error for various photoelectric systems in polarimetry are considered. It is shown that it is possible to develop a photoelectric scale which is linear to 1 p.p.m. having a limit of error of about 5 X degree.
T
of optical rotation of solutions by manual equipment based on the angular scale is being replaced by electronic systems. With the trend toward automation, the conventional polarimetric method is not amenable to simple mechanization because it involves a subjective observation of a critical balance point. Also, there has been great interest ( 4 ) in measuring optical rotation in the absorption band of substances. It appears that a complex substance may be characterized by its optical rotatory dispersion in its absorption band. Unfortunately, light absorption introHE MEASUREMENT
594
ANALYTICAL CHEMISTRY
duces ellipticity into the initial linearly polarized light beam as it traverses the sample, thus making it difficult to obtain a visual balance point with precision. Also absorption reduces the intensity of light, frequently necessitating the use of extremely dilute solutions in the spectral region of greatest interest. Hence the conventional visual instrument using the angular scale is of little value for many substances and for all substances with rotations of degree or less. With the advent of excellent light transducers it was to be expected that their application to polarimetry would be considered in due time. Several publications have appeared recently (2, 8-10) describing various optical and electronic systems for using light transducers; however, the general limitations and a critical comparison of these various systems have not been considered. This has led to some confusion. It is the purpose of this paper
to point out the various sources of error and to estimate their magnitude. Such a survey should be of value in guiding the analyst in the design and use of future instruments in this field of reawakened interest. The problems considered deal mainly with the sensitivity of single beam systems and the deviation of the output of these systems from linearity with respect to optical rotation. The latter aspect is pivotal in the design of an analog polarimeter. Some aspects for split beam optical arrangements may be easily deduced from simple extensions of the analyses presented in this paper. However, the special problems created by the split beam polarimeter are being considered in a subsequent paper. ELEMENTARY ANALOG TYPE, SINGLE BEAM
Consider the simplest photoelectric polarimeter as illustrated in Figure 1A. The light beam emitted by the light
