Process for the application of IR spectroscopy for modulation and monitoring of diazonium ion concentration

The invention relates to a method of determining and controlling the diazonium ion concentration in diazotization solutions and/or in azo coupling reactions, which comprises determining said diazonium ion concentration by means of IR spectroscopy from the intensity of the N.tbd.N stretching frequency. The method is extremely selective and virtually trouble-free and permits the simultaneous determination of the concentration of different diazonium salts in mixtures.

The present invention relates to a method of determining the diazonium ion 
concentration in diazotisation solutions and/or in azo coupling reactions 
by means of infra-red (IR) spectroscopy. 
In recent years, increasing efforts have been made to automate dye 
manufacture both with respect to the actual preparatory process itself and 
to the working up. As regards the process for the preparation of one of 
the most important classes of dye, namely azo dyes, there is still a need 
for simple methods of monitoring the two basic synthesis steps, i.e. the 
diazotisation and coupling reactions. Common to both reactions is a change 
in the diazonium ion concentration during the reaction course. It should 
therefore be possible to control diazotisation as well as coupling by 
determining the diazonium ion concentration. 
It has now been found that such control is possible in simple and elegant 
manner by infrared spectroscopic analysis of the respective reactive 
mixtures. The -N.sub.2.sup.+ group of the diazonium salts is detected in 
the infra-red spectrum by a strong band in a wave number range from 
.nu.=2150 to 2350 cm.sup.-1, the intensity of which band is a function of 
the diazonium ion concentration. 
Accordingly, the present invention relates to a method of determining and 
controlling the diazonium ion concentration in diazotisation solutions 
and/or in azo coupling reactions, which process comprises determining said 
diazonium ion concentration by IR spectroscopy from the intensity of the 
N.tbd.N stretching frequency. 
The IR spectroscopic measurement can be made by taking samples during the 
diazotisation and coupling reactions at specific intervals and examining 
them spectroscopically in the respective wave number range, or by passing 
small amounts of the diazotisation solution or coupling suspension over a 
by-pass continuously in a flow cell and continuously recording the 
intensity of the N.tbd.N stretching frequency. The continuous measurement 
is preferred, as it permits the reactant streams to be influenced 
immediately, even when there are insignificant changes in concentration. 
If, as in azo coupling is often the case, the reaction mixture is in the 
form of a suspension, it is advantageous to effect filtration before the 
IR measurement, for example by subjecting a partial stream of the 
suspension to continuous clarifying filtration and analysing the filtrate 
by IR spectroscopy. In this manner the process is always carried out in 
homogeneous phase and the result cannot be influenced by the solids 
content. Filtration is conveniently effected by means of a membrane 
separating method, e.g. by microfiltration, ultrafiltration or 
hyperfiltration. 
During the diazotisation, the flow of nitrite or amine will conveniently be 
controlled via the intensity of the N.tbd.N stretching frequency, i.e. 
whenever the signal of the N.tbd.N stretching frequency has reached a 
nominal value determined by prior experimentation, the flow of nitrite or 
amine is corrected accordingly. By this means it is possible to avoid an 
overaddition of nitrite and also to control the concentration of nitrite 
during diazotisation such that undesirable secondary reactions do not 
occur. 
The diazotisation will normally be carried out continuously or batchwise in 
a solution containing mineral acid. If diazotisation is effected 
batchwise, the amine will be suspended or dissolved, e.g. in aqueous 
hydrochloric acid, and then reacted with an aqueous nitrite solution, e.g. 
an aqueous solution of sodium nitrite. The reaction proceeds almost 
quantitatively. 
In the preferred continuous method, the procedure is for example that the 
diazo component is diazotised continuously with sodium nitrite in a tube 
reactor. Simultaneously, the intensity of the N.tbd.N stretching frequency 
is measured with an IR spectrophotometer. The fluctuations in intensity 
measured with the IR spectrophotometer are processed by a process control 
computer which controls the reactant streams after comparison with the 
nominal value via a corresponding signal. 
The method of this invention is susceptible of application to all 
diazotisable amines. It is preferred to carry out the method of 
determining and controlling the diazonium ion concentration in 
diazotisation solutions and/or during azo coupling reactions in the 
presence of a polar solvent, e.g. water, an alcohol such as methanol, 
ethanol, propanol, isopropanol or butanol, or a ketone such as methyl 
isopropyl ketone. It is particularly preferred to carry out the 
determination and control of the diazonium ion concentration in aqueous 
solution. 
Examples of suitable diazo components are: aniline and derivatives thereof 
such as 4-nitroaniline, 3-nitroaniline, 2-chloro-4-nitroaniline, 
4-chloro-2-nitroaniline, 2,6-dichloro-4-nitroaniline, 4-aminoacetanilide, 
2,4-dinitroaniline, 4-chloroaniline, 2,4,5-trichloroaniline, 
2,5-dimethoxyaniline, o-anisidine, p-anisidine, o-phenetidine, 
p-phenetidine, o-toluidine, p-toluidine, 4-nitro-2-aminoanisole, 
2-nitro-4-aminoanisole, p-phenoxyaniline, or also 4-methylsulfonylaniline, 
4-amino-2,4-dichlorobenzophenone, 4'-amino-2,4-dinitrobenzophenone, 
2-nitroaniline, 2-chloro-4,6-dinitroaniline, 2,5-dichloroaniline, 
3,3'-dichlorobenzidine, 5-nitro-2-aminoanisole, 3-nitro-4-aminotoluene, 
2,4-dichloroaniline, 3-nitro-4-aminoanisole, 
2-aminoanisole-4-sulfodiethylamide, 5-chloro-2-aminotoluene, 
4-chloro-2-aminotoluene, 4-nitro-2-aminotoluene, 5-nitro-2-aminotoluene, 
4-nitro-2-aminoanisole, 3,3'-dimethoxybenzidine, 
3,3'-dimethoxy-6,6'-dichlorobenzidine, 2-amino-4-chlorophenol, 
2-aminophenol-4-sulfamide, 2-aminophenol-5-sulfamide, 
2-aminophenol-4-sulfomethylamide, 3-amino-4-hydroxyphenylmethylsulfone, 
2-amino-5-nitrophenylmethylsulfone, 4-amino-3-nitrophenylmethylsulfone, 
2-(N-methyl-N-cyclohexylsulfamoyl)aniline, 
2-amino-4,2',4'-trichlorodiphenyl ether and 4-aminoazobenzene; .alpha.- or 
.beta.-napthylamine, and derivatives thereof, such as 
2-naphthylamine-6,8-disulfonic acid, 1-naphthylamine-3,6,8-trisulfonic 
acid, 4-naphthylamino-5-hydroxy-1,7-disulfonic acid or 
2-naphthylamino-7-hydroxy-6-sulfonic acid. 
Examples of heterocyclic amines are: 3-amino-1,2,4-triazole, 
2-aminothiazole, benzthiazoles such as 2-aminobenzthiazole, 
2-amino-4-chlorobenzthiazole, 2-amino-4-cyanobenzthiazole, 
2-amino-4,6-dinitrobenzthiazole, 2-amino-4-methoxy-6-nitrobenzthiazole, 
2-amino-6-methoxy-1,3-benzothiazole or aminobenztriazoles, which may also 
be appropriately substituted. 
The solution of the diazonium salt may be kept in a storage vessel or, 
without being stored, reacted with a suitable coupling component to give 
the azo dye. The azo coupling can also be carried out continuously or 
batchwise. Continuous coupling is conveniently carried out in a tube 
reactor or multi-compartment reactor. Such a reactor may be provided at 
one or more sites with a sampling means. Sampling can in turn be effected 
at specific intervals or, preferably, continuously. The intensity of the 
N.tbd.N stretching frequency is here employed as controlled variable for 
controlling the rate of addition of the diazo or coupling component; or, 
in other words, the ratio of diazo component to coupling component is 
controlled in this manner and, as required, freshly adjusted. Hence it is 
possible to achieve a virtually quantitative conversion of the starting 
materials to the desired azo dye. Secondary reactions can thus be reduced 
to a minimum and a standard type dye is obtained. 
Depending on the diazo and coupling components, the azo coupling is carried 
out in an acid or basic reaction medium, with the optional addition of 
surfactants if it is desired to obtain a substantially finely particulate 
dye dispersion. In addition to the amines already mentioned as suitable 
diazo components, the following compounds may be cited as examples of 
suitable coupling components: N-substituted anilines, e.g. 
N,N-dimethylaniline, N,N-di-(.beta.-carbomethoxyethyl)aniline, 
N-(.gamma.-methoxypropyl)-3-acetylaminoaniline, 
N,N-di-(.beta.-hydroxyethyl)-2,5-dimethoxyaniline, 
N-ethyl-N-(.beta.-hydroxyethyl)aniline, 
N,N-di-(.beta.-hydroxyethyl)aniline; and also phenol and substituted 
phenols such as o-, m- and p-cresol, resorcinol, 
4-phenylazo-1,3-dihydroxybenzene and 3-acetylaminophenol, naphthols such 
as 1- or 2-naphthol, 6-bromo-2-naphthol, 4-methoxy-1-naphthol and 
2-naphthol-6-sulfonamide; as well as acylacetoarylamides, 
2,6-dihydroxypyridines or 5-pyrazolones. 
In principle, the intensity of the N.tbd.N stretching frequency can be used 
for controlling the individual reactions, i.e. the diazotisation and 
coupling, as well as for monitoring both reactions. 
The IR spectroscopic analysis is carried out with a commercially available 
IR spectrophotometer. Plates made of inorganic salts which are 
IR-permeable are normally used as window material, e.g. calcium fluoride 
plates. A compensating window is usually inserted into the reference path 
of rays of the spectrophotometer to compensate for the energy absorption 
and the losses in reflectance of the measuring cell. The processing of the 
intensity fluctuations measured in the N.tbd.N stretching frequency range 
can be made direct on-line with the process control computer, which is 
this case controls the reactant streams. 
It must be borne in mind that C--C triple bonds, the cyanide group and the 
isocyanate group absorb in the same wave number range as the diazonium 
group (q.v. "Spektroskopische Methoden in der organischen Chemie", Georg 
Thieme Verlag 1984, page 53). When using diazo components and coupling 
components which contain triple bonds or corresponding substituents, it 
must be established by prior experimentation whether the possibility 
exists--depending on the position of the bands--of monitoring their 
conversion by means of the method of this invention. In principle, 
however, the method described herein is exceedingly selective and 
substantially trouble-free, and furthermore permits the simultaneous 
monitoring of different diazonium salts--a feature which is advantageous 
when diazotising mixtures of amines. 
The method of this invention is susceptible of application in particular to 
all amines which can be diazotised in acid aqueous medium and to all azo 
dyes which can be obtained by azo coupling. 
In particular, the method of this invention can be used in continuous 
diazotisation and coupling reactions for the preparation of azo dyes. The 
preferred utility is as part of an on-line control of a 
computer-integrated, automated process for the preparation of azo dyes.

The invention is illustrated by the following Example in which percentages 
are by weight. 
EXAMPLE 
(Diazotisation reaction) 
A mixture of 4-aminoacetanilide and aniline (molar ratio 0.45:0.55) is 
diazotised in dilute hydrochloric acid with 4N aqueous sodium nitrite 
solution. The diazotisation is carried out at three different amine 
concentrations of 0.071, 0.142 and 0.284 mole/l. After complete conversion 
of the mixture of amines to the corresponding diazonium salts, the 
individual reaction mixtures are analysed by IR spectroscopy. This is done 
in each case by recording the N.tbd.N stretching frequency in the 
wavelength range from 4 to 5 nm (.ltoreq. a wavelength range of .nu.=2000 
to 2500 cm.sup.-1). In the illustration, the absorption bands are 
superimposed. The transmission in percent (% T) is indicated as scale of 
ordinates. This corresponds to the percentage radiation which is absorbed 
by the sample at the respective wavelength. The comparison radiation 
serves as reference. The abscissa is calibrated in nm (wavelength 
.lambda.). Band (a) corresponds to a total diazonium ion concentration of 
0.07, band (b) corresponds to a concentration of both diazonium salts of 
altogether 0.28 mole/l, and band (c) corresponds to a concentration of 
altogether 0.28 mole/l, provided the diazotisation is quantitative. All 
three bands have a shoulder relative to the absorption peak at slightly 
shorter wavelength, caused by the N.tbd.N stretching frequency of the 
diazotised aniline. 
The bands show clearly the dependence of the concentration of the IR signal 
on the N.tbd.N stretching frequency. In diazotisation reactions, the 
addition of amine or nitrile is controlled by means of the intensity of 
the IR signal as controlled variable. 
The diazotisation reaction of the following amines is controlled by the 
same method: 2-chloro-4-nitroaniline, 4-chloro-2-nitroaniline, 
3-amino-1,2,4-triazole.