Analytical reagent for cyanide

In a method for the determination of cyanide wherein the cyanide is reacted with chloramine-T to form cyanogen chloride, said cyanogen chloride is reacted with pyridine to form glutaconic dialdehyde, and said dialdehyde is reacted with an organic compound to form a colored compound, Wherein the improvement comprises employing as said organic compound, N,N'-dialkyl barbituric acid, the alkyl groups being preferably each of 1-6 carbon atoms, and especially N,N'-dimethylbarbituric acid. The latter in a solution of dimethyl sulfoxide has a shelf life of at least one year.

BACKGROUND OF THE INVENTION 
This invention relates to an analytical reagent suitable for the 
colorimetric determination of cyanide ions in liquids, and to the 
production of new colored compounds. 
The determination of cyanide has been based on a reaction, known for a long 
time, wherein cyanide and the active chlorine from chloramine-T (sodium 
para-toluenesulfonic acid chloramide) are reacted to cyanogen chloride 
which splits a pyridine ring with the formation of glutaconic dialdehyde. 
This dialdehyde condenses with two molecules of a suitable organic 
compound to a colored polymethine compound. Organic compounds which have 
been recommended for this purpose are barbituric acid [Z. analyt. Chem. 
[Journal of Analytic Chemistry] 138, 414 [1953]), pyrazolones (Anal. Chem. 
19, 272 [1947])], and benzidine ("Deutsche Einheitsverfahren zur 
Wasseruntersuchung" [German Standard Methods for Water Testing] D 13, 
1971, p. 5). These organic compounds have, however, a number of 
disadvantages which render them essentially useless as a ready-to-use 
analytical reagent for the detection of cyanide for general purposes, for 
example, outside of the laboratory. 
Specifically, solutions of these organic compounds have a short shelf life: 
after about two months, barbituric acid in water and pyridine fail to 
yield exact analytical results, and after about three months, at room 
temperature, barbituric acid in dimethyl sulfoxide results in 
sedimentation. (These sediments form in only a few days if the reagent is 
stored at 42.degree. C.) As for the pyrazolone reagent, it must be 
prepared fresh daily, since a pink color appears only after two days, and 
the color of the thus-produced polymethine dye is unstable. Lastly, the 
benzidine reagent is not only too insensitive, but also carcinogenic. 
SUMMARY OF THE INVENTION 
One object of this invention is to provide an improved analytical reagent 
for the determination of cyanide ions, and method for using same. Another 
object is to provide novel colored compounds which are useful for the 
determination of cyanide ions. Upon further study of the specification and 
appended claims, further objects and advantages of this invention will 
become apparent to those skilled in the art. 
To attain these objects, there is provided N,N'-dialkyl-barbituric acid as 
the key to solving the problems of the prior reagents. 
In a comprehensive aspect of this invention, there is provided an 
analytical reagent for the colorimetric determination of cyanide in 
liquids, comprising N,N'-dialkyl-barbituric acid, and chloramine-T and 
pyridine. 
Preferably, the N,N'-dialkyl barbituric acid is substituted in the 
1,3-position by lower alkyl groups, each being different or preferably the 
same, such as, for example, methyl, ethyl, propyl, butyl, pentyl, or hexyl 
groups, especially N,N'-dimethylbarbituric acid. The 
N,N'-dialkyl-barbituric acid is preferably dissolved in dimethyl sulfoxide 
as the solvent. Other aprotic solvents such as dimethylformamide, 
dimethylacetamide, hexamethylphosphoric acid triamide, methylpyrrolidone, 
tetramethylurea and sulfolane are also suitable but a yellow color appears 
after several months. It could not be foreseen that, for example, 
N,N'-dimethylbarbituric acid, as contrasted to barbituric acid, has a 
shelf life of at least one year in dimethyl sulfoxide, even at a 
temperature of 42.degree. C. Accordingly, solvent solutions of 
N,N'-dialkyl-barbituric acid are especially useful per se in the context 
of this invention. The preferred concentration range of 
N,N'-dialkyl-barbituric acid is from 3 - 20, especially from 5 - 12% by 
weight. 
The analytical reagent of this invention preferably also contains pH 
adjusting means, namely bases, acids and/or buffer substances to adjust 
the pH of the sample solutions to be investigated. For example, the pH 
value, suitable for the determination, is first adjusted to about 3 and 
then to 5 in subsequent steps of the reaction. Suitable bases are, for 
example, alkali metal hydroxides, such as sodium or potassium hydroxide; 
suitable acids are, for example, mineral acids and carboxylic acids, such 
as hydrochloric acid, sulfuric acid, tartaric acid, oxalic acid, citric 
acid, etc. Buffers are, for example, phosphate buffers and EDTA 
(ethylenediaminetetraacetic acid). EDTA is also utilized for its chelating 
properties for the elimination of the interfering influence of, for 
example, copper, zinc, iron(II), and chromium(III) ions. 
The preferred weight ratios in the comprehensive analytical reagent 
including chloramine-T : 1,3-dimethylbarbituric acid : pyridine are 1 : 1 
- 10 : 100 - 1000, especially about 1 : 5 : 500. In this connection, it 
should be kept in mind that pyridine is used not only as a reagent but 
also as a base for increasing the pH value. If the pH is adjusted by 
another base, then the amount of pyridine can be reduced by a factor of 
about 100, so that for example, the range above of 1 : 5 : 500 is reduced 
to 1 : 5 : 5. 
To effect the cyanide determination, the sample solution to be analyzed is 
first adjusted to a pH of about 2 - 13 with the aid of acids and/or bases. 
Subsequently, the ready-to-use reagent solutions of chloramine-T, 
1,3-dialkyl-barbituric acid, and pyridine are added thereto. After a 
reaction time of about 5 minutes, the color intensity is measured by 
conventional techniques, e.g., photometrically at 588 nm. or compared with 
a calibrated color scale. 
The structural formulae of the condensation products of the barbituric 
acids of the present invention with glutaconic dialdehyde are as follows: 
##STR1## 
R = alkyl group with up to 6 carbon atoms. All these products are novel.

Without further elaboration, it is believed that one skilled in the art 
can, using the preceding description, utilize the present invention to its 
fullest extent. The following preferred specific embodiments are, 
therefore, to be construed as merely illustrative, and not limitative of 
the remainder of the disclosure in any way whatsoever. In the following 
examples, all temperatures are set forth uncorrected in degrees Celsius; 
unless otherwise indicated, all parts and percentages are by weight. 
EXAMPLE 1 
A measuring vessel is rinsed with the sample solution to be tested, having 
a pH of 2 - 13, and is then filled with 10 ml. of sample solution. Ten 
drops of a 10% tartaric acid solution and thereafter 10 drops of a 5% EDTA 
solution are added to the sample solution and the latter is shaken after 
each addition. Thereafter, 10 drops of a 1% chloramine-T solution are 
added, the solution is shaken, and then allowed to stand for about 30 
seconds. Then, in succession, 5 drops of 10% 1,3-dimethylbarbituric acid 
solution in dimethyl sulfoxide and 5 drops of pyridine are added and the 
mixture is shaken after each addition. 
After about 5 minutes, the solution containing the thus-produced colored 
condensation product of glutaconic dialdehyde with 1,3-dimethylbarbituric 
acid is compared with a color scale or measured photometrically at 588 nm. 
In this way, a solution is analyzed which contains 0.05 mg. of cyanide 
ions per liter. The same value is obtained when using a 
1.3-dimethylbarbituric acid solution which has been stored for 8 months. 
EXAMPLE 2 
A self-sufficient, ready-to-use reagent kit contains the following 
solutions: 
1. 10% Tartaric acid solution 
2. 5% EDTA solution 
3. 1% Chloramine-T solution 
4. 10% 1,3-Dimethylbarbituric acid solution in dimethyl sulfoxide 
5. Pyridine. 
The preceding examples can be repeated with similar success by substituting 
the generically or specifically described reactants and/or operating 
conditions of this invention for those used in the preceding examples. 
From the foregoing description, one skilled in the art can easily ascertain 
the essential characteristics of this invention, and without departing 
from the spirit and scope thereof, can make various changes and 
modifications of the invention to adapt it to various usages and 
conditions.