Reagent for Colorimetrically Indicating the Presence of Formaldehyde

The present invention relates to a class of reagents for colorimetrically determining formaldehyde in solution or in air, and to the use of such a reagent for monitoring exposure to formaldehyde.

Formaldehyde is an important chemical which is widely used in various industries, such as plastics, textiles, etc. Because of increasing concern over safety and health hazards presented by exposure to formaldehyde vapors in air and governmental regulations limiting the permissible level of formaldehyde vapors in the workplace environment, industries require an effective and reliable way to monitor formaldehyde levels in the workplace environment and to control exposure to hazardous formaldehyde concentrations.

Various monitoring devices have been developed for monitoring exposure by individuals to formaldehyde. One popular type of monitoring device is a dosimeter badge, which is intended to be worn by persons who work in an area which may be contaminated with formaldehyde vapors. Various ways are provided for obtaining a reading of the formaldehyde exposure from the badge. However, the commercially available badges which have been developed for monitoring formaldehyde have been subject to one or more disadvantages or limitations, including expense, lack of reliability, and inability to provide a continuous direct readout of formaldehyde exposure. Many of these disadvantages and limitations are due to the chromophoric reagent used in the dosimeters.

Several reagents for colorimetric determination of formaldehyde have been described in the literature. The most important ones are described in Table 1 below.

However, the above-noted known colorimetric reagents all suffer from at least one of the following disadvantages:

1. They form color very slowly.

2. The reagents are unstable.

3. The reagents cannot be easily used in the field.

4. The reagents cannot be adapted to dry-chemistry application.

5. The reagents are non-selective to formaldehyde.

The present invention provides a colorimetric reagent which can visibly show the presence of formaldehyde, and which is useful in various applications, such as for example in a direct reading colorimetric gas dosimeter. The availability of a colorimetric reagent which can visibly show the presence of formaldehyde can help even unskilled personnel determine whether they are present in a dangerous environment.

The present invention is based upon a colorimetric reaction involving two chemicals: rubeanic acid (dithiooxamide) or a derivative thereof and a metal-cyanide complex, preferably a cyano complex of a metal selected from the group consisting of silver, copper, cadmium, lead, iron, bismuth, cobalt, thallium, nickel, gold, palladium, platinum, and ruthenium.

The essence of the colorimetric reaction is the formation of colored compounds due to the reaction between a metallic ion and rubeanic acid or a derivative of rubeanic acid. If sufficient cyanide ions are present, a metal-cyanide complex is formed which does not form color. However, when formaldehyde is added to a solution which contains the metal-cyanide complex, the cyanide ion reacts with the formaldehyde according to the reaction:

which destroys the complex and releases metallic ions. The metallic ions, in turn react with the rubeanic acid or rubeanic acid derivative and form color. The degree of color change is quantitatively related to the number of formaldehyde molecules reacted.

The colors formed with different metal complexes are: Ag--yellow-brown to black; Bi--yellow-brown to black; Pb--yellow; Cu--green-black; Cd--yellow; Fe (II)--blue; Co (II)--brown to orange-yellow; Ni (II)--blue-violet; Pd--orange-yellow to red; Au--orange; Ru (III and IV)--blue; Pt--rose-red; Tl--yellow.

The colorimetric reaction may effectively employ either rubeanic acid (dithiooxamide) or derivatives of rubeanic acid in which one or both of the hydrogen atoms of one or both of the amino groups is replaced with other organic moieties, such as for example alkyls (e.g. methyl, ethyl, hydroxyalkyls, aryls (e.g. benzyl, halobenzyl, pyridyl).

The concentration and relative proportions of the rubeanic acid and the metal-cyanide complex in the reagent are not critical, and may be varied over a wide range depending upon the sensitivity desired.

The reagent composition is preferably maintained, typically in excess of pH 10.5, by including in the reagent composition suitable bases or alkaline compounds, such as alkali metal cyanides for example.

Various additives may be included in the reagent in addition to the rubeanic acid and metal-cyanide complex, such as for example, a buffer--typically a borate, a stabilizer, an electrolyte, a solvating media, a gelling media, and polymers or prepolymers such as polyvinyl alcohol, polyvinylidene ethylene oxide, polyethylene glycol, polyvinyl acetate and functionalized polystyrene.

The reagent may be used for detecting the presence of formaldehyde in a gas mixture such as air or in an aqueous solution. The reagent may be used in the form of a solution or it may be dried on a suitable carrier or substrate.

An exemplary formulation of chromophoric reagent for the detection of traces of formaldehyde in solution is as follows:

1. KAg(CN).sub.2 : 20 ml. of 0.75 molar in H.sub.2 O.

2. KCN: 1 gm.

3. Rubeanic acid: 20 ml. Sat. solution in CH.sub.3 OH at 21.degree. C.

4. KNO.sub.3 : 1.5 gm.

5. KBO.sub.3 : 0.7 gm.

6. water: as solvent for the KAg(CN).sub.2

7. methanol: 100 ml.

An exemplary formulation of the chromophoric reagent for the detection of traces of formaldehyde in air is as follows:

1. KAg(CN).sub.2 : 20 ml. of 0.75 molar in H.sub.2 O

2. KCN: 1 gm.

3. Rubeanic acid: 20 ml. Sat. solution in CH.sub.3 OH at 21.degree. C.

4. KNO.sub.3 : 1.5 gm.

5. KBO.sub.3 : 0.7 gm.

6. glycerol: 10 gm.

7. polyethylene glycol: 10 gm.

8. polyvinyl alcohol: 5 gm.

9. water: as solvent for the KAg(CN).sub.2

10. methanol: 100 ml.

The reagent of Example 2 is used in a dosimeter badge for detection of formaldehyde by applying drops of the reagent to a carrier media, such as filter paper or a granular sorbent material such as silica or alumina, and allowing the reagent to dry. The chromophore-treated carrier is then encapsulated in a case formed of a gas impermeable plastic material having an opening formed therein to allow the ambient gas to enter the case and contact the carrier. Preferably, a permeable membrane is disposed between the opening and the carrier to reduce the effect of wind superfacial velocity on the reading. Upon exposure to formaldehyde vapors, the chromophore-treated carrier changes color, forming a yellow to brown-black color which is quantitatively related to the number of formaldehyde molecules reacted.

A chromophoric reagent is produced with the formulation as set forth in Examples 1 and 2, but substituting for ingredient #3 (Rubeanic Acid) 20 ml. of N,N' Bis(2-hydroxy-ethyldithiooxamide) at a concentration of 0.08 molar.