Stabilization of tetrazolium salts with cyclodextrins

In a process for stabilizing a tetrazolium salt used as a color indicator in the determination of a substrate which is an oxidizable biochemical material by oxidizing said substrate with an oxidase, wherein said tetrazolium salt is selected from the group consisting of compounds of the formulas II, III, and IV:

• ##STR9## wherein X is halogen;
• and each of R.sup.1, R.sup.2 and R.sup.3 is independently an organic residue selected from the group consisting of phenyl, phenyl substituted with at least one halogen, nitro and ##STR10## wherein each of Y.sup.1 and Y.sup.2 is independently --NO.sub.2 or --H;
• Y.sup.3 is --OCH.sub.3, --I or --H;
• and X is a halogen;
• ##STR11## wherein each R4 is nitro or halogen, provided that at least one R.sup.4 is nitro;
• R.sup.5 is hydrogen;
• a lower alkyl group, a lower alkoxy group or a halogen;
• R.sup.6 is hydrogen or --OR.sup.7 ;
• R.sup.7 is a straight- or branched-chain aliphatic hydrocarbon having 1 to 4 carbon atoms which has at least one functional group selected from the group consisting of sulfonic acid, sulfonic acid salt, carboxyl, and carboxyl salt, and can also have a hydroxyl group, provided that the sum of the number of sulfonic acid group, sulfonic acid salt group, carboxyl group, and carboxyl salt group contained in R.sup.6 is 2 or more and one of said functional groups forms an intramolecular salt with a tetrazolium ring;
• and X is a halogen,
• the improvement wherein said tetrazolium salt is stabilized against chemical decomposition by adding to an aqueous solution of said tetrazolium salt at least one member selected from the group consisting of:
• (i) .beta.-cyclodextrin;
• (ii) a derivative of .beta.-cyclodextrin represented by the formula:
• .beta.-CD(--OH).sub.21-m (--OZ.sup.1).sub.m (V)
• wherein CD is a cyclodextrin residue, Z.sup.1 is --NO.sub.2, --PO.sub.3 H, --SO.sub.3 H or a group of the formula --(CH.sub.2).sub.n Z.sup.2 ;
• Z.sup.2 is --SO.sub.3 H or --CO.sub.2 H;
• n is an integer of 1 to 4;
• and m is a positive number having a value of 1 to 5,
• or represented by the formula:
• .beta.-CD(--OH).sub.21-k (--OCH.sub.3).sub.k (VI)
• wherein k is a positive number having a value of 1 to 21;
• and
• (iii) .gamma.-cyclodextrin.