A diluent and a method of assaying are described, by which the ionic strength range of the liquid that is to be assayed potentiometrically, is compressed. The diluent comprises a predetermined concentration of (a) a protonated water-soluble organic amine having (i) a pKa greater than or equal to 9.3, and (ii) a hydroxyalkyl of 1 to 5 carbon atoms, bonded to the nitrogen of the amine, wherein the oxygen-to-carbon bond of the hydroxy is no more than two bonds removed from the nitrogen, and (b) an acid anion.

A single, stable reagent and automatable methods utilizing the reagent for the assay of calcium in serum and aqueous samples, such as urine. (Aqueous samples contain little or no protein.) The reagent includes arsenazo III, 8-hydroxyquinoline, a buffer that does not bind calcium competitively with arsenazo III and that provides a reagent pH from about 8.5 to about 10, the buffer being in sufficient amount to maintain pH within +/- 0.1 pH units of the reagent pH when the reagent is added to a sample of unknown calcium content, dimethylsulfoxide and water. Importantly, dimethylsulfoxide permits the incorporation of 8-hydroxyquinoline into a single, stable reagent, which is at an alkaline pH that is optimal for calcium measurement by the calcium-arsenazo III complex. 8-Hydroxyquinoline eliminates assay interference due to magnesium ion. Also, at least one non-ionic surfactant in sufficient amount to minimize lipid turbidity may be added to the reagent. Spectral measurements of calcium-arsenazo III are made at wavelengths that minimize interference from lipemia, hemolysis, and bilirubinemia. Spectral interference from lipemia and hemolysis is further reduced by employing bichromatic measurements of the calcium-arsenazo III complex and by employing serum blank measurements.

The invention relates to a dry chemical method and a test strip for determining calcium and magnesium in aqueous samples. Said test strip contains o-cresolphthalein complex (o-CPC; CAS Registry No. 2411-89-4) as a colour-causing reagent in a buffer system with a pH value of below 8.5. By using in particular an aqueous ethanolic imidazole buffer with a pH value of 6.75, conditions can be set up in such a way that the o-CPC has an identical molar sensitivity with respect to Ca and Mg. The total water hardness of aqueous samples can thus be easily determined.

An agent and process are used for the determination of calcium in liquids. The agent contains essentially orthocresolphthalein complexone, an acid and a zwitterionic buffer and is characterized in that the buffer contains at least one sulphonic acid amine of the general formula ##STR1## wherein R.sub.1 and R.sub.2 are identical or different and denote H or alkyl, hydroxyalkyl or cycloalkyl having in each case up to 8 C atoms and R.sub.3 denotes alkyl or hydroxyalkyl having up to 8 C atoms.

An improved multilayer analytical element for the determination of calcium comprising a liquid impermeable, optically transparent support; at least one reagent layer positioned with a first side on said support and containing at least one reagent capable of reacting with a calcium component in a fluid sample; and at least one porous developing layer on the second side of said reagent layer, said porous developing layer being permeable to calcium from said fluid sample thereby transporting said calcium to said reagent layer. The improvement comprises at least one of said reagent layer and developing layer containing a basic substance of the formula M.sub.v H.sub.w PO.sub.4 .multidot.ZH.sub.2 O wherein M is lithium, sodium or potassium; v is an integer of 2 or 3 and w is an integer of 0 or 1 provided that v plus w equals 3; and Z is 1/2 when M is lithium, 0, 2 or 12 when M is sodium and 0 or 8 when M is potassium; and said reagent layer contains (i) 3,3'-bis[N,N-di(carboxymethyl)aminomethyl]-o-cresolphthalein and (ii) 8-hydroxyquinoline or a derivative thereof.