5661 Tin(II) compounds are important in 99m Tc radiop harmaceutical kits as stabilizing agents [1], in dental gels and food as preservatives [2][3][4], and in electroless plating as electrochemical catalysts [5][6][7][8]. Common methods to determine tin(II) are limited by some combination of the range and/or limit of detection, ease of application, reproducibility, and inability to distinguish tin(II) from tin(IV). A need to rapidly quantitate tin(II) concentration in an electroless plat ing sensitization solution at concentrations less than 0.3 µg/mL to evaluate its effectiveness motivated the development of the present colorimetric approach. A spectrophotometric approach was selected which complements the existing spectrophotometric meth ods for quantification of boron [9], vanadium(IV) [10], and mercury(II) [11]. Common methods for determining tin(II) concen tration are summarized in Table 1. These methods are: electrochemistry [1-4, 12-19], membrane sensor [18], inductively coupled plasma-optical emission spectrometry (ICP-OES) [19], flame atomic absorp tion spectrometry (FAAS) [20], fluorescence [21], and spectrophotometry [22-29]. The electrochemi cal method, which includes anodic stripping voltam metry (ASV) [1-4, 12] and differential pulse polarog 1 The article is published in the original.raphy (DPP) [13][14][15][16][17], has been widely used because of selective determination of tin(II) in the presence of tin(IV). The ASV method has a low LOD. For exam ple, a LOD equal to 0.00026 µg/mL was achieved by Hutton et al. by using the ASV method with a bismuth film electrode [3]. However, the limitation of the ASV method is that it requires either formation of a tin(II) complex such as a tin(II)-oxine [1] or a tin(II)-tropolone [2], or use of a modified electrode such as an epoxy carbon electrode or the BiFE [3, 4, 12]. The DPP method does not require a tin(II) complex or a modified electrode but it is limited by the detection range and the LOD. Decristoforo et al. [14] reported a DPP method with a LOD of 0.005 µg/mL but the con centration range was from 10 to 15 µg/mL. Similarly, using the DPP method, Sebastian et al. [15] quantified tin(II) within the concentration range 0 to 10 µg/mL but the LOD was 0.5 µg/mL.Recently published methods based on membrane sensor, ICP, FAAS, and fluorescence quenching im proved the detection concentration range and the LOD, but the constraints in the methods limit their usage. The tin(II) selective potentiometric membrane sensor method was reported to have a concentration range of 0.013 to 1190 µg/mL and a low LOD of 0.0025 µg/mL, but preparation of the membrane elec trode was work intensive [18]. ICP-OES was used to Abstract-A rapid, straightforward spectrophotometric method based on the redox reaction of tin(II) with a mixture of N bromosuccinimide (NBS) and 3,3',5,5' tetramethylbenzidine dihydrochloride (TMB) was developed for determining low concentrations of tin(II). The redox method improved sensitivity by 2.3 fold relative to the existing spectrophotometric methods by t...