Nonbleeding Indicator Papers

It can be prepared either by using appropriate direct dyes or reactive dyes. In the latter case, pure linters (the raw material for high-quality paper) are suspended in water, and the solution of the reactive dye is added. For example, the dye 15 reacts via the sulfonic acid group in the side chain. The reaction mixture is then made alkaline and the dye reacts with the linters. After completion of the reaction, the fiber pulp is centrifuged, washed electrolyte-free, and processed on a paper machine to form the pH paper. Paper produced in this way is mostly bonded onto a plastic material and used as pH test strip. 

Experiments have been performed with other carriers such as hydrophilic plastic foils and even textiles, but none of these products has come to market. 

Naumann, W. Fischer, W.-D. Mayer, G. Wieland, E. J. Newman, C. M. Wilson, Indicator Reagents , Ullmann s Encyclopedia of Industrial Chemistry, 6th ed., Electronic Release, Wiley-VCH, Weinheim, 2002. 

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In order to overcome these difficulties, E. Merck Co. has marketed nonbleeding pH paper, which has the indicator dye covalently bound to the paper (cellulose) via its 2-sulfoxyethylsulfonyl derivative. The paper-bound indicators, although better than the conventional pH papers, were subject to bacterial attack and were less stable to acids and bases. 

Most probably, the first - but non-fiberoptic - sensors for continuous use where those for pH and for oxygen. It has been known for decades that cellulosic paper can be soaked with pH indicator dyes to give pH indicator strips which, however, leached and thus were of the "single-use" type. The respective research and development is not easily traced back since it is not well documented in the public literature. However, in the 1970s, indicator strips became available where they pH indicator dye was covalently linked to the cellulose matrix, usually via vinylsulfonyl groups. These "nonbleeding" test strips allowed a distinctly improved and continuous pH measurement, initially by visual inspection. In the late 1980 s instruments were made available that enabled the color (more precisely the reflectance) of such sensor strips to be quantified and related to pH. Respective instruments are based on the use of LEDs and are small enough to be useful for field tests in that they can be even hand-held. This simple and low cost detection system is still superior to many of the complicated, if not expensive optical pH sensors that have been described in the past 20 years. 

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