Bleeding Indicator Papers

These papers are obtained when uncoated unfilled paper is soaked with solutions of pH indicators and then carefully dried. Depending on the end use, one or more indicator dyes can be applied. Three types of indicator papers are commercially available. 

Simple indicator papers only show if a solution is acidic, neutral, or alkaline. Examples of this type are litmus paper, phenolphthalein paper, and congo paper. 

Universal indicator papers permit pH determinations over the total pH range from 0 to 14. The determination of the pH is made by comparing the color of the paper with a reference color scale. The accuracy is ca. 1 pH unit. 

Special indicator papers cover ranges from 2 to 5 pH units. The determination method is the same as with universal indicator papers. The accuracy is better 0.5 units is regularly attained under favorable conditions, 0.2-0.3 units is possible. 

The pH papers which are produced by impregnation are prone to bleeding , that is, a certain portion of the indicator dyes dissolves in the test solution during immersion of the paper or the strip. This occurs particularly in alkaline medium, where the solubility of the dyes reaches a maximum, and in weakly buffered solu- 

---

We recorded the surface pH by pressing a moistened non-bleeding indicator paper to the fabric for a short period, after which the resultant colour change was interpreted. These revealed pH values between 3.3 and 4.0 in the various locations tested, whereas for new linen sailcloth the value was around 6.0. While it is difficult to deduce the true acid content from this type of assessment, nevertheless values below pH 4 are generally regarded as a cause for some concern over the longer term, leading to recommendations for remedial action. 

As indicated when K = kXjjT0MCp was first introduced just before Eqn. (12), K is constant with respect to decisions within each feedwater heater or condenser, but is a variable with respect to associated costs. Thus, for example, the dimensionless cost k includes the cost of cooling water inlet filters in the case of the condensers—while for feedwater heaters it includes the cost of the bleed steam duct and tap required for the addition of a new heater. It also includes the cost of end effects (such as headers and water boxes) which will be added automatically to k when the non-linearity correction factors mentioned above are included. When all these corrections are considered, the results of Eqns. (A2) and (A3) appear to be in good agreement with the values found in well-optimized plants (such as the Wisconsin Electric Plant from which the data for this work was taken), but it is beyond the scope of this paper to pursue this matter further. 

Conventional pH indicators must be dissolved in a liquid to be tested. Their presence may be undesirable in some applications, e.g., in food and pharmaceuticals. Even when pH papers are used to indicate the pH of a solution, lengthy immersion of the paper can result in bleeding of the indicator dye into solution, as well as promoting chromatographic effects that cause the dye to creep and form uneven color zones. 

---