Indicator papers sensitivity

The checkers found that pH indicator paper (sensitivity +/-1 pH unit) can also be used to monitor the acidity of the solution to pH 3. 

Low temperature steam and formaldehyde Reactive chemical Indicator paper impregnated with a formaldehyde-, steam- and temperature-sensitive reactive chemical which changes colour during the sterilization process Gas concentration, temperature, time (selected cycles)... 

Studies of the ect of structure are much needed. Initial studies correlated selectivities to the relative numbers of different crystal faces in crystallites and indicated structure sensitivity (21-23), Such studies may need refinement (24), and one important direction is the use of single crystals. A number of papers in this volume deal with this topic (Wickham, etaL Aniga, et al, Schulz and Cox Chen, et oL). 

There are several sensitive methods for detecting cyanide in blood samples, so murder and suicide cannot easily be disguised. One simple method uses a special indicator paper which changes colour in the presence of cyanide. 

Indicator papers serve the same purpose as do indicator solutions in that they reveal the reaction of a liquid. As we shall see farther on, the sensitivity of the papers depends on so many conditions that as a rule they do not permit a very precise measurement of the hydrogen ion concentration. It is true that the pH of buffer mixtures can be measured approximately with indicator papers. They may be employed to greater advantage, however, for qualitative purposes such as the testing of gases for acidic or basic constituents (ammonia, acetic acid, etc.). 

The sensitivity of indicator papers depends upon a number of factors which will be discussed below. Suffice it to say that this sensitivity is always smaller than that of the indicator solution when determined with strong acids or bases. When buffer mixtures are used, however, the sensitivity of the paper towards hydrogen or hydroxyl ions is the same as that of the solution of the corresponding indicator. 

The influence of the concentration of congo red on the sensitivity of Congo paper is illustrated below. It appears from the table that the most sensitive paper is obtained when filter paper is soaked in a 0.1 or 1% congo red solution. A solution of HCl as dilute as 0.0002 N can then be detected. The 0.1% paper is to be preferred because the color change is more distinct. The use of filter paper permits of greater sensitivity than does a sized indicator paper. 

It frequently happens that hydrogen ions are removed from solution by impurities in the indicator or in the paper, or as a result of adsorption by the paper. In other words, the solution apparently is neutralized. The use of a buffer mixture, however, counteracts the influence of traces of impurities. The sensitivity of indicator papers is found to be the same as that of the corresponding indicator solutions when measured in the presence of buffer solutions. For strong electrolytes, the indicator papers measure the titration acidity rather than the hydrogen ion concentration. 

Procedure according to Behrens. The indicator paper is drawn for 30 seconds through the solution being examined, and the liquid allowed to drain off. The color of the moist paper is then observed on a white background. A black background is better for bromphenol blue paper. A gray coloration corresponds to the pH sensitivity of the list given above. A sensitivity of 0.2 pH unit may be assumed. 

The capillary phenomena in filter paper have been the subject of numerous investigations. It is possible to tell when the sensitivity limit of an indicator paper is being approached simply from the appearance of the reactions taking place. A drop of 0.001 N HCl placed upon congo paper will, of course, diffuse radially. The center of this region shows the red alkaline color of the indicator. This is surrounded by a circle showing the acid reaction, which is followed in turn by a water circle. Other indicator papers such as dimethyl yellow, azolitmin, litmus papers, etc., behave in similar fashion. 

B. Pratt and H. 0. Swartout have prepared indicator papers from fruit and vegetable extracts (cf. page 165) but mention no sensitivity limits. Filter paper was soaked in the aqueous extracts, dried, moistened with ammonium hydroxide, and again dried in air. The paper then has a neutral color. These natural indicator papers arc listed below. 

The limits of sensitivity of indicator papers towards strong acids and bases. 

A supply of bromophenol/blue indicating paper or equivalent, sensitive to 5-10 ppm ammonia or greater over a two-minute period of exposure. QdH 3.0 fellow) to pH 4.6 (blue)]... 

In this paper, we described a method for preparing alkanethiol-based monolayer protected gold-nanoparticles as filler for conductive polymer. Our results indicate that sensitivity of PDMS/Au for organic vapor is superior to conventional CB or CN filler. Both response and recovery time of sensor is faster and the reproducibility of sensor was excellent for ON/OFF cycles by switching between dry and organic vapors. 

Chemical indicators for dry-heat steriliza tion are available either in the form of pellets enclosed in glass ampuls, or in the form of paper strips containing a heat-sensitive ink. The former displays its end point by melting, the latter by a color change (see Chromogenic materials). 

A less accurate but more colorful way to measure pH uses a universal indicator, which is a mixture of acid-base indicators that shows changes in color at different pH values (Figure 13.5, p. 359). A similar principle is used with pH paper. Strips of this paper are coated with a mixture of pH-sensitive dyes these strips are widely used to test the pH of biological fluids,... 

Saturated solutions of some reagents (T) 829 Schoniger oxygen flask see Oxygen flask Schwarzenbach classification 53 Screened indicators 268 Sebacic acid 469 Secondary pH standards 831 Selective ion meters 567 Selectivity coefficient, 559 in EDTA titrations, 312 in fluorimetry, 733 of analytical methods, 12 Selenium, D. of as element, (g) 465 Semi-log graph paper 572 Sensitivity (fl) 834, (fu) 732 Separation coefficient 163, 196 Separations by chromatographic methods, 13, 208. 233, 249... 

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