Visible reaction changes

The reason the two forms of phenolphthalein are different colors is because the chemical reaction changes the shape of the phenolphthalein molecule. In other words, the molecule has a different shape under acidic conditions than it does under basic conditions. The different shapes absorb and reflect different wavelengths of light. Our eyes see different wavelengths of visible light as different colors. 

This reaction is accompanied by a visible colour change, as orange dichromate ions become green chromium(lll) ions. The concentration of alcohol in the blood is determined by measuring the intensity of the final colour. 

Chemical resistance could easily be called the heart of colorant stability in plastic systems When colorants do not perform as expected, there is usually a visible color change. This implies that a chemical reaction has taken place with the colorants. For colorants to perform in a system, they must remain unaffected by all the steps of processing and life cycle. 

Stability of (BN) iSO F in a sealed Pyrex glass tube. A powder sample (BN) 3S03F was prepared in a 3 in. Pyrex tube with a Teflon valve. After the reaction was complete and excess SjO Fj (confirmed by IR (19) to be S2O5F2 free) had been removed, the Pyrex tubing was drawn down and sealed under vacuum. There was no visible color change in this sealed sample over several months. 

A qualitative test for BTA analytes in urine, termed the BTA stat. test, has been developed. BTA analytes are high molecular weight polypeptides composed of complexes of basement membrane proteins. The presence of BTA analytes in urine is thought to involve either invasion of the basement membrane by tumor, production by the tumor itself, or a combination of these, which maybe linked with the bodjfs immune response. If BTA analytes are present in a significant level, they will combine with latex particles to produce an agglutination reaction, which produces a visible color change on the... 

Synchrotron radiation studies [92] have identified systematic changes in the interplanar spacing in the vicinity of the microscopically visible reaction interface, showing that the thickness of the reaction zone is about 150 pm. There is apparently significant loss of water fi-om the zone ahead of the recrystallization plane identified as the advancing reaction interface. 

When the reaction of Fe(III)TMP(Cl) (17) and mCPBA in dichloromethane at low temperature was directly monitored by UV-Vis spectroscopy, it was found that the kinetic profile for the formation of 14 showed a pronounced induction period (79). Figure 7 shows the time-dependent visible spectral changes in the reaction of 17 and mCPBA. Absorbance changes monitored in the reaction are also illustrated, and the inset in Fig. 7 shows the effect of acid on the kinetic behavior. The addition of acid (mCBA in this case) clearly accelerated the reaction. Inspection of spectral changes, especially the absorbance changes around 390 nm, indicates the absence of isosbestic points in the spectral changes. These observations are indicative of the formation of an intermediate in the reaction of... 

Visible spectral changes observed in the reaction of 18 with peracid at -46°C in dichloromethane are shown in Fig. 8 (78). It is clear that rapid formation of an intermediate whose visible spectrum is characteristic of a high-spin Fe(Ill)... 

By investigating the visible spectral change of CoPc in a PVP membrane coated on a graphite electrode by in situ potential step chronospectrometry (PSCS) during CO2 reduction, it has been shown that, after CO2 coordination to the two-electron reduced CoPc, one more electron is injected from the electrode to form CO, recovering one-electron reduced CoPc (Path II of Fig. 13-9) [25], different from the previously proposed reaction scheme (Path I). 

The secondary electrode redox reaction is chosen to be a system where there is little perceptible visible color change or as an electrochromic system where the color change is complementary to that of the color change at the primary electrochromic electrode. 

Other bio-responsive hydrogels have been developed to give an optical output that use different actuation methods. These include sensors for a-cyclodextrin, in which a reaction with polydiacetylene liposomes embedded in poly(ethylene glycol) diacrylate (PEGDA) leads to a visible colour change of the hydrogel from blue to red [17]. 

In a 500 ml. three-necked flask, fitted with a reflux condenser and mechanical stirrer, place 121 g. (126-5 ml.) of dimethylaniline, 45 g. of 40 per cent, formaldehyde solution and 0 -5 g. of sulphanilic acid. Heat the mixture under reflux with vigorous stirring for 8 hours. No visible change in the reaction mixture occurs. After 8 hours, remove a test portion of the pale yellow emulsion with a pipette or dropper and allow it to cool. The oil should solidify completely and upon boiling it should not smell appreciably of dimethylaniline if this is not the case, heat for a longer period. When the reaction is complete, steam distil (Fig. II, 41, i) the mixture until no more formaldehyde and dimethylaniline passes over only a few drops of dimethylaniline should distil. As soon as the distillate is free from dimethylaniline, pour the residue into excess of cold water when the base immediately solidifies. Decant the water and wash the crystalline solid thoroughly with water to remove the residual formaldehyde. Finally melt the solid under water and allow it to solidify. A hard yellowish-white crystalline cake of crude base, m,p. 80-90°, is obtained in almost quantitative yield. RecrystaUise from 250 ml. of alcohol the recovery of pure pp -tetramethyldiaminodiphenylmethane, m.p. 89-90°, is about 90 per cent. 

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