Thymol-blue, as indicator

In general,it may be stated that weak acids(Kfl > 5 x 10-6) should be titrated with phenolphthalein, thymolphthalein, or thymol blue as indicators. 

Walash and Rizk (32) reported a non-aqueous titration of several analgesics in dosage form including salicylamide in tetramethylurea using 0.1 N sodium methoxide as the titrant, the end point was measured either potentio-metrically or with thymol blue as indicator. The results were comparable with those obtained using dimethyl-formamide as a solvent. 

The end-point of the titration is found visually with thymol blue as indicator, or potentiometrically with glass and saturated calomel electrodes. The error of the determination does not exceed 1.4%. Alkoxy groups do not interfere in the procedure. 

If it is difficult to obtain consistent results with certain polyesters, an alternative method is where the acetic anhydride is mixed with perchloric acid and 1,2-dichloroethane. The polyester sample is added to this solution and the mixture warmed gently. A second mixture of A, A -dimethyl-formamide, pyridine and water is then added and the excess acid titrated with standard potassium hydroxide using thymol blue as indicator. 

Standardisation of silicotungstic acid Dissolve 3 g of the acid in water and titrate with 0 1 N sodium hydroxide, using thymol blue as indicator, until a definite, permanent, yellow colour appears. Let themlofO-lN alkali required be... 

Ryan, Yanowski and Pifer titrated the sample in dimethylformamide with 0-lN lithium methoxide using thymol blue as indicator. Small volumes of dimethylformamide were used (favouring a high ratio of benzene to solvent at the end-point) as it had been found that this sharpened the end-point slightly. 

Magnesium Nitrate Solution. Adjust the pH of a 50 per cent w/v solution of magnesium nitrate to 9 5 with ammonium hydroxide, using thymol blue as indicator, and extract with successive portions of di-thizone solution in chloroform until the dithizone layer remains green. 

Procedure B. The experimental details for the preparation of the initial solution are similar to those given under Procedure A. Titrate 25 or 50 mL of the cold solution with standard 0.1M hydrochloric acid and methyl orange, methyl orange-indigo carmine, or bromophenol blue as indicator. Titrate another 25 or 50 mL of the cold solution, diluted with an equal volume of water, slowly with the standard acid using phenolphthalein or, better, the thymol-blue cresol red mixed indicator in the latter case, the colour at the end point is rose. Calculate the result as described in the Discussion above. 

Other SC- and BSC-PEG derivatives were prepared using the same general protocol. The active carbonate contents of the products were determined according to Kalir et al. (77) by reacting aliquots of the polymers with an excess of benzylamine and back titration of the latter with perchloric acid in dioxane using thymol blue as an indicator. The results of these determinations are summarized in Table I. 

For the non-aqueous titration of weak acids a solvent such as an alcohol or an aprotic, solvent is used that does not compete strongly with the weak acid for proton donation. Typical titrants are lithium methoxide in methanol or tetrabutyl ammonium hydroxide in dimethylformamide. End-point detection may be carried out with thymol blue as an indicator or potentiomctrically (see p. 65). 

Fluorouracil may be assayed by dissolving an accurately weighed sample in dimethylformamide and titrating it with O.IN tetra-n-butyl-ammonium hydroxide in benzene to a blue end point using thymol blue as the indicator. 

Sulfamethoxazole can also be titrated in dimethylformamide with O.IN LiOCH3 using thymol blue as the indicator (17). The proton on the sulfonamide function is replaced by the lithium ion. Each ml of O.IN lithium methoxide is equivalent to 25.33 mg of sulfamethoxazole. 

The concentration of hydroxyl ions was determined by means of titration by 0,1N nitric acid solution with metliylorange as an indicator, that of Er" anions and thiourea - by 0,05N Hg(N03)2 titration (in the presence of ethyl alcohol) with diphenylcarbazone as an indicator, that of F anions - by direct potentiometric method [14], that of hydroquinone - by 0,1N Ce S0i )2 solution titration in the presence of sulfur acid with phe-nylanthranilic acid as an indicator, that of formic and acetic acids -by 0,0TN NaOH solution titration with thymol blue as an indicator. 

Table VI. A Comparison of the Results Obtained Using Two Titrants With Benzene-Methanol as Solvent and Thymol Blue as the Indicator... 

Pb + may be directly titrated with complexone(III) in hexamethylenetetramine medium with xylenol orange as indicator. The pH value is located about 5.6. A similar titration of Zn + can be carried out at pH about 6 with xylenol orange or methyl thymol blue as the indicator. Hexamine is then the buffer. This titration is also possible in ammonia buffer (pH about 10) in the presence of eriochrome black T. 

Bi is also directly titrated by complexone(III) in the pH = 0-1 range with xylenol orange or methyl thymol blue as the indicator. 

Leavitt and Autian investigated the use of tetra-w-butylammonium hydroxide as titrant and employed both a potentiometric and a visual method of determining the end-point. They found that the potentiometric method gave a sharp inflection at the end-point when benzene-wopropyl alcohol or benzene-chloroform solvent was used and a modified calomel electrode enhanced the inflection in a majority of the determinations carried out. They state that the visual method, employing thymol blue as the indicator, gave the desired accuracy and precision necessary for routine control purposes. 

Prepare 0.1 M sodium carbonate using the purest anhydrous salt. Pipette 25.0 cm in a conical flask, add a few drops of thymol blue indicator. Cool to 0 C and stir magnetically as you titrate with standardised 0.2 M HCl until the colour changes from yellow to blue. Repeat and calculate the average titre. Repeat two titrations using bromophenol blue as indicator when the colour changes at the end-point from blue to yellow. Compare the two titres with the different indicators. 

Thymol blue is used extensively as an indicator for titrations of substances acting as acids in dimethylformamide solution. It is used as a 0.2 per cent w/v solution in methanol with a sharp colour change from yellow to blue at the end point. 

A slight excess of 10 per cent barium chloride solution is added to the hot solution to precipitate the carbonate as barium carbonate, and the excess of sodium hydroxide solution immediately determined, without filtering off the precipitate, by titration with the same standard acid phenolphthalein or thymol blue is used as indicator. If the volume of excess of sodium hydroxide solution added corresponds to timL of 1M sodium hydroxide and u mL 1M acid corresponds to the excess of the latter, then v — v = hydrogencarbonate, and V— v — v ) = carbonate. 

CHARACTERIZATION. The intrinsic viscosity of the soluble fractions was determined in toluene at 30 C. The MAH content of the soluble fractions was determined by heating a 0.5-1.0g portion in refluxing water-saturated xylene for 1 hr and titrating the hot solution with 0.05N ethanolic KOH using 1% thymol blue in DMF as indicator. 

Although litmus paper, cabbage juice, and phenolphthalein can indicate whether a substance is acidic or basic, they have limitations in that they cannot determine an exact pH. To do this, an acid-base indicator called universal indicator can be used. Universal indicator is actually a mixture of several different acid-base indicators (usually phenolphthalein, methyl red, bromthymol blue, and thymol blue). This mixture produces a wide range of colors to indicate different pHs. Under very acidic conditions, universal indicator is red. It turns orange and then yellow between the pHs of 3 to 6. It is green at neutral pH and turns greenish-blue as a solution becomes more alkaline. In very basic conditions, universal indicator turns a dark purple color. 

The titration of nalidixic acid in DMF with lithium methoxide has been reported(1)(2) with thymolphthalein as the indicator. It has also been titrated with sodium methoxide in ethylene-diamine or DMF methanol 1 2 with thymol blue indicator.(31) An error for this titration was reported as + 0.7%. A titration with sodium borohydride followed potentiometrically or with thymol blue indicator has also been reported by Bachrata and co-workers. The standard deviation was reported as + 0.60%.(32)... 

Fluorescent pH indicators offer much better sensitivity than the classical dyes such as phenolphthalein, thymol blue, etc., based on color change. They are thus widely used in analytical chemistry, bioanalytical chemistry, cellular biology (for measuring intracellular pH), medicine (for monitoring pH and pCC>2 in blood pCC>2 is determined via the bicarbonate couple). Fluorescence microscopy can provide spatial information on pH. Moreover, remote sensing of pH is possible by means of fiber optic chemical sensors. 

Determination of Acidity or Alkalinity of NG. Place a lOg sample of the NG by pipet into a tared beaker and retare. Add 100ml of benzene and transfer to a 250-ml separatory funnel when dissolved. Wash twice with 50-ml portions of neutral distd w, combining the washes in a 250-ml beaker. Add several drops of bromo-thymol blue indicator and immediately titrate with 0.01N NaOH if yellow, or with 0.01 N sulfuric acid if blue. Run a blank on 100ml of benzene and correct the first titratiaQ value. Then, % acidity (as sulfuric acid) or alkalinity (as sodium carbonate) = kVN/W, where k is... 

Consider the titration in Figure 11-2. for which the pH at the equivalence point is calculated to be 9.25. If thymol blue is used as an indicator, what color will be observed through most of the titration prior to the equivalence point At the equivalence point After the equivalence point ... 

These salts have sharp and reproducible decomposition temperatures but no true melting points. They act as dibasic acids when titrated with standard alkali, thymol blue being used as indicator. 

Alcohol oxidation requires release of a proton, which formally comes from the alcohol. In other dehydrogenases such as lactate dehydrogenase, proton release occurs simultaneously with hydride transfer. In liver ADH proton release can be demonstrated, by reaction of the proton with an indicator such as thymol blue or phenol red in stopped-flow spectrophotometry, to be faster than hydride transfer, 270 vs. 150 s and unaffected by use of deuterated substrate, so it occurs before hydride transfer. Binding of the NAD+ nicotinamide ring is accompanied by a conformational change of ADH bringing the catalytic zinc about 0.1 nm closer to the... 

Add exactly 50 mL 0.2 N HC1 to each Erlenmeyer flask from the buret. Also, add a few drops of thymol blue indicator. Gently stir with the stirring rods to disperse the tablets. (Some of the inactive ingredients may not go into solution and will settle as a fine powder on the bottom of the flask.) At this point the solution should be red (the color of thymol blue at acidic pH). If either of your solutions does not have red coloration, add 10 mL 0.2 N HC1 from the refilled buret and make certain that the red color will persist for more than 30 sec. Record the total volume of 0.2 N HC1 added to each flask on your Report Sheet (3). 

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