M r range % acrylamide Deionized . cells in the range of 190-1100 nm were made on a Shimadzu (Kyoto, Japan) 1601 UV/VIS Spectrophotometer controlled by an UVProbe-2.5 so ware. Beyond the ranges shown, the color of the indicator will remain the same. METHYL RED: The IUPAC name for methyl red is 2 -(N, N-dimethyl-4-aminophenyl) azobenzene carboxylic acid. At this point, the bromothymol is already blue, and a few drops of BTB are used on a water slide. The bromophenol blue is also used as a dye. (adsbygoogle = window.adsbygoogle || []).push({}); Want chemistry games, drills, tests and more? pH < 6.0 the solution appears to be yellow, at the end point, between pH 6.2 and 7.6, the solution appears to be green (an equimolar mixture of blue and yellow), pH < 6.8 the solution appears to be yellow, at the end point, between pH 6.8 and 8.4, the solution appears to be orange (an equimolar mixture of red and yellow), pH < 8.0 the solution appears to be yellow, pH > 9.6 the solution appears to be blue, at the end point, between pH 8.0 and 9.6, the solution appears to be green (an equimolar mixture of yellow and blue), pH < 8.3 the solution appears to be colourless, pH > 10.0 the solution appears to be magenta, at the end point, between pH 8.3 and 10.0, the solution appears to be pale pink (an equimolar mixture of colourless and magenta), pH > 6.0 the solution appears to be yellow, at the end point, between pH 4.4 and 6.0, the solution appears to be orange (an equimolar mixture of pink and yellow). Bromophenol blue (3,3,5,5-tetrabromophenolsulfonphthalein, BPB),[2] albutest[3] is used as a pH indicator, an electrophoretic color marker, and a dye. Bromophenol blue changes color at pH less than 7 Antacid titration is an acid-base titration in which antacid that acts as bases and is mainly composed of metal hydroxides like NaOH reacts with excess amount of acid such as HCl. Consider bromothymol blue (pH range 6.2 - 7.6) and phenol red (pH range 6.8 - 8.4) as possible indicators for this neutralisation reaction: The equivalence point for the reaction is represented by the blue line at pH=7. to light purple to dark purple. The table below shows the color titration has been reached. All rights reserved. Xylene cyanol and orange G may also be used for this purpose.[6]. At the equivalence point neither the acid nor the base is the limiting reagent. They are used to visually signal the acidity or alkalinity of an aqueous (water-based) solution. To use all the functions on Chemie.DE please activate JavaScript. Reaxys. However, the apparent color shifts mostly depend on the concentration and/or path length through which the solution is observed. (using the StoPGoPS approach to problem solving). Which indicator would be best to Ideally: pH at the end point = pH at the equivalence point. An aqueous solution of hydrochloric acid, HCl(aq), is a strong acid. Find out more about the company LUMITOS and our team. Animal Cell Culture. 1 equivalent of a base is the quantity which supplies 1 mole of OH-. F. Chevalier, in Encyclopedia of Dairy Sciences (Second Edition), 2011 Gel Staining. Since the equivalence point for the titration (pH=7) occurs within the pH range for the visible colour change of the indicator (the end point between pH 6.8 and 8.4), this indicator can be used for this titration. On this Wikipedia the language links are at the top of the page across from the article title. Copyright 2023 SIBATA Class A Graduated Glass Cylinders, 0.2 (red) - 1.8 (yellow) and 7.2 (yellow) - 8.8 (red), 1.2 (red) - 2.8 (yellow) and 7.4 (yellow) - 9.0 (purple), 7.3 (colorless/reddish) - 8.7 (blue/green). Preparation of 6X DNA Loading Dye (Bromophenol blue, Xylene Cyanol FF, Ficoll 400) - Laboratory Notes. 3,3-bis(3,5-dibromo-4-hydroxyphenyl)-3H-2,1-benzoxathiole-1,1-dione, OC1=C(Br)C=C(C=C1Br)C1(OS(=O)(=O)C2=CC=CC=C12)C1=CC(Br)=C(O)C(Br)=C1, Visual transition interval pH 3.0 (yellow) to pH 4.6 (blue). Other indicators work in different pH ranges. Bromothymol Blue - C 27 H 28 Br 2 O 5 S What is Bromothymol blue? Better weighing performance in 6 easy steps Daily Visual Balance Check How to quickly check pipettes? color change in this pH range. occur until the pH gets closer to pH 11. Bromothymol blue (also known as bromothymol sulfone phthalein and BTB) is a pH indicator. The molecular weight of bromophenol blue is around 670 Daltons or grams per mole. Alizarine yellow will be . [3] Highly acidic Bromothymol blue is magenta in color. Some content on this page could not be displayed. Please enable javascript and pop-ups to view all page content. The pH ranges between 3.0 to 4.4. Since the equivalence point for the titration (pH=8.7) occurs within the pH range for the visible colour change of the indicator (the end point between pH 8.0 and 9.6), this indicator can be used for this titration. Bromophenol Blue sodium salt (3,3,5,5-Tetrabromophenolsulfophthalein); Undefined, for electrophoresis; Suitable for use with agarose or polyacrylamide gel electrophoresis (PAGE); Bromophenol blue sodium salt is used as tracking dye in electrophoresis; Bromophenol Blue is a pH indicator, and a Leave a Reply Cancel reply. changed from eight to 11. This process was done very fast, causing a lot of bubbles to be formed. Which indicator is blue at a pH of 5? The original Alfa Aesar product / item code or SKU reference has not changed as a part of the brand transition to Thermo Scientific Chemicals. Copyright 2023 NagwaAll Rights Reserved. In an acid-base titration, the following chemical reaction (Arrhenius neutralisation) takes place: When all the acid has reacted with all of the base the resultant solution is an aqueous salt solution, MA(aq). Which indicator is blue at a pH of Bromophenol blue is also used as a dye. over different pH ranges. In PH. In general, predicted pH ranges are 2 pH units but the published results for pH range are smaller than this. As CO2 is absorbed from the breath into the solution, forming carbonic acid, the solution changes color from green to yellow. (Its peak absorbance is 600nm at a basic pH of 12.) each of the three remaining indicators. BUT phenolphthalein changes colour between pH 8.3 and 10.0, so, at the equivalence point the phenolphthalein remains colourless. pH of five is bromophenol blue. As we slowly add NaOH (base) to the acid, the pH gradually increases. Membership IS NOT required to shoot at our range. The indicator that would be best to show that the pH of a solution has changed from 8 to 11 will be an indicator that undergoes a color change in this pH range. Sodium hydroxide is a base, and it was in the pitcher at the beginning, so when added to the phenolphthalein in beakers 2 and 4, it turned pink . The balanced chemical reaction below represents the reaction between HCl(aq) and NH3(aq): NH4Cl is the salt of a strong acid and a weak base, so a solution of NH4Cl will have a pH < 7 (NH4+ is a weak acid) changes in pH that occur as HCl(aq) is added to NaOH(aq). . need to select the indicator that would best show that the pH of a solution has Please do not block ads on this website. The equivalence point for the reaction is represented by the blue line at pH = 8.7. What caused the foam in this experiment? NaCl(aq) will have a pH=7 The background colour represents the colour of the solution containing the phenolphthalein indicator over the same range of pH values. Chris has the following acid-base indicators currently available in the lab: Which indicator should Chris use for the titration? Looking at pH five, we see that The deprotonation of the neutral form results in a highly conjugated structure, accounting for the difference in color. [9] However, a recent study suggests that methyl red is more useful in determining activity due to the bright yellow ring formed in the zone of enzyme activity.[10]. Call 800-372-6726. Bromothymol blue (also known as bromothymol sulfone phthalein and BTB) is a pH indicator. "Bromophenol Blue (CAS 115-39-9) Properties", "Bromphenol Blue - Use and Manufacturing", "Product Specification: Bromophenol Blue", "Agarose gel electrophoresis (basic method)", "Quantification of dichromatism: a characteristic of color in transparent materials", https://en.wikipedia.org/w/index.php?title=Bromophenol_blue&oldid=1142525474, Chemical articles with multiple compound IDs, Multiple chemicals in an infobox that need indexing, Chemical articles with multiple CAS registry numbers, Pages using collapsible list with both background and text-align in titlestyle, Articles containing unverified chemical infoboxes, Creative Commons Attribution-ShareAlike License 3.0, This page was last edited on 2 March 2023, at 22:14. It can thus be in protonated or deprotonated form, appearing yellow or blue, respectively. It is employed as an electrophoresis tracking dye and also finds its uses as an industrial dye, a laboratory acid-base indicator and a biological stain to detect proteins and nucleic acids (absorbance at 610 nm), particularly when staining living tissues. It is typically sold in solid form as the sodium salt of the acid indicator. In solution at pH 3.6 (in the middle of the transition range of this pH indicator) obtained by dissolution in water without any pH adjustment, bromophenol blue has a characteristic green red color. Search Jul 2018 - Present4 years 9 months. (2) What is the relationship between what you know and what you need to find out? A common use is for measuring the presence of carbonic acid in a liquid. The pH of blood ranges from 7.35 to 7.45, for instance. An aqueous solution of hydrochloric acid, HCl(aq), is a strong acid. The indicator that is blue at a pH of 5 is bromophenol blue. Reagents; Culture Medium; Insect Cell Culture An aqueous solution of acetic acid (ethanoic acid), CH3COOH(aq), is a weak acid. The approach utilizes purified BPB dye whose absorbance characteristics have been determined over a range of temperatures and salinities. For bromophenol blue: pH 3 = yellow; pH 4.6 = blue; See the graphic below for colors and pH ranges. Copyright 2023 that undergo a color change over a specific pH range. At low pH, the dye absorbs ultraviolet and blue light most strongly and appears yellow in solution. below a pH of three and blue above a pH of approximately 4.6. pH range > 7 therefore the base must be stronger than the acid. [8] This means it has the largest change in colour hue, when the thickness or concentration of observed sample increases or decreases. It is less soluble in nonpolar solvents such as benzene, toluene, and xylene, and practically insoluble in petroleum ether. In gel electrophoresis it can be used as a color marker and as a biological stain it can be used to stain proteins and nucleic acids. Bromophenol blue (BPB) is most commonly used as an agarose gel electrophoresis size marker. Part 2. Most living tissues prosper at a near-neutral pHthat is, a pH close to 7. The original Alfa Aesar product / item code or SKU reference has not changed as a part of the brand transition to Thermo Scientific Chemicals. In solution at pH3.6 (in the middle of the transition range of this pH indicator) obtained by dissolution in water without any pH adjustment, bromophenol blue has a characteristic green red colour, where the apparent colour shifts depending on the concentration and/or path length through which the solution is observed. Categories: PH indicators | Triarylmethane dyes. At neutral pH, the dye absorbs red light most strongly and transmits blue light. want to know which indicator is blue at a pH of five. pH No information available Melting Point/Range 279 C / 534.2 F Boiling Point/Range No information available Flash Point No information available Evaporation Rate Not applicable Flammability . Approximate pH range for color change: 3.0-4.6: Color of acid form: yellow: Color of base form: purple: CAS Number: 115-39-9: ChEBI: CHEBI:59424: ChEMBL: ChEMBL2103765: . Expert Solution. [7][8] A common demonstration of BTB's pH indicator properties involves exhaling through a tube into a neutral solution of BTB. Copyright 2006-2023 Thermo Fisher Scientific Inc. All rights reserved, Colonnes et cartouches de chromatographie, Consommables en plastique et fournitures de laboratoire, Afficher toutes les catgories de produits, Spectroscopie, analyse lmentaire et isotopique, Voir toutes les applications et techniques, Services aux organisations de dveloppement et de fabrication sous contrat (CDMO) et pour les essais cliniques, Consultez toutes les rubriques d'aide et d'assistance. Your browser does not support JavaScript. Senior Scientist at Sorrento Therapeutics, Inc. since July 2018, specializing in antibody discovery and engineering using high-throughput . At the equivalence point neither the acid nor the base is in excess. To answer the second question, we This phenomenon is called dichromatic color. Titrate to pH 6.8 with conc. The bottle became warm throughout this process creating . All rights reserved. The background colour represents the colour of the solution containing the phenol red indicator over the same range of pH values. Why is litmus paper not a very good indicator? Bromothymol blue is the most commonly used pH indicator and is in low concentration and size container and low toxicity. As a biological stain, it plays an important role to stain proteins and nucleic acids. They work only in colorless solutions in which a color change can be observed and can only give an approximate pH value. [2], The presence of one moderate electron-withdrawing group (bromine atom) and two moderate donating groups (alkyl substituents) are responsible for bromothymol blue's active indication range from a pH of 6.0 to 7.6. Use our request page at this link. Description. The litmus test is quick and simple, but it suffers from a few limitations. In a 1% gel it runs at around 500bp but the higher % of the gel the lower it will run. It is mostly used in applications that require measuring substances that would have a relatively neutral pH (near 7). * B = blue, C = colorless, O = orange, R = red, V = violet, Y = yellow. We will have to add an excess of NaOH(aq) to the HCl(aq) to make phenolphthalein change colour, in other words, the end point as indicated by the indicator will occur AFTER the equivalence point for the acid-base reaction. Since bromophenol blue carries a slight negative charge at moderate pH, it will migrate in the same direction as DNA or protein in a gel; the rate at which it migrates varies according to gel density and buffer composition, but in a typical 1% agarose gel in TAE buffer or TBE buffer, bromophenol blue migrates at the same rate as a DNA fragment of approximately 500 base pairs. Ground-only (No-Fly) chemicals are limited to the 48 contiguous U.S. states and Canada. (ii) Decide on the pH range of the indicator and hence name the most suitable acid-base indicator to use: Ideally, indicator's colour change at the end point should occur at the same pH as the equivalence point of the neutralisation reaction. The table below shows the color range for several different indicators. A suitable indicator for the titration of the weak acid CH3COOH(aq) and the strong base NaOH(aq) would be either thymol blue (pH range 8.0 - 9.6) or phenolphthalein (pH range 8.3 - 10.0). Here, we present a method utilizing purified bromophenol blue (BPB) as an end-point indicator for making simple one-point alkalinity measurements with spectrophotometric detection. would be best to show that the pH of a solution has changed from eight to 11 is Acid-base pH indicators are solutions that have a characteristic color at certain pH levels. For example, common indicators such as phenolphthalein, methyl red, and bromothymol blue are used to indicate pH ranges of about 8 to 10, 4.5 to 6, and 6 to 7.5 accordingly. They would both appear yellow. Required fields are marked *. . At the equivalence point the only species in solution is NaCl(aq) which has pH=7. A suitable indicator would be methyl red (pH range 4.4 - 6.0), The equivalence point for the reaction is represented by the blue line at pH=5.28. An aqueous solution of sodium hydroxide, NaOH(aq), is a strong base. Bromophenol blue is also used as a color marker to monitor the process of agarose gel electrophoresis and polyacrylamide gel electrophoresis. five? Beyond the ranges shown, the color Thus, BTB is commonly used in science classes to demonstrate that the more that muscles are used, the greater the CO2 output. So bromophenol blue will be yellow [2], Bromothymol blue is sparingly soluble in oil, but soluble in water, ether, and aqueous solutions of alkalis. It changes from yellow at pH 3.0 to purple at pH 4.6; this reaction is reversible. Strong bases: Hydroxides of Group 1 and 2 elements, so sodium hydroxide is a strong base (sodium is a Group 1 element) Despite its larger molecular weight of 670g/mol it runs faster than Xylene cyanol (539g/mol) because of its larger negative charge (hydroxyl groups).. Bromophenol blue is also a pH indicator which is yellow below pH=3 . The solution was poured (20 mL) into Petri dishes and 1 mM ampicillin was added to . (b) You may be able to approximate the pH of the salt solution using the relative strength of acid and base as shown above. Bromothymol blue may be used for observing photosynthetic activities, or as a respiratory indicator (turns yellow as CO2 is added). Chemicals with hazard fees are limited to the 48 contiguous U.S. states. Beer's Law Plot for Bromophenol Blue y = 0.0256x + 0.02 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 It can be prepared by slowly adding excess bromine to a hot solution of phenolsulfonphthalein in glacial acetic acid.[4]. . hospital patients (CDC, 2016). Demonstration experiments using universal indicators. Below is the balanced chemical reaction for the reaction between CH3COOH(aq) and NaOH(aq): CH3COOH(aq) + NaOH(aq) CH3COONa(aq) + H2O(l). (d) You may be given a titration curve to use to determine which indicator you would use (examples of this are shown in the next section). Which indicator would be best to show that the pH of a solution has changed from 8 to 11? The test may be false-positive in the presence of other alkaline substances such as blood or semen, or in the presence of bacterial vaginosis. Eppendorf 10-100 and 100-1000 mL vary- . Some documentation and label information may refer to the legacy brand. This 0.1% aqueous bromothymol blue solution (also known as Bromthymol Blue) is a commonly used pH indicator. Reference: [1] Dhananasekaran S,Palanivel R,Pappu S. Adsorption of Methylene Blue, Bromophenol Blue, and Coomassie Brilliant Blue by -chitin nanoparticles. Bromophenol blue is used as a tracking dye in electrophoresis, as a color marker in gel electrophoresis and polyacrylamide gel electrophoresis. show that the pH of a solution has changed from eight to 11? Nagwa uses cookies to ensure you get the best experience on our website. While the conjugation is responsible for the length and nature of the color change range, these substituent groups are ultimately responsible for the indicator's active range. Bromophenol blue: yellow: 3.0-4.6: purple: Congo red: blue-violet: 3.0-5.0: red: Methyl orange: red: 3.1-4.4: . Bromophenol Blue. Bromophenol blue, Tetrabromophenolsulfonephthalein, is an acid-base indicator whose useful range as an indicator lies between pH 3.0 and 4.6. This phenomenon is called dicromatic colour[1].. Bromophenol blue is commonly used in entry-level lab courses to stain proteins in wet-mount slides. Bromthymol blue changes color over a pH range from 6.0 (yellow) to 7.6 (blue). All products from TargetMol are for Research Use Only. Bromophenol blue is an acid phthalein dye. and Bromophenol Blue using a digital camera . Indicators are weak acids and bases (1) 1 equivalent of an acid is the quantity of that acid which will donate 1 mole of H+. (see Calculating pH of Aqueous Salt Solutions). It acts an intermediate and an acid-base indicator in the pH range 3.0 to 4.6. Healthcare-associated infections (HAIs), i.e., infections while receiving care at a healthcare facility, remain as one of the most important public health challenges for affecting 1 in 25 United States. 3,3-Bis(3,5-dibromo-4-hydroxyphenyl)-2,1, InChI=1S/C19H10Br4O5S/c20-12-5-9(6-13(21)17(12)24)19(10-7-14(22)18(25)15(23)8-10)11-3-1-2-4-16(11)29(26,27)28-19/h1-8,24-25H, InChI=1/C19H10Br4O5S/c20-12-5-9(6-13(21)17(12)24)19(10-7-14(22)18(25)15(23)8-10)11-3-1-2-4-16(11)29(26,27)28-19/h1-8,24-25H, Brc1cc(cc(Br)c1O)C3(OS(=O)(=O)c2ccccc23)c4cc(Br)c(O)c(Br)c4, Except where otherwise noted, data are given for materials in their. . When the pH increases to about 3.1, the colour of the indicator starts to look more orange than red so the end point of the titration as indicated by the indicator has been reached. values in which four different indicators change color. BUT the equivalence point of the titration will not occur until well after this colour change, at pH=7, so the end point occurs BEFORE the equivalence point. Bromothymol blue has been used in conjunction with phenol red to monitor the fungal asparaginase enzyme activity with phenol red turning pink and bromothymol blue turning blue indicating an increase in pH and therefore enzyme activity. Clinical trial: N/A. Certificates of Analysis for chemicals are available upon request. The below shows the approximate colour of some acid-base indicators at different pH values and the type of titrations they are useful for: Not all acid-base indicators are suitable for use in acid-base titrations: There are two steps in deciding which indicator to use for a particular acid-base titration: (a) You may be told the pH of the solution (eg, in an exam question). pH Indicator Ranges (Ascending pH) Acid-base pH indicators are solutions that have a characteristic color at certain pH levels. Since the equivalence point for the titration (pH = 7) occurs within the pH range for the visible colour change of the indicator (the end point between pH 6.2 and 7.6), this indicator can be used for this titration. Briefly, we prepared 1% (w / v) agarose (Biozym Scientific, Oldendorf, Germany) in 0.1 M citrate buffer (pH 3.5) by boiling and then cooling to 50 C before mixing with the proteins (1 mg/mL in 0.1 M citrate buffer, pH 3.5) isolated from a Silvaner Franken wine. Let us know if you liked the post. Different indicators change color over different pH ranges. What would happen if we used methyl orange? However, the color change doesnt The gel must firstly be immersed in a fixation solution containing acid (phosphoric acid or acetic acid) and alcohol (ethanol or methanol) as a . [5] Bromophenol blue is structurally related to phenolphthalein (a popular indicator). range for several different indicators. Bromophenol blue is also used as a pH indicator with a transition range of pH 3 to 4.6. Solutions of the dye, therefore, are blue. Consider bromothymol blue (pH range 6.2 - 7.6) and phenol red (pH range 6.8 - 8.4) as possible indicators for this neutralisation reaction: A suitable indicator for this strong acid - strong base titration would be bromothymol blue (pH range 6.2 - 7.6) or phenol red (pH range 6.8 - 8.4). It is a good indicator of dissolved carbon dioxide (CO2) and other weakly acidic solutions. It is a good indicator of dissolved carbon dioxide (CO 2) and other weakly acidic solutions. Advantec MFS 7010030 pH Test Paper, Narrow Range, Booklet, Bromophenol Blue, pH 2.8-4.4, 7mm Width, 70mm Length (Pack of 200) The table shows us the range of pH values in which four different indicators change color. Comes packed in convenient squeeze bottle for dispensing by the drop. Your email address will not be published. Copyright 2006-2023 Thermo Fisher Scientific Inc. All rights reserved, Spectroscopy, Elemental and Isotope Analysis. T21481 CAS 115-39-9. This means that the indicator that To answer the first question, we : B392-5; BP115-25 CAS No 115-39-9 . An intermediate of the deprotonation mechanism is responsible for the greenish color in neutral solution.[2]. Weak acid + strong base salt + water Save my name, email, and website in this browser for the next time I comment. Uses of bromophenol blue indicator include testing for soap in unwashed biodiesel and glycerol. Indicator Thymol Blue Bromophenol Blue pKa 1.65 4.10 5.00 Methyl Red Bromothymol Blue Phenophthalein The K values for citric acid are: 7.30 9.50 4 Ka1 = 8.4 x 10-4 Ka2 = 1.8 x 10-5 Ka3 = 4.0 x 10-6. Strong acids are: hydrohalic acids (except HF), sulfuric acid, nitric acid and perchloric acid, so acetic acid is a weak acid. There is a wide range of Mini-PROTEAN TGX Precast Protein Gels, . For a given solute, there is often one wavelength (or a small range of wavelengths) at which the absorbance is greatest (and therefore the transmittance the least). Bromthymol blue changes color over a pH range from 6.0 (yellow) to 7.6 (blue). Bromophenol blue is used as a tracking dye for electrophoresis. Bromothymol blue acts as a weak acid in a solution. This mixture displays the colors red, orange-yellow, green, blue, and indigo-violet: How pH Indicators Work Most pH indicators are weak acids or weak bases. They are used to visually signal the acidity or alkalinity of an aqueous (water-based) solution. As vaginal pH normally is acidic, the blue color indicates the presence of amniotic fluid. The PH range of Xylenol Orange indicator is between 3.2 and 4.4. HCl, then make up to 500 ml. Initially there is a large excess of acid, the solution is acidic, and the methyl orange indicator is red. Thats the only way we can improve. It acts an intermediate and an acid-base indicator in the pH range 3.0 to 4.6. An aqueous solution of sodium hydroxide, NaOH(aq), is a strong base. of the indicator will remain the same. Solutions of the dye, therefore, are blue. The equivalence point for the reaction is represented by the blue line at pH=8.7. Chris should use the phenolphthalein indicator. ChemLmy Dcpaztmem ofFuzhou UniversityFuzllou350002 2Chemisty Department ofXiamen UniversityXiamen361005 AbstaaetThe characteristics ofPVC membranes response pHweinvestigatKlThe pHsemifive dyebromophenol bluewimmobilized asion pair cetyllrimethylammoniumbromide plastlzedPVC membraneThe measurement range ofpH is4 . Determine the pH of the solution at the equivalence point: Use a table of indicator colour and pH range to choose an indicator which changes colour over a pH range that includes the equivalence point. Despite its name, bromothymol blue solution may sometimes appear . In general, we can apply the following generalisation for aqueous solutions at 25C based on the relative strength of the acid and strength of the base used in the titration: An appropriate acid-base indicator will change colour at the equivalence point of the titration.