How do you determine the titrant concentration

USER COM. Titering of custom solutions DL-TIP. Spring Dear Readers. Introduction. Table of Contents


1 USER CM Information for users of METTLER TLED titration and ph systems Dear Readers, With this new customer magazine we would like to offer you an even better service. In the DL-Tip section, we make our practical experience available to you. In addition, you will find information about new products and their application, as well as applications from all areas of titration and electrochemistry. Titering of measurement solutions Spring DL-TIP Introduction The history of measurement analysis goes back to Lavoisier and his contemporaries in the 18th century. At that time, the first titrations were carried out, initially with solid reagents (e.g. potash), but soon with so-called standard solutions of known concentration. We hope you enjoy reading! Table of contents DL-TIP - titering of custom-made solutions How is it done? How often? What has to be considered? Accessories - Wide range for titration An overview New in the sales program - MPC227 conductivity meter Especially for conductivity titration Applications - Titration in the mineral oil industry Simply and precisely determine quality-relevant criteria - New vitamin C determination Revolutionary amperometric indication A titration determines how much standard solution is required to convert the entire amount of substance in the sample. The analyte concentration can be calculated based on the consumption in ml, the titrant concentration in mol / l and the molar mass M of the substance to be determined. It should therefore be noted that the titration is not an absolute method, because the result is decisively influenced by the titrant concentration. The effective titrant concentration usually deviates from the target concentration, be it because of inaccurate production or because of changes due to chemical resistance. How is the titer determined? The determination of the effective concentration, generally also called titer, is carried out by titrating a substance of precisely known composition. If possible, primary substances are used for this purpose. These are chemicals that meet the following conditions [1, 2, 3, 4]: Clearly defined composition and high purity Large molar mass (to avoid weighing errors) Easy to dry without difficulty weighing (no reaction with oxygen or C 2, not hygroscopic) Quickly and easily soluble in the solvent used. Fast and stoichiometric reaction with the titrant. USER CM February 98 1

2 In practice there are of course very few ideal substances that meet all the conditions. For example, sodium carbonate is hygroscopic and must be dried again before use [2]. In cases in which no suitable primary substance can be found, a so-called secondary standard can be used, i.e. a substance whose exact composition is known by comparison with a primary substance [4]. How often should the titer be determined? The titer must be determined for each newly prepared titrant. This also applies to commercially available reagents with guaranteed accuracy, as not only concentration errors but also burette inaccuracies are compensated for in the titer factor. (When using aggressive reagents, e.g. sodium hydroxide solution, the buret diameter can change.) Depending on the stability of the titrant used, the titer determination should be repeated more or less often. Table 1 gives some examples of commonly used titrants. In this context it is interesting that in the new DL50 titrators (see Figure 1) from METTLER TLED the titer determination interval can be programmed: The device informs the user when a determination has to be carried out. This of course leads to increased accuracy of the result. Methodology The METTLER TLED application brochures 8, 9 and 18 contain methodology, results and tips for determining the titer of the most common titrants from acid and alkalimetry, redox titration, complexometry, turbidimetry and precipitation titration. A selection of titer determination methods is stored in each device so that the most common determinations can be started at the push of a button. Titrant Standard substance Interval Remarks Sodium hydroxide Potassium hydro- weekly Protect against penetration of NaH genphthalate from C 2 sulfuric acid Tris- (hydroxymethyl) - every 14 days H 2 S 4 aminomethane dichlorophenol- ascorbic acid daily Store cool and dark indophenol (vitamin C) Karl Fischer reagent sodium tartrate - Protect daily from moisture Composite 5 dihydrate Table 1: Titrant base material Figure 1: METTLER TLED DL55 titrator Importance It must be emphasized that the titer determination must be carried out with particular care. [3] Each error has a systematic effect on all determinations that are carried out with the respective measurement solution. In the case of titer determinations, multiple determinations should always be carried out and statistically evaluated. The use of a mathematical test to find outliers (e.g. Grubbs) is recommended [1]. Professional devices such as the DL70 titrators from METTLER TLED enable this test to be carried out automatically after each series. Titre determinations are also very suitable as system suitability tests and thus provide quick information on whether the system used is still suitable for titration [5]. Literature [1] METTLER TLED, Basics of Titration, 1993 [2] Vogel, Textbook of Quantitative Inorganic Analysis, 4th edition, 1978 [3] Jander Jahr, Massanalyse, 14th edition, de Gruyter, 1986 [4] E. Merck , Primary Volumetric Standards [5] METTLER TLED Application Brochure 15, Result Control, Method Validation and Device Certification 2 USER CM February 98

3 MPC227 The new combined pH / conductivity measuring device Several in one go! Even the tailor in the fairy tale was proud when he succeeded in "killing flies". METTLER TLED is breaking new ground with the introduction of the first combined pH / conductivity meter MPC227. Three different conductivity sensors are available, so that measurements can always be made with a suitable sensor in the ms range and even aggressive media can be "dealt with" without any problems. A sensor is also particularly suitable for conductometric titration. Many customers need to measure both the pH and conductivity of a sample. For example, in the water and wastewater industry, both pH and conductivity are important indicators for water quality. Where previously separate devices were required, the MPC227 works for two, which of course lowers investment costs and saves valuable laboratory space. The two independent measuring channels can be easily and conveniently calibrated with ph or conductivity standards, with the device accepting any sequence of calibration solutions thanks to the automatic buffer recognition. Three-point calibration for the ph channel ensures high results accuracy. One of the greatest challenges in the chemistry laboratory today is quality assurance and the tasks associated with fulfilling legal requirements. Gapless documentation is just as important as precise adherence to the standard operating instructions. The MPC227 reminds the user to calibrate after a certain period of time, saves the results of the last calibration, automatically displays the electrode quality and documents the measurement results seamlessly on a matrix printer. GLP requirements are thus easily met. Combination titrator / MPC227 A connection from the conductivity meter to the titrator is ideal for two extremely interesting applications: Automated direct measurement of conductivity in connection with other analyzes, e.g. in the water industry (calcium, chloride, alkalinity, fluoride) Conductometric indication for titrations as an alternative to potentiometry Thanks to the calibrated and switchable analog output, the MPC227 is very suitable in connection with DL50 and DL70 titrators from METTLER TLED. A special, logarithmic data transmission ensures a consistently high level of accuracy over the entire measuring range. With the MPC227 the user acquires a quality device for the problem-free measurement of pH values ​​and conductivity. The combined device saves costs and space, particularly when one parameter is frequently measured and the other parameter is occasionally measured. Titration users and water analysts will appreciate the combination of the two devices as an interesting new addition to the METTLER TLED range. The MPC227 is available from the middle of the combination of titrator DL55 / MPC227 ph / conductivity meter / printer GA42 USER CM February 98 3

4 High-performance analysis thanks to the right accessories A wide range of accessories is now available for METTLER TLED titrators. Depending on the application, suitable accessories can improve the accuracy of the results, reduce the workload or open up new applications. Automation Preparing and adding samples or processing different determinations one after the other unattended: Needs that can be satisfied with the ST20A sample changer. Up to 20 samples can be analyzed one after the other; The titration stand is rinsed and the electrode is conditioned between the samples. Cleaning In order to avoid carry-over of the sample, the titration stand should be carefully cleaned after each analysis. The use of a flushing unit makes this easier. Simply press it on and the titration stand is clean and ready for the next determination. Application brochures Method development can be time-consuming. The choice of suitable conditions, processes and calculations often requires lengthy trials. METTLER TLED provides knowledge and experience in the form of detailed application brochures. Here you will find methodology, results, curves but also tips and tricks, for example for sample preparation or disposal. Sensors Karl Fischer accessories beaker The titration electrodes from METTLER TLED prove themselves in tough routine use. The service life of the electrode depends heavily on the right choice. The ground-joint diaphragm models in particular are characterized by their great robustness and fast response time. A special feature are the phototrodes, actual photometers, which measure the light absorption in the sample and thus enable automatic color change titrations. The Karl Fischer booth DV705 converts a potentiometric titrator into a device for determining water. Drying ovens enable Karl Fischer titration of samples that are difficult to access. Various beakers (glass, plastic, 100 ml, 250 ml, thermostatically controlled) are available as well as suitable stoppers, weighing boats, drying tubes, burettes, cables and printers. Overview of the sensors Sensor type Diaphragm Application DG101-SC ph ceramic micro applications, DG111-SC ph ceramic standard acid / base DG113-SC ph ground, movable Non-aqueous titrations, TAN / TBN DG114-SC ph ground, movable Cloudy, dirty samples, proteins DG115-SC ph ground joint, fixed Low ionic strength, fast reaction DM140-SC Pt ceramic redox titrations (Cer, KMn 4, I 2) DM141-SC Ag ceramic precipitation titrations (AgN 3) DM142 Pt voltametry (KF, VitC) DP550 Photo photometric titration (Ca / Mg, surfactants) DP660 Photo Photometric Titration (Al, Cu) 4 USER CM February 98

5 Titration in the mineral oil industry Crude oil is the source of 40% of the energy produced worldwide and thus one of the most important commodities in the world. The quality control of the crude oil is essential and is regulated by various regulations such as ASTM, IP or BSI. METTLER TLED developed methods for the titrator line of the DL50 family in order to determine the most important quality-relevant criteria simply and precisely. In the first half of 1996, barrels of crude oil were produced every day around the world; in the first half of 1997 this number rose to This corresponds to an average growth of 3.6%. The USA, Saudi Arabia and the countries of the former Soviet Union together produce around 42% of all crude oil. Countries such as Venezuela, Nigeria, Algeria and China increased their production volumes above average compared to the first half of 1996 and the first half of 1997 (see Fig. 1). The oil is partly processed in the country of origin itself, and partly exported abroad. Refineries around the world process crude oil into gasoline, diesel fuel, motor oil, heating oil, kerosene and other petroleum products. The waste from this production is mainly used as asphalt. From the listed economic data it can be seen that a quality check of the crude oil is necessary. In order to simplify quality control, various organizations have published analytical regulations, including the ASTM (American Society for Testing and Materials) is probably the best-known source of standards in the petroleum industry. The ASTM was founded in 1898 with the intention of creating a uniform basis for the control and assessment of the quality of mineral oil products. Today, exactly 100 years later, this is still the goal of the ASTM regulations. The environmental aspects are also becoming increasingly important: with the help of the SBN and SAN standards, for example, the quality of engine oil is controlled. Unnecessary oil changes and the associated pollution can be avoided in this way. Among the various ASTM standards for the mineral oil industry, the most frequently performed determinations are the SAN and AN (strong acid number and acid number), the SBN and BN (strong base number and base number) and the bromine number. Country Growth USA, 0% Saudi Arabia, 9% Former USSR, 5% Iran, 1% Mexico, 8% Norway, 7% China, 4% Venezuela, 1% Canada, 7% England, 8% Nigeria, 6% Emirates (UAE), 5% Kuwait, 9% Indonesia, 5th % Libia, 5% Algeria, 6% world, 6% Figure 1: Crude oil production in 1000 barrels per day, first half year [1] These relatively complex investigations are all carried out with the help of potentiometric titration. METTLER TLED has developed methods for the titrators of the DL50 family that enable the above-mentioned criteria to be determined easily and reliably. In addition to the methods that correspond in every detail to the standard regulations ASTM D (SAN and AN), D and D (SBN and BN) and ASTM (bromine number), some additional methods have been developed: Rapid methods for the determination of SAN / AN and SBN / BN, methods for determining the bromine index, the mercaptan content in oil products and the chloride in crude oil. All of these methods are published in application brochure no. 20 «Regulations in the mineral oil industry». USER CM February 98 5

6 What is the purpose of determining the acid and base numbers? The SBN is a measure for the content of strong bases in mineral oil products. By definition, a strong base is a basic component whose dissociation constant is at least 1000 times higher than that of the next weaker group. Basic components are mainly organic and inorganic bases, including amino components, but also salts of weak acids, basic salts of polyacids and some additives such as inhibitors and detergents. Strong acids are defined according to strong bases. Acid components in mineral oil products are mostly organic and inorganic acids, esters, phenols, lactones, resins, ammonium salts and others. Both the acid and the base number provide information about the mv changing state of an oil under oxidizing conditions. Relative changes in the oil are determined independently of the color or other properties. In this way, the point in time for a necessary oil change can be determined. The bromine number The bromine number is the calculated number of grams of bromine that reacts with 100 g of sample - under the conditions of the standard ASTM D method. The level of the bromine number stands for the number of bromine-active components in the oil, mainly double bonds. Rapid methods and other methods The rapid methods can be used for routine examinations. They allow a rough estimate of the true value in about a quarter of the time that is required for the ASTM method. The determination of the mercaptans corresponds to ASTM regulation D 3227 and is also important for the assessment of the oil quality. The chloride content of the crude oil is an important criterion with regard to corrosion processes in the oil pipeline. Summary Most ASTM regulations require a few pre-titrations before the actual titration is started - this is an ideal task for the automatic METTLER TLED titrators, for which the corresponding methods have already been developed and published in application brochure no. 20 «Regulations in the mineral oil industry» were. With the help of these methods and the METTLER TLED titrators, a simple and exact determination of the quality-relevant criteria of mineral oil products is possible without special specialist knowledge. Sample 200 sample blank value A HCl SAN buffer value A 0 LM AN 200 KH blank value B titration curve acid number AN AN AN buffer value B ml Literature [1] Petroleum Economist, Volume 64 Number 10, London 1997 [2] Applications Brochure 20, Determinations in the Petroleum Industry , METTLER TLED 6 USER CM February 98

7 New method for determining vitamin C Ascorbic acid (vitamin C) is vital for humans. Vitamin C deficiency leads to scurvy, a condition that was once common among seafarers. Ascorbic acid is said to protect against viral infections and even cancer [1]. Although these effects have not been proven, the vitamin C concentration in foods and beverages is often reported as an ingredient and must therefore be analyzed quantitatively. How can the vitamin C content be determined? A wide variety of methods for determining vitamin C are mentioned in the literature, including redox titrations in which iodine or dichlorophenolindophenol (DPI) is used as the titrant. Titration is an accurate, quick and easy analytical technique and DPI is an ideal oxidant because of its selectivity for ascorbic acid. Ascorbic acid is easily oxidized. The endpoint indication can be potentiometric, photometric, voltametric or - as described here - amperometric. H H H H 2e Fig. 1: Oxidation of ascorbic acid Voltametric or amperometric titration? With each titration, the consumption of titrant up to the complete reaction with the analyte is determined. The exact detection of the equivalence point (same amount of titrant and analyte in the solution) is the prerequisite for an exact determination of the content. H H In voltammetry, a constant current I is applied to a double platinum pin electrode and the change in potential U is measured over the course of the titration. During the titration of ascorbic acid with DPI, the potential remains constant until all of the ascorbic acid has been oxidized. The subsequent oxidation of the excess DPI on the electrode creates a large potential jump, which can be evaluated with the help of the Tubbs [2] or the standard method [3]. [mv] [ml] Fig. 2: Example of a voltametric titration curve [4] An alternative is the amperometric method. The potential is kept constant and the change in current is measured. According to Ref. [5], the potential must be chosen so that it is close to the half-wave potential π 1/2. The half-wave potential shows how easily a substance can be oxidized or reduced and depends on the acidity of the solution as follows: π 1/2 (25 C) = 0.059 lg [h +], 92 [H +] where 10 5.92 is the first dissociation constant of ascorbic acid. In the course of the titration, the current remains constant during the reaction of ascorbic acid with DPI. Starting at the equivalence point, the current increases proportionally to the excess of DPI. The titration curves of amperometric titrations show a segmented course, that is, two straight lines intersect at the equivalence point. METTLER TLED has developed an algorithm for evaluating segmented curves. [µa] Fig. 3: Example of an amperometric titration [4] [ml] Device requirements METTLER TLED titrators have a special method concept that allows individual work steps to be freely combined within a method. Starting with the sample preparation and dilution over the actual titration and calculation up to the documentation, the method is put together; it thus allows the analysis to be fully automated and frees the user from routine work. Comparison between the amperometric and voltametric method The amperometric titration has two major advantages over the voltametric method: - The amperometric titration is faster because the equilibrium is established more quickly after adding an increment. - The amperometric indication is more sensitive, so that vitamin C can also be determined in lower concentrations. Continued on page 8 USER CM February 98 7

8 The table opposite shows the results of several sample series in which the vitamin C concentration in various fruit juices was determined using amperometric titration with segmented evaluation. The low relative standard deviation (RSD) with small sample volumes and short analysis time (2-3 minutes) proves the precision of the method. Samples Origin Samples- Vitamin C RSD n amount Content [%] [g] [mg / 100g] Kiwi fruit freshly squeezed, 79 0.979 4 orange juice bottle, 16 0.501 3 apricot juice bottle, 51 0.477 4 lemon juice freshly squeezed, 00 0.339 6 Schwarzer Bottle 10 53.06 0.044 3 currant juice Table 1: Example of various vitamin C determinations [4] Events, conferences and courses Forum Laboratoire March 31, April 3, 1998 Paris Analytica April 1998 Munich InCom March 1998 Düsseldorf 3 rd Intern. Symp. On Humidity & Moisture April 1998 London Customer titration courses and seminars (CH) Information and registration with: Yvonne de Buhr Tel Fax DL Course 45, Oil Analysis (English) November 1998 Greifensee, Switzerland Customer titration courses and seminars (Germany) User seminar DL5x and DL7x March 31, April 1, 1998 Giessen KF / Feuchteseminar (InCom) March 26, 1998 Düsseldorf For further information please contact Mettler-Toledo GmbH in Giessen Tel. If you have any questions about other conferences, the products or applications, please contact please contact your local METTLER TLED representative. Editorial office Mettler-Toledo GmbH, Analytical Sonnenbergstrasse 74 CH-8603 Schwerzenbach, Switzerland Tel Fax Internet: U. Bauer, J. Maag, V. Schindler, Dr. T. Fritz, Ch. Walter Layout and production, market support AnaChem Schwerzenbach, V. Mahler ME