Food Potentiometric Analysis Collection Food PAC 6.003 Methods for the Titrimetric/Potentiometric Analysis of Foodstuffs Dear User, You have decided to purchase a Metrohm Titrator which, with its special collection of methods for your own particular applications, is intended to meet all your requirements. Metrohm always attempts to provide customers with as wide a range of application sup- port as possible in order to make daily work easier. In this Application File you will find descriptions of the analytical methods together with the necessary comments and explanations and – specially for you – printouts of the instru- ment parameters and examples of curves. All these methods are loaded on the method memory card.
All that you need to do is to «feed» your titrator with the card, load the required method into the working memory and off you go!!! For Titrando users: a conversion program ensures that you can use Titrino parameters in the Titrando without any problems. This conversion program is contained in the 6.XXX PC Control program. We wish you lots of pleasure and success in your work, Your Metrohm 8.1911 1 Some additional information – The methods described here have been drawn up taking the current state of knowledge into account. – All the methods are formulated so that you can use them as SOPs (Standard Operating Procedures) in your laboratory.
– Many of the methods described here can be automated even further; see the proposal given in the annex. For details please consult your local Metrohm distributor, which can be found on the Internet under: www.com ⇒ Distributors – The method memory card supplied can be used with the 798, 799, 785 and 751 Titrinos (from program version 20). With the Metrodata VESUV Light 3. – Among other things, the supplied CDs contain: • The VESUV backup file, which allows you to copy the 96 methods into 716, 736, 751, 785, 794, 798 and 799 Titrinos.
For further information please consult the section «Restoring methods» in the Instructions for Use supplied or contact your local Metrohm distributor. If the VESUV software is used for data acquisition instead of the printer then the report «curve» must be deleted at the Titrino, which is set instead to «mplist» (VESUV can only process measuring point lists). • A conversion program for taking over Titrino parameters into the Titrando. This conversion program is contained in the 6.XXX PC Control program.
• Acrobat® Reader® for installation on your PC so that you can read PDF files. • Application Bulletins nos. – We recommend that you only load the methods that you require in your instrument. – The method overview can be removed and kept near your instrument together with the Food PAC card.
Important symbols used in the methods c(X) molar concentration of substance X in mol/L M(X) molar mass of substance X in g/mol w(X) mass fraction of substance X, e. w(NaCl) = 10% σ(X) volume fraction of substance X, e.) ρ ((X) mass concentration of substance X, e. Environmental Protection Agency (EPA) Remarks The determination of the water content of foodstuffs and semi-luxuries is not described in Food PAC. Please consult the Metrohm monograph «Water determination by Karl Fischer titration», which can be obtained free of charge from your local Metrohm distributor.
2 Contents A pH value H Soft drinks, lemonades A 1 Calibrating the pH glass electrode H 1 Citric acid, citrates A 2 Measuring the pH value H 2 Phosphoric acid (cola drinks) H 3 Potassium B Preparing the most important titrants and H 4 Total phosphorus determining their titer J Fruit and vegetable juices, fruit nectars and B 1 Alkaline titrants (NaOH, KOH) B 2 Acidic titrants (HCl, H2SO4) jams B 3 Iodine solution J 1 pH value and titratable total acidity B 4 Thiosulfate J 2 Ascorbic acid (vitamin C) B 5 Permanganate J 3 Sulfurous acid (sulfite) B 6 Silver nitrate J 4 Chloride B 7 Na2EDTA J 5 Total phosphorus J 6 Sulfate C Drinking water and mineral water J 7 Calcium and magnesium C 1 pH value and acid capacity (carbonate hard- J 8 Potassium ness) J 9 Ash alkalinity C 2 Calcium and magnesium (Ca hardness, Mg J 10 Kjeldahl nitrogen, total protein hardness and total hardness) J 11 Formol number C 3 Chloride J 12 Reducing sugars in jams C 4 Sulfate C 5 Sulfides and hydrogen sulfide K Beer, vinegar, spirits and wine C 6 Total and residual chlorine (free chlorine) K 1 Beer, pH value C 7 Permanganate index (oxidizability) K 2 Beer, CO2 content C 8 CO2 content of carbonated water K 3 Beer, ascorbic acid (vitamin C) C 9 Oxygen content according to Winkler K 4 Beer, total sulfurous acid K 5 Vinegar, pH value and total acidity D Milk and dairy products K 6 Vinegar, volatile acidity D 1 pH value K 7 Vinegar, ascorbic acid (vitamin C) D 2 Titratable acidity, degree of acidity K 8 Vinegar, free sulfurous acid D 3 Chloride K 9 Vinegar, total sulfurous acid D 4 Calcium K 10 Vinegar, ash alkalinity D 5 Vitamin C (ascorbic acid) K 11 Vinegar, chloride D 6 Kjeldahl nitrogen K 12 Vinegar, sulfate K 13 Spirits, pH value and total acidity E Edible fats and oils K 14 Spirits, volatile acidity K 15 Spirits, total esters E 1 Acid number and free fatty acids (FFA) E 2 Hydroxyl number L Coffee, cocoa and chocolate E 3 Iodine number E 4 Peroxide number L 1 pH value and degree of acidity (green and E 5 Saponification number roasted coffee, coffee extracts) L 2 Ash alkalinity (green and roasted coffee, cof- F Cereals, flour milling products, dry pasta fee extracts) products L 3 Chloride (green and roasted coffee) L 4 Directly reducing sugars (roasted coffee, F 1 pH value and degree of acidity coffee extracts) F 2 Chloride and cooking salt content L 5 pH value (cocoa and chocolate powder) F 3 Kjeldahl nitrogen, total protein L 6 Ash alkalinity (cocoa and chocolate pow- F 4 Calcium and magnesium (in ash) der) L 7 Kjeldahl nitrogen (cocoa and chocolate G Honey, sugar and sweets powder) G 1 pH value and free acids L 8 Free fatty acids in cocoa butter G 2 Formol number L 9 Iodine number of cocoa butter G 3 Reducing sugars L 10 Saponification number of cocoa butter 3 Contents M Artificial sweeteners, gelling and thickening agents M 1 Methoxy and ethoxy groups in gelling and thickening agents M 2 Cyclamate in artificial sweeteners M 3 Saccharin in artificial sweeteners N Preserved fruits, vegetables and mushrooms N 1 Oxalic acid in fruits and vegetables N 2 Total sulfurous acid in dried fruits and veg- etables N 3 Cooking salt content of mushroom extracts and concentrates O Cooking salt, spices, pickling salt, seasoning, herbal and flavored salts O 1 Chloride content O 2 Total iodine in cooking salt O 3 Fluoride in cooking salt O 4 Tricalcium phosphate in cooking salt O 5 Nitrite in pickling salt P Meat products, meat extracts, bouillon preparations, aspic, seasonings, soups, sauces P 1 Chloride (NaCl) in meat products P 2 Kjeldahl nitrogen and raw protein in meat products P 3 Sulfurous acid in meat products P 4 Chloride (NaCl) in meat extracts, aspic and bouillon P 5 Kjeldahl nitrogen in meat extracts, aspic and bouillon P 6 Chloride (NaCl) in seasonings, soups, sauc- es P 7 Kjeldahl nitrogen in seasonings, soups, sauces Q Qualifying the titrator, validating a titration method Index 4 Method 1 – A 1 Calibrating the pH glass electrode Recommended • Comb. pH glass electrode with built-in Pt 1000 temperature sensor, e.000 Unitrode Reagents • Buffer solutions pH = 4.120 General Free protons (H+ ions) occur in solutions just as little as free electrons. They combine with water to form oxonium ions: H+ + H2O → H3O+ The pH value is defined as the negative logarithm of the oxonium ion activity, i. of the concentration of free, dissociated oxonium ions in mol/L: pH = –log [H3O+] Strictly speaking, the term pH only applies to purely aqueous solutions.
The pH scale ranges from 0 to 14 with the neutral point at pH = 7.0, where the H3O+ and OH – ions are present in equilibrium. pH values below 7 result from an H3O+ excess, pH values above 7 from an OH – ion excess. The more acidic a A pH value solution the higher its H3O+ ion concentration and the lower its pH value. tartaric acid, do not dissociate completely.
This means that only a small fraction (approx.3%) of their acid ions are released. This also means that only on very rare occasions can the pH value be used as a measure of the concentration of acids or bases. As the pH scale is logarithmic this means that small differences in pH corre- spond to large differences in the concentration of H3O+ ions. For example, at pH = 3.0 there are ten times more H3O+ ions present than at pH = 4.0, and at pH = 3.1 there are twice as many H3O+ ions present than at pH = 3.
The measuring setup for potentiometric measurements always consists of two electrodes – a measuring or indicator electrode and a reference electrode. For practical reasons these two electrodes are usually contained in a single com- bined electrode. The indicator electrode (in this case the pH glass electrode) produces a poten- tial that is dependent on the composition of the sample solution. The reference electrode (usually Ag/AgCl) has the task of providing a potential that is as independent as possible of the sample solution (reference potential).
The potential measurement itself takes place virtually current-free by using a «voltmeter» (in this case a Titrino) with a high-impedance measuring input (this is necessary to avoid unwanted potential drops). The measured potential U is made up from the individual potentials produced by the indicator and reference electrodes. The following illustration shows a schematic diagram with a sepa- rate pH glass electrode (left) and a reference electrode (right): U1: Galvani potential between measuring electrode and measuring solution U2: Galvani potential between internal buffer and glass membrane U3: Galvani potential between internal reference electrode and internal buffer U4: Galvani potential of reference electrode U5: Diffusion potential at the diaphragm 1 Method 1 – A 1 Calibrating the pH glass electrode The individual potentials U2, U3 and U4 are determined by the construction of the electrodes and are therefore constant for a given electrode pair. The diffu- sion potential U5 should be kept relatively constant and low by taking suitable measures.
These measures include an optimal and clean diaphragm, constant stirrer speed during the measurements as well as a suitable reference electro- lyte solution whose anions and cations have similar ionic mobilities – e. In this way the potential U1 measured between the electrodes depends only on the sample solution. In the pH measurement this potential is again dependent on the activity ai of the measuring ion (H3O+ ion / OH – ion). This relationship is described by the Nernst equation: U = U0 + * log ai = U0 + UN * log a where: U measured difference in potential between indicator and reference electrode U0 standard potential of the combined electrode (depends on its construction) R gas constant (8.31441 J / (K mol) T absolute temperature in K (273.15 + t / °C) zi charge on the measuring ion i including its sign (+1 for H3O+ and –1 for OH–) ai activity of measuring ion UN Nernst slope (59.16 mV at 25 °C and z = 1) 2.303 conversion factor from natural to common logarithm The Nernst slope UN describes the theoretical electrode slope and corresponds to the change in potential produced by altering ai by a factor of ten.
It depends on the temperature and charge z of the measuring ion. Please note: The instru- ment compensates the effect of temperature on UN but not on the pH value of the solution!