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Titration with step motor burette Science Background CHEMISTRY Acids and bases What does it mean for a solution to be acidic or basic? An acid is a substance that donates hydrogen ions. Because of this, when an acid is dissolved in water, the balance between hydrogen ions and hydroxide ions is shifted. Now there are more hydrogen ions than hydroxide ions in the solution. This kind of solution is acidic. A base is a substance that accepts hydrogen ions. When a base is dissolved in water, the balance between hydrogen ions and hydroxide ions shifts the opposite way. Because the base "soaks up" hydrogen ions, the result is a solution with more hydroxide ions than hydrogen ions. This kind of solution is basic (alkaline). The pH scale The pH scale was first used by the Danish biochemist Sören Sörensen in 1909 to create a more efficient notation for the enormous range of the H 3O+ concentrations confronted within his experiments. He defined pH as the negative logarithm (with base 10) of the hydronium ion concentration and in that way reduced an inconvenient number to a simple number. Henceforth we will write log instead of log10 𝑝𝐻 = −log([𝐻3 𝑂+ ]) [H3O+] is expressed in powers of 10: [𝐻3 𝑂+ ] = 10−𝑝𝐻 In neutral solution: [H3O+] = [OH-] = 10-7mol/L In acidic solution: [H3O+] In basic solution: [H3O+] < [OH-] > [OH-] pH = pOH =7. (pure water at 25°C) pH < 7. pH>7. Conductivity The Conductivity sensor measures the ability to conduct electricity in water solutions. When salts and other inorganic chemicals dissolve in water, they break apart into electrically charged ions. Ions increase the water’s ability to conduct electrical current. Common ions in water that conduct electrical current include sodium, chloride, calcium, and magnesium. Organic compounds, such as sugars, oils, and alcohols, do not form ions. The principle by which the sensor measures conductivity is simple - two graphite plates (cells) are placed in the sample, a potential is applied across the plates and the current is measured. The Conductivity sensor actually measures the conductance of the solution (the inverse of the resistivity R), which is determined from the voltage and current values according to Ohm's law (G = 1/R = I/V). Titration with step motor burette – Science background 1 The specific cell constant (K) of the conductivity electrode is used to determine the conductivity (C). The conductivity is the cell conductance multiplied by the cell constant, C=G*K. The electrode separation distance divided by the electrode area determines the cell constant. The supplied electrode has a nominal cell constant K of 1.0 cm-1. The SI unit of conductance is Siemens (S). Since S is a very large unit, conductance of aqueous samples is commonly measured in S, and conductivity in S/cm. Some typical conductivity ranges of hydrous solutions are: Sample Conductivity (μS/cm) Pure water 0.055 Distilled water 0.5 Deionized water 0.1 – 10 Rain water 20 - 100 Drinking water 50 - 200 Tap water 100 - 1500 River water 250 - 800 Brackish water 1000 - 8000 KCI 0.01 M 1410 MgSO4 5810 KCI 0.1 M 12900 Ocean water 53000 H2S04 82600 KCI 1.0 M 112000 Titration Titration is an analytical method in which a standard solution with the known concentration is used to determine the concentration of another solution. During titration the standard solution (also known as titrant) is slowly added to the solution of unknown concentration by means of a burette. The endpoint of the reaction can be observed by the colour change when using an indicator (for example phenolphthalein) or detected by pH measurement. At the end point an amount of standard solution has been added that just completely reacts with the solution titrated. 2 CMA Learning and Teaching Resources The moles of standard solution can be calculated by multiplying the volume of standard solution used by its molarity. nstandard solution = Vstandard solution * cstandard solution The moles in the titrated solution of unknown concentration are then found using the coefficient in the chemical equation. Then, dividing the moles of the titrated solution by the volume of that solution gives the concentration of the titrated solution. ctitrated solution = ntitrated solution / Vtitrated solution. Step motor burette The step motor burette (titrator) is an actuator with which a liquid can be added to a solution automatically and relatively accurately. That way, a certain volume of a liquid can be added at constant speed, allowing for the creation of a graph in which you can plot pH versus volume added. This might also be achieved by using a drop counter and photo gate, but the step motor burette is much more convenient. More information about the function of the step motor burette, connecting it to an interface or tips about possible experiments can be found in the actuator’s manual. Conductivity titration using step motor burette – Science background 3