ASTM D7795 Standard Test Method for Acidity in Ethanol and Ethanol Blends by Titration
TEST METHOD A - POTENTIOMETRIC TITRATION
8. Apparatus
8.1 Potentiometric Titrator - Automatic titration systems capable of adding fixed increments of titrant at fixed time intervals (monotonic) or variable titrant increments with electrode stability between increment additions (dynamic) with endpoint seeking capabilities as prescribed in the method. At the very least, the automatic titration system shall meet the performance and specification requirements as warranted by the manufacturer.

8.2 A monotonic or dynamic mode of titrant addition shall be used. During the titration, the speed and volume of the addition may vary depending on the rate of change of the system. The recommended minimum volume increment is 0.05 mL for low acidity samples such as E30, and the recommended maximum volume increment is 0.1 mL. A signal drift of 10 mV/min and endpoint recognition set to last is recommended to ensure endpoint detection. When using a monotonic titrant addition the waiting time between increment additions shall be sufficient to allow for mixing and a stable electrode response. Wait at least 10 s between additions.

8.3 Buret, 5 mL capacity, capable of delivering titrant in 0.02 mL or larger increments. The buret tip shall deliver titrant directly into the titration vessel without exposure to the surrounding air. The buret used for base solutions shall have a guard tube containing carbon dioxide absorbent.

8.4 Titration Stand, suitable for supporting the electrode, stirrer and buret tip.

8.5 Sensing Electrode, standard pH, suitable for non-aqueous titrations.

8.6 Reference Electrode - Silver/Silver Chloride (Ag/AgCl) Reference Electrode, filled with 1M-3M LiCl in ethanol.

8.7 Combination pH Electrodes - Sensing electrodes may have the Ag/AgCl reference electrode built into the same electrode body, which offers the convenience of working with and maintaining only one electrode. A combination pH electrode designed for nonaqueous titrations of organic solvents is needed for titration of ethanol and ethanol blends. The combination pH electrode shall have a sleeve junction on the reference compartment and shall use an inert ethanol electrolyte, 1 to 3 mol/L (M) LiCl in ethanol. Combination pH electrodes shall have the same or better response than a dual electrode system. They shall have a movable sleeve for easy rinsing and addition of electrolyte.

8.8 Titration Beaker, borosilicate glass or plastic beaker of suitable size for the titration.

8.9 Sparging System, a gas delivery system suitable to deliver directly into the liquid sample, with an external pressure of 10 psi (69 kPa).

8.10 Variable-Speed Mechanical Stirrer, a suitable type, equipped with a propellertype stirring paddle. The rate of stirring shall be sufficient to produce vigorous agitation without spattering and without stirring air into the solution. A propeller with blades 6 mm in radius and set at a pitch of 30 to 45° is satisfactory. A magnetic stirrer and stirring bars is also satisfactory.
8.10.1 If an electrical stirring apparatus is used, it shall be electrically correct and grounded so that connecting or disconnecting the power to the motor will not produce a permanent change in the instrument reading during the course of the titration.

9. Reagents and Materials
9.1 Purity of Reagents - Reagent grade chemicals shall be used in all tests. Unless otherwise indicated, it is intended that all reagents shall conform to the specifications of the committee on Analytical Reagents of the American Chemical Society, where such specifications are available. Other grades may be used, provided it is pure enough to be used without lessening the accuracy of the determination.
9.1.1 Commercially-available solutions may be used in place of laboratory preparations provided the solutions have been certified as being equivalent.

9.1.2 Alternate volumes of the solutions may be prepared, provided the final solution concentration is equivalent.

9.2 Purity of Water - Unless otherwise indicated, reference to water shall be understood to mean reagent water that meets the requirements of either Type II or III of Specification D1193.
9.2.1 Prepare CO2 free water by sparging 1 L of water (9.2) with nitrogen (9.8) for no less than 3 min.

9.3 Potassium Acid Phthalate (KHC8H4O4), primary standard, dried - Place 4-5 g of primary standard potassium acid phthalate KHC8H4O4 of 100-mesh fineness, in a weighing bottle at 120°C for 2 h. Stopper the container and cool it in a desiccator.

9.4 Potassium hydrogen phthalate (KHP) Solution - Accurately weigh approximately 1.0 g of dried KHP and record the mass to the nearest +/- 0.0001 g and transfer it to a 500 mL Class A volumetric flask swept free of carbon dioxide. Add 200 mL of Type II DI water that is free of carbon dioxide, stopper and swirl gently until it is dissolved. Dilute to 500 mL and mix thoroughly. Express the concentration of KHP in solution as Molarity in moles of KHP per litre of solution. The use of a volumetric flask can be avoided by weighing 1.0 g of dried KHP to the nearest 0.0001 g into a beaker and adding 500 g of Type II carbon dioxide free DI water. Record the total mass of water and KHP to the nearest 60.01 g and express the concentration of KHP in the solution as mg KHP per gram of solution. Mix thoroughly to dissolve the KHP and store it in a closed container.

9.5 Sodium Hydroxide, Standard Solution (0.01 N) - Prepare and standardize a 0.01N sodium hydroxide (NaOH) solution in accordance with the Preparation and Standardization of Solutions, Precision and Bias, Preparation of 50 % of NaOH Solutions and standardizations section of Practice E200.
9.5.1 Alternatively, KOH (0.01 N) in isopropyl alcohol solution may be used instead of 0.01 N) NaOH.

9.6 Alcohols, refined, ethyl or isopropyl.

NOTE 1 - Isopropyl alcohol (99 % grade) conforming to Specification D770, or 190 proof ethyl alcohol conforming to formula No. 3A of the U.S, Bureau of Alcohol, Tobacco, and Firearms as defined in Title 27, Code of Federal Regulations (or equivalent regulations in other jurisdictions) is suitable for use as the solvent. The use of methyl alcohol is not recommended.

9.7 Lithium Chloride Electrolyte - Prepare a 1-3 mol/L (M) solution of lithium chloride (LiCl) in ethanol per the electrode manufacturer's recommendation.

9.8 Nitrogen, 99.9 % pure.

9.9 Commercial Aqueous pH 4 and pH 7 Buffer Solutions - These solutions shall be replaced at regular intervals consistent with their stability or when contamination is suspected. Information relating to their stability is provided by the manufacturer.

10. Preparation of Apparatus
10.1 Prepare the titrator in accordance with the manufacturer's instructions. Any visible air bubbles in the buret tip shall be eliminated prior to titration since this can lead to errors.

10.2 Preparation of Electrodes - When the combination pH electrode contains Ag/AgCl reference with an electrolyte which is not 1 to 3 mol/L (M) LiCl in ethanol, the electrolyte shall be replaced. Drain the electrolyte from the electrode (vacuum suction), wash away all the salt (ifpresent) with water and then rinse with ethanol. Rinse several times with LiCl electrolyte solution. Finally, replace the sleeve and fill the electrode with the LiCl electrolyte to the filling hole. When refitting the sleeve, ensure that there will be a free flow of electrolyte into the system.

10.3 Maintenance and Storage of Electrodes:
10.3.1 Follow the manufacturer's instructions for storage and use of the electrode

10.3.2 Prior to each titration soak the prepared electrode in water for at least 2 min. Rinse the electrode with deionized water immediately prior to use. The glass membrane needs to be rehydrated after titration of non-aqueous solutions.

10.3.3 When not in use, immerse the lower half of the combination electrode in LiCl electrolyte. Do not allow electrodes to remain immersed in a titrated sample solution for any longer than it is necessary. While the electrodes are not extremely fragile, handle them carefully at all times.

11. Calibration and Standardization
11.1 Calibration of Electrode:
11.1.1 Select the correct electrode for the analysis (see 10.2).

11.1.2 Verify that the electrode is filled with 1 to 3 mol/L (M) LiCl in ethanol solution (see 10.2).

11.1.3 Prepare the two buffer solutions, pH 7.0 and pH 4.0 by placing approximately 50 mL of each solution in individual 125-mL disposable beakers.

11.1.4 Calibrate the electrode using the two buffer solutions according to the manufacturer's instructions. Immerse the electrode in each buffer solution, adjust the stirring speed so that adequate mixing occurs without forming a vortex and wait for the instrument reading. When the reading is complete, rinse the electrode in high purity water, wipe gently and repeat the measurements with the other buffer solution. Record the pH value with an accuracy of 0.01 and the temperature with an accuracy of 0.1°C. The measured pH values should be within +/- 0.05 pH units of the buffer's certified value.
11.1.4.1 Verify that the calibration slope is between 0.95 and 1.02. An ideal pH glass electrode has a slope of 1.00 (100 % of the Nernst slope) and an electrode zero point of 0 mV for pH 7 at 25°C. In practice, the electrode zero point potential shall be within +/- 15 mV (corresponding to pH 6.75 to 7.25) and the slope shall be >0.95 (>56.2 mV per pH at 25°C). The electrode zero point and the electrode slope may change as a result of the aging of the glass membrane or contamination of the diaphragm. If the electrode slope falls below 0.95 follow the electrode manufacturer's instructions for electrode maintenance or replace the electrode. The pH electrode shall be calibrated at regular intervals using fresh buffer solutions.

11.1.5 The slope is automatically stored in the titrator.

11.1.6 The slope is not used for sample analysis, but rather, it provides information on the responsiveness of the electrode. An electrode not meeting the stated criteria is not suitable to perform this method.

11.2 Standardization of the 0.01 mol/L (M) NaOH Titrant:
11.2.1 Weigh 2 g of the KHP solution and record the mass to the nearest 0.0001 g (or pipet 2 mL of KHP solution using a Class Apipette) into a beaker and add 50 +/- 5 mL of CO2 free water. Place vessel on the magnetic stirrer and titrate the KHP standard with the 0.01 mol/L (M) NaOH. Record the volume of titrant used to neutralize the KHP at the inflection point. This volume of KHP solution may use approximately 2 mL of the 0.01 M NaOH.

11.2.2 Prepare two additional KHP solutions and standardize the titrant as in 11.2.1.

11.2.3 Use the three determinations to calculate the average concentration (molarity) of the NaOH. The average of the titrant molarity determinations shall agree within +/- 0.0005 mol/L (M). Use this average to calculate the acidity of the titrant (see 17.1.3).

12. Procedure - Potentiometric Titration
12.1 Weigh 60 +/- 5 g to the nearest 0.001 g of ethanol or ethanol blend into a titration beaker and record the mass.

12.2 Prepare the titrator in accordance with the manufacturer's instructions. Immerse the electrode, sparging tip, and buret tip into the sample taking care such that the reference junction is immersed in the sample. Adjust the stirring speed so that adequate mixing occurs without forming a vortex.

12.3 Sparge the sample with nitrogen at a flow rate of 200 +/- 20 mL/min for 60 s prior to titration, and continue to sparge throughout the entire analysis.

12.4 Record the volume of titrant used to reach the last well defined end point (inflection point).