TITLE: Determination of the total acid number (TAN) in crude oils
AUTHORS: Jonas Sundberg

[DESCRIPTION]
<div class = "text-blocks"><div class = "text-block"><span style = "font-weight:bold;">Description</span></div><div class = "text-block">Titrimetric determination of total acid number (TAN) in crude oil. Protocol is applicable to crude oils of medium acidity (approximately 0.5 mg KOH/g). Sample and reagent volumes should be adjusted if the TAN value is expected to be outside of this range. </div><div class = "text-block"><span style = "font-weight:bold;">Instrumentation</span></div><div class = "text-block">The analysis is carried out using a slightly modified version of ASTM 664 using a SI Analytics TitroLine 7750 (see attached documents for details of the original procedure). </div><div class = "text-block"><span style = "font-weight:bold;">Reporting</span></div><div class = "text-block">TAN values are reported in milligram of potassium hydroxide per gram of sample (mg KOH/g).</div><div class = "text-block"><span style = "font-weight:bold;">Quality control (QC) / system suitability test (SST)</span></div><div class = "text-block"><span>A synthetic oil containing a mixture of C</span><span style = "vertical-align:super;vertical-align:sub;">13</span><span>, C</span><span style = "vertical-align:sub;">15</span><span> and C</span><span style = "vertical-align:sub;">17</span><span> linear fatty acids (in 80:20 hexadecane:toluene) corresponding to to 0.5 mg KOH/g is prepared and analyzed with each batch (maximum 6 oils). The value should be within 10% of the true value.</span></div><div style = "text-align :; float : ;"><img style = "" src = "https://s3.amazonaws.com/pr-journal/bci7a.png" /></div></div>

[STEPS]
?. [Setup]
Assemble the titrator inside a ventilated fume-hood as samples are stored in open containers. Keyboard is optional, but highly recommended for entering of sample details. To store results as a PDF document, insert a USB memory stick (without encryption) into a USB-port in the back of the titrator. To avoid data loss, record results manually on paper. To ensure proper mixing, stirring should be used during all steps of the analysis. The stirrer bar should not touch the electrode, and spin at a low speed while stimulating proper mixing.
?. [Instrument initiation]
Rinse titrant bottle with MilliQ H2O followed by 2-propanol and fill with suitable amount of titrant solution.NoteApproximately 50 mL is used for initiation followed by 5 - 6 mL per analysis.
?. [Instrument initiation]
Rinse solvent syringe, tubing and automatic pipette with titrant solution;Click methodSelect 'Rinsing'Perform 3 rinses in total.
?. [Sample preparation and analysis]
Weigh approximately 20 g (corresponding to 25 mL, adjust if acid content deviates from XXX) of crude oil and record the exact weight (two decimals). ¨¨Dissolve sample in 125 mL of titration solution (500:495:5 toluene:2-propanol:H2O).Click 'Mode'Select 'TAN ASTM 664'.Click 'Start'Enter measured mass. Record results presented on screen.Discard sample, rinse electrode with titrant solution, water and titrant solution again.
?. [Sample preparation and analysis]
Dissolve the sample in titration solvent XXX, Y mL)
?. [Titrant calibration, blanking and QC]
Calibrate the titrant solution against freshly prepared aqeous potassium hydrogen phtalate (0.2 g per 80 mL);Fill a 25 mL beaker with 20 mL of KHPT solution.Click 'Mode'.Select 'Titer KOH'. Click 'Start' and enter the measured mass of KHPT.Discard the solution, and use a new clean beaker for next calibration.   Repeat the calibration three times. The average value is automatically stored in the unit.  NoteThe calculated concentration should be approximate 0.1 M. If the value deviates heavily, something is wrong either with titrant, KHPT or instrument.
?. [Instrument initiation]
As titration will be carried out using organic solvents in open containers, instrument must be placed inside a ventilated fume hood. To allow editing of sample names, connect the associated keyboard. If you want to store results (high recommended), connect a USB memory stick to one of the backside USB ports. Results will be stored as PDF in a folder named "result". Connect the power cable and verify that system starts.
?. [Titrant calibration, blanking and QC]
Blank the instrument against titration solvent;Add approximately 125 mL of titration solvent to a 250 mL beaker with a stirrer. Click 'Mode'Select 'Blank TAN-TBN'Repeat blanking once.
?. [Titrant calibration, blanking and QC]
Verify instrument performance against QC sample;Weigh approximately 20 g (corresponding to 25 mL, adjust if acid content deviates from XXX) of crude oil in a 250 mL beaker. Record the exact weight (to two decimals).Dissolve sample in 125 mL of titration solution (500:495:5 toluene:2-propanol:H2O).Click 'Mode'Choose 'TAN ASTM 664'Click ... and enter measured sample mass.Discard sample, rinse electrode with titrant solution, water and titrant solution again.NoteThe result should lie within 10% of 0.5 mg KOH/g. If not, troubleshoot!
?. [Sample measurement]
Determine TAN for crude oil(s)Weight 20 grams (approximately 25 mL) of crude oil sample in a 250 mL beaker with a stirring rod. Record the exact mass to two decimals. Dissolve the oil in 100 mL of titration solvent.Click 'MODE'.Select 'TAN ASTM 664'.Click 'START' to start.Enter weight, date and sample information.After the measurement is done, discard the solution and rinse the electrode with titration solvent, water and titration solvent again.Repeat until all samples have been measured.
?. [Prepare instrument for analysis]
The system is stored under H2O. Remove water bottle, and replace with bottle filled with a suitable amount of titrant solution (6 g/L KOH in 2-propanol).
Approximately 50 mL of titrant is used in calibration and blanking, followed by 5 -6 mL per sample.
?. [Prepare instrument for analysis]
Rinse syringe, tubing and pipette with titrant solutionClick 'MODE'.Select 'Rinsing'.Place empty beaker under pipette. Perform three consecutive rinses (10 mL each).Discard titrant waste.
?. [Prepare instrument for analysis]
Calibrate titrant solution against freshly prepared KHP (aq., 12 mM)Prepare three separate solutions of approximately 125 mg potassium hydrogen phtalate (KHP) in H2O (50 mL) in 100 mL beakers. Record the exact mass to two decimals.Irrigate the beakers in a ultrasonic bath to facilitate dissolution. When fully dissolved, place the first beaker on the titrator base. Click 'MODE'.Select 'Titer KOH'.Click 'START'Enter the exact measured weight of KHP.After the measurement is finished, discard the solution and rinse the electrode with water.Repeat the calibration measurement on all three solutions. The average value is automatically saved in the unit.
0.09 - 0.11 M (%RSD
The experimentally measured value should be 0.1 M. A large deviation indicats an issue with either the titrant / KHP solution or instrument. Troubleshoot and continue when the issue has been identified and fixed.
?. [Prepare instrument for analysis]
Blank instrument againt titration solvent (500:495:5 toluene:2-propanol:H2O)Transfer 100 mL of titration solvent to a 250 mL beaker with a stirring rod.Click 'MODE'.Select 'Blank TAN-TBN'.Click 'START'After the measurement is done, discard the solution and rinse the electrode with titration solvent, water and titration solvent again.Repeat the blank measurement twice.
?. [Prepare instrument for analysis]
Validate instrument performance using QC sampleWeight 20 grams (approximately 25 mL) of QC sample in a 250 mL beaker with a stirring rod. Record the exact mass to two decimals. Dissolve the sample in 100 mL of titration solvent.Click 'MODE'.Select 'TAN ASTM 664'.Click 'START'Enter 'YYMMDD_QC' as sample name.After the measurement is done, discard the solution and rinse the electrode with titration solvent, water and titration solvent again.
0.5 mg KOH/g
The measured value must be within 10% of 0.5 mg KOH/g. If the value deviates, identify the issue before continuing. See attached documents under protocol description for a example QC titration.
?. [Titrant]
Measure 6.0 grams of potassium hydroxide (KOH) and transfer to a 1 L glass flask. Use a volumetric flask to measure 1000 mL of 2-propanol and transfer to the flask containing KOH. Stir overnight at room temperature. If the solution is cloudy or a precipitate is visible, filter through glass filter funnel. Label flask according to contents and date of preparation.
The solution has limited shelf-life, and should be used within a month or discarded.
?. [Titration solvent]
Measure 500 mL of toluene in a 500 mL volumetric flask and transfer to a 1 L glass bottle. Using a pipette, transfer 5.0 mL of de-gassed H2O (Milli-Q) to a 500 mL volumetric flask and top up with 2-propanol. Transfer contents to the toluene containing flask. Label flask according to contents and date of preparation.
?. [Quality control sample]
Weigh and transfer tridecanoic acid (53.6 mg, 0.25 mmol), pentadecanoic acid (60.6 mg, 0.25 mmol) and heptadecanoic acid (67.6 mg, 0.25 mmol) to a 100 mL glass flask.Measure 20.0 mL of toluene using a volumetric flask and transfer to a 100 mL volumetric flask.Top-up with hexadecane to mark, and transfer solvent mixture to glass bottle containing fatty acids. Irrigate in ultrasonic bath for 10 minutes to ensure compelte dissolution.
The density of 80:20 hexadecane:toluene has been experimentally determined to approximately 820 g/L. 100 mL equals 82 grams. The molar amount of fatty acids is 0.75 mmol which corresponds to 42.08 mg KOH. 42.08/82 = 0.51 mg KOH/gram.
?. [Finishing]
When all samples have been measured, proceed with the following steps to prepare the instrument for storage: Rinse electrode with titration solvent and water. If any (semi)-solid material have deposited on the electrode, remove it carefully using a lint-free Kimwipe. Remove the titrant bottle and replace with bottle containing freshly prepared H2O (Milli-Q).Place empty beaker under pipette. Click 'MODE'.Select 'Rinsing'.Click 'START'.Perform three consecutive rinses (10 mL each).Turn off and disconnect system.