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IT IS THE USER'S RESPONSIBILITY TO ESTABLISH APPROPRIATE PRECAUTIONARY PRACTICES AND TO
DETERMINE THE APPLICABILITY OF REGULATORY LIMITATIONS PRIOR TO USE. EFFECTIVE HEALTH AND
SAFETY PRACTICES ARE TO BE FOLLOWED WHEN UTILIZING THIS PROCEDURE. FAILURE TO UTILIZE THIS
PROCEDURE IN THE MANNER PRESCRIBED HEREIN CAN BE HAZARDOUS. MATERIAL SAFETY DATA SHEETS
(MSDS) OR EXPERIMENTAL MATERIAL SAFETY DATA SHEETS (EMSDS) FOR ALL OF THE MATERIALS USED IN
THIS PROCEDURE SHOULD BE REVIEWED FOR SELECTION OF THE APPROPRIATE PERSONAL PROTECTION
EQUIPMENT (PPE).


© COPYRIGHT 1964, 1967, 1987, 1989, 2005 UOP LLC. All rights reserved.

Nonconfidential UOP Methods are available from ASTM International, 100 Barr Harbor Drive, PO Box C700, West
Conshohocken, PA 19428-2959, United States. The UOP Methods may be obtained through the ASTM website,
www.astm.org, or by contacting Customer Service at [email protected], 610.832.9555 FAX, or 610.832.9585 PHONE.


HYDROGEN SULFIDE AND MERCAPTAN SULFUR
IN LIQUID HYDROCARBONS
BY POTENTIOMETRIC TITRATION

UOP Method 163-05

SCOPE

This method is for determining hydrogen sulfide and mercaptan sulfur in hydrocarbons by
potentiometric titration. Typical samples include gasoline, naphtha, light cycle oils and similar
distillates that are liquid at ambient temperature and pressure. The lower limit of detection is 0.1
mass-ppm mercaptan sulfur and 1.0 mass-ppm hydrogen sulfide.

REFERENCES


UOP Method 41, “Doctor Test for Petroleum Distillates,” www.astm.org


UOP Method 999, “Precision Statements in UOP Methods,” www.astm.org

OUTLINE OF METHOD

The liquid hydrocarbon sample is weighed into 2-propanol containing a small amount of
ammonium hydroxide. The solution is titrated potentiometrically with alcoholic silver nitrate using a
glass reference and silver-silver sulfide indicating electrode system. Hydrogen sulfide and mercaptan
sulfur concentrations are calculated as mass-ppm. Free sulfur complicates the potentiometric curve,
and instructions are given for interpreting the curve when free sulfur is present. The presence of free
sulfur is reported with the results.

APPARATUS

References to catalog numbers and suppliers are included as a convenience to the method user.
Other suppliers may be used.



Balance, readability 0.1-mg


Beaker, Berzelius, high form, 400-mL, Fisher Scientific, Cat. No. 02-546D


Beakers, electrolytic, 250-mL, Brinkmann Instruments, Cat. No. 020212209, two or more required



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163-05

Cleaning pad, synthetic, mildly abrasive, Scotch-Brite™, Runco Office Supply, Cat. No. MMM-96


Cylinder, graduated, 100-mL, Class B, Fisher Scientific, Cat. No. 08-550E

Desiccator, 160-mm ID, and porcelain plate, Fisher Scientific, Cat. Nos. 08-632 and 08-641A,

respectively


Electrode, combination silver/glass titrode, Brinkmann Instruments, Cat. No. 020948507. The
electrode should be dedicated to sulfur analysis.



Flasks, volumetric, Class A, 500-mL, Fisher Scientific, Cat. No. 10-210-5F, two required


Oven, drying, capable of operation at 120ºC, Fisher Scientific, Cat. No. 13-246-506G


Pipets, volumetric transfer, Class A, 1-, 8-, and 50-mL, Fisher Scientific, Cat. Nos. 13-650-2B, -2J,
and -2S, respectively



Regulator, nitrogen, two-stage, high-purity, delivery pressure range 15-200 kPa (2-30 psi),
Matheson Tri-Gas, Model 3121-580



Titrator, potentiometric, recording, ± 2000-mV range, 1-mV resolution, capable of reducing the
titration rate to a minimum in the vicinity of the endpoint, with dispenser having a volume
readout of 0.00 - 99.99 mL, 0.0001 of the buret volume resolution, Metrohm Model 836 Titrando
system with optional sample changer, and two 20-mL burets, Brinkmann Instruments



Automatic titration using the titrator is recommended. However, titration can be performed
manually using the following equipment.



Buret, 50-mL, Class A, with PTFE stopcock, Fisher Scientific, Cat. No. 03-701-25C

pH meter, digital, Mettler Toledo Model S20K, Fisher Scientific, Cat. No. 01-913-807

Stirrer, magnetic, with stir bar, Fisher Scientific, Cat. No. 14-493-120S


REAGENTS AND MATERIALS


References to catalog numbers and suppliers are included as a convenience to the method user.
Other suppliers may be used. Unqualified references to solutions mean aqueous solutions.


Ammonium hydroxide, concentrated, Certified ACS Plus, Fisher Scientific, Cat. No. A669-212


Desiccant, 8-mesh, indicating, Drierite, Fisher Scientific, Cat. No. 07-578-3A


Detergent, LiquiNox, Fisher Scientific, Cat. No. 04-322-15B


Nitrogen, high purity, 99.99%


Paper, test, lead acetate, strips in vials, Fisher Scientific, Cat. No. 14-862


Potassium chloride, Certified ACS, Fisher Scientific, Cat. No. P217-500. Oven dry before each use
at 105ºC for 2.5 hours and cool in a desiccator.



2-Propanol, Certified ACS grade, Fisher Scientific, Cat. No. A416-4. Purge with nitrogen for 10 to
15 minutes before using.



Silver nitrate, 0.1-M solution, standardized at 25ºC against NIST potassium chloride, lot analysis
supplied, Fisher Scientific, Cat. No. SS72-4

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163-05



Silver nitrate, alcoholic solution, 0.01-M. Prepare a 0.01-M alcoholic silver nitrate solution by
pipetting 50 mL of the standardized 0.1-M solution into a 500-mL volumetric flask. Dilute to the
mark with 2-propanol, cap and invert several times to mix thoroughly. Calculate and record the
molarity of this solution: the lot analysis of the 0.1-M solution, divided by 10.



Silver nitrate, alcoholic solution, 0.001-M. Prepare a 0.001-M alcoholic silver nitrate solution by
pipetting 50 mL of the standardized 0.01-M solution into a 500-mL volumetric flask. Dilute to
the mark with 2-propanol, cap and invert several times to mix thoroughly. Calculate and record
the molarity of this solution: the lot analysis of the 0.1-M solution, divided by 100.



Sodium sulfide nonahydrate, Fisher Scientific, Cat. No. S425-500


Sodium sulfide nonahydrate, 1% solution. Dissolve 1.0 g ±1.0 mg of sodium sulfide in 100 mL of
water.



Sulfuric acid, concentrated, 95-98%, Fisher Scientific, Cat. No. A484-212


Sulfuric acid, dilute. Prepare by mixing equal volumes of concentrated sulfuric acid and water.


Toluene, 99.8% minimum purity, Fisher Scientific, Cat. No. T290-4. Purge with nitrogen for 10 to
15 minutes before using.



Water, deionized or distilled


Wipers, Kimwipes Ex-L, Fisher Scientific, Cat. No. 06-666A

PROCEDURE


The analyst is expected to be familiar with general laboratory practices, the technique of titration,
and with the equipment being used.


Electrode Preparation


1. Prepare the silver electrode by cleaning the surface of the silver rod electrode with a mild
detergent.


• Proper electrode preparation is essential to obtain reproducible and noise-free titration curves having

good distinguishable endpoints.


2. Rinse the electrode with water and wipe dry with a wiper.


3. Polish the surface of the electrode gently with a cleaning pad until the surface is smooth to the
touch.


4. Rinse the electrode with water again, and wipe dry with a wiper.


5. Immerse the electrode in a solution consisting of 1 mL of concentrated ammonium hydroxide,
96 mL of 2-propanol and 8 mL of 1% aqueous sodium sulfide solution. Add slowly from a
buret over a period of about 10 minutes, with stirring, approximately 10 mL of 0.01-M
alcoholic silver nitrate solution. A film of silver sulfide will be deposited on the silver. Wipe
off the excess silver sulfide on the electrode with a wiper. Prepare this electrode fresh daily or
as required.


• During the titration of samples containing sulfur, the electrode will be coated with AgS. It is necessary to

repeat the electrode preparation when the coating begins to peel, becomes rough and uneven, or if the
titration curves become noisy.

Copyright by ASTM Int'l (all rights reserved);
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163-05


• To remove any residue from previous titrations, it is good practice to dip and swirl the electrode in mild

detergent and rinse with water at the at the beginning of each set of samples. This aids in keeping the
titration curves noise free, and reduces the chance of the appearance of false endpoints.


Titration of Sample


Mercaptans oxidize easily. It is important that samples are handled quickly in order to minimize

their exposure to air. Titrate samples immediately after they are pipetted while covering the beaker
with a nitrogen blanket during the analysis.


1. Prepare the titrator for operation with the proper electrode in place and with the appropriate
molarity alcoholic silver nitrate solution (see Table 1) as the titrant. Set the operating
parameters on the instrument to reduce the rate of titrant addition in the vicinity of the
endpoints.


2. Before titrating, check the contents of the sample qualitatively for hydrogen sulfide by

suspending lead acetate paper in the vapor space above the sample.


• Moisten a piece of lead acetate paper with water and fasten one end of the paper to the stopper of a
bottle containing the sample. Do not allow the paper to contact the hydrocarbon sample. Allow it to
stand for about one minute.


• If the paper turns silver-black, hydrogen sulfide is present, and the test is positive. This test is sensitive
to approximately 1 mass-ppm of hydrogen sulfide. Some mercaptans will cause lead acetate paper to
darken with a tan to yellow coloration. This is not to be confused with a positive hydrogen sulfide test.


• If the lead acetate paper test is difficult to read, UOP Method 41, “Doctor Test for Petroleum Distillates,”

may be run as a more rigorous test.

3. Add the recommended volume of 2-propanol to the appropriate size beaker (see Table 1). Add

approximately 1 mL of concentrated ammonium hydroxide.

4. Weigh a portion of the sample, to the nearest 0.1-mg, into the beaker while maintaining the

solution under a nitrogen blanket. Use Table 1 as a guide in selecting the proper sample size.


• The sample size should be selected according to Table 1 to give a titration of at least 2 mL. When the
sample contains more than 500 ppm sulfur, select a sample size sufficient to give a 5- to 10-mL titration.


• If the sample is insoluble in the 2-propanol, add enough nitrogen-purged toluene to the beaker containing
isopropyl alcohol to obtain a solution.



• Solutions containing high levels of mercapans and low levels of hydrogen sulfide are titrated separately
for the two components. A sample is titrated as above to the mercaptide endpoint, and then a larger
sample is titrated only to the hydrogen sulfide endpoint. The mercaptan concentration is determined by
difference.


• If the ratio of sulfide sulfur to mercaptan sulfur is greater than 10:1, then the mercaptan result is

considered to be qualitative.


Table 1
Selection of Equipment, Reagents, and Sample Volumes



Expected mercaptan
concentration,

ppm, S

Beaker
size,
mL


2-Propanol,

mL

Suggested
sample
mass, g


Titrant,

M


0.5-1 400 150 100 0.001
1-100 250 100 50 0.01
100-300 250 100 10 0.01
300-500 250 100 5 0.01

Copyright by ASTM Int'l (all rights reserved);
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163-05


5. Add a stirring bar, and place the beaker in the titration apparatus. Insert the electrode and stir,

covering the solution with a nitrogen blanket to prevent mercaptan oxidation. Adjust the stirring
speed to create a vortex in the solvent such that bubbles do not develop at its center. Titrate
with the alcoholic silver nitrate solution, manually or automatically depending on the
equipment available. Observe the respective inflections and note the corresponding volumes of
titrant.


• If the titration is to be performed manually, add the silver nitrate solution in 1-mL increments, recording

the volume and emf after each addition. Near the endpoints, add the silver nitrate solution in 0.1-mL
increments. Allow enough time for the titration cell to reach equilibrium before recording the volume of
silver nitrate solution and the emf. The endpoints are found where the rate of change in potential with
respect to the volume of added titrant is a maximum.


• Plot the titration data as emf vs. volume of silver nitrate solution. Estimate the mid-point of each inflection
by inspecting the titration curve. Alternatively, the endpoints may be determined by examining the titration
data for the emf/mL maxima, thereby obviating the task of plotting the data. This procedure can be
followed by an experienced operator.


Interpretation of Titration Curves


The Figure shows titration curves that can be obtained for gasolines having different ratios of
hydrogen sulfide, mercaptan and free sulfur (including polysulfides). Curve A is an example of a
titration where only hydrogen sulfide is present. Curve B is an example of a titration where only
mercaptan is present. Curve C is an example of a titration where both hydrogen sulfide and
mercaptan are present. Curves A, B, and C are the titration curves most commonly encountered.


When free sulfur is present in the sample, it reacts with some of the mercaptan forming polysulfide,
the titration of which complicates the interpretation of the titration curve. Curve D is an example of a
titration where free sulfur has reacted with a portion of the mercaptan to form polysulfide and, thus,
three inflections are observed: hydrogen sulfide, polysulfide and the remaining mercaptan. Curve E
is an example of a titration where the free sulfur has reacted with a portion of the mercaptan to form a
polysulfide but no hydrogen sulfide was present and, thus, only two inflections are observed: the first
for polysulfide and the second for mercaptide.


• Disulfides do not show an inflection.





Copyright by ASTM Int'l (all rights reserved);
Reproduction authorized per License Agreement with Monique Tyree (IHS); Mon Mar 7 12:16:11 EST 2005

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163-05

One Inflection (Curve A or B)

When the lead acetate test is positive for hydrogen sulfide and only one inflection occurs, the only
species present is hydrogen sulfide. The hydrogen sulfide content is determined from the total
titration, from zero volume to the inflection point.

When the lead acetate test is negative, the only species present is mercaptan. The mercaptan
content is determined from the total titration, from zero volume to the inflection point.

Two Inflections (Curve C or E)

When there are two inflections and the lead acetate test is positive, the first inflection is hydrogen
sulfide, and the second inflection is mercaptan. The hydrogen sulfide content is determined from the
volume of titrant from zero volume to the first inflection point. The mercaptan content is determined
from the volume of titrant used from the first inflection point to the second inflection point.

When the lead acetate test is negative, and the absence of H2S is confirmed by UOP Method 41,
Doctor Test, disregard the first inflection point. The mercaptan content is determined from the total
titration, from zero volume to the second inflection point.

Three Inflections (Curve D)

When there are three inflections, and the lead acetate test is positive, the first inflection point is
hydrogen sulfide. Disregard the second inflection point. The mercaptan content is determined from
the volume of titrant from the first inflection point to the third inflection point.


Three inflections are not normally seen when the lead acetate test is negative. Verify the sulfur
types in the sample using UOP Method 41, Doctor Test, and check the equipment for proper
operation.

CALCULATIONS


Calculate the concentration of hydrogen sulfide and mercaptan, as sulfur, using Equations 1 and 2,
respectively. Report the results to the nearest 1 mass-ppm unless the 0.001-M titration solution was
used and the result is less than 1 mass-ppm. Then report to the nearest 0.1 mass-ppm.


Hydrogen sulfide, as S, mass-ppm =
W
AM16

103 )1(


Mercaptan, as S, mass-ppm =
W

M)AB(32
103


)2(



where:


A = volume of silver nitrate solution used to reach the sulfide ion endpoint, mL
B = volume of silver nitrate solution used to reach the mercaptide ion endpoint, mL
M = molar concentration of the alcoholic silver nitrate solution, moles/L
W = mass of sample, g
16 = molecular weight of sulfur divided by 2, the number of reactive hydrogen sites in

hydrogen sulfide
32 = molecular weight of sulfur divided by 1, the number of reactive hydrogen sites in

mercaptan
103 = factor to convert L to mL and g/g to µg/g

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163-05


NOTES


1. Whenever there is a question regarding the identity of an inflection point, it is advisable to

verify the sulfur types in the sample using the lead acetate paper or the Doctor Test.


2. Hydrogen cyanide, if present, titrates with two inflection points and may cause confusion in the
interpretation of the titration curves. Generally, the mercaptan inflection is observed between
the two cyanide inflections.


3. Some recording titrators will produce a small inflection that occurs a few seconds after the
titration is begun. This has been attributed to electrode conditioning and is not to be interpreted
as a hydrogen sulfide inflection point. To distinguish between this phenomenon and a true
hydrogen sulfide inflection point, repeat the titration with twice the sample volume. If the
inflection is due to hydrogen sulfide, the titration volume will double.


PRECISION

Precision statements were determined using UOP Method 999, from precision data obtained using a
Brinkmamm Metrohm Model 751GPD Titrino titrator with an automatic sample changer.


Repeatability and Intermediate Precision


A nested design was carried out for determining mercaptan in two samples with two analysts in one
laboratory. The two analysts carried out tests on the two samples on two separate days, performing
two tests on each sample each day. The total number of tests at each concentration was eight. In
addition, one analyst in one laboratory performed eight replicate analyses on each of two low-level
mercaptan samples. The precision data are summarized in Table 2. Two tests performed by the same
analyst on the same day should not differ by more than the repeatability allowable difference with
95% confidence. Two tests performed in one laboratory by different analysts on different days
should not differ by more than the intermediate precision allowable difference with 95% confidence.


Table 2
Repeatability and Intermediate Precision, mass-ppm


Repeatability

Intermediate
Precision




Component

Mean
Concentration

Within-
Day esd

Allowable
Difference

Within-
Lab esd

Allowable
Difference



Mercaptan 5 0.25 1.0 0.23 0.9
Mercaptan 303 1.7 6.7 1.4 5.4
Mercaptan 0.6 0.023 0.08
Mercaptan 1.0 0.026 0.09


The data in Table 2 are a short term estimate of repeatability and intermediate precision. When the

test is run routinely, a control standard and chart should be used to develop a better estimate of the
long term intermediate precision.


Reproducibility

There is insufficient data to calculate the reproducibility of the test at this time.

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