Is it sour because of Mercaptans or does it ACT sour because of Poly Sulfides

Determination of type and amounts of volatile sulfur compounds in crude acid

Instrumentation setup

The gas chromatographic system used in this procedure is:

Notice the black tubing. This connects the probe to the Sievers detector module. This tubing must not be kinked. The vacuum pump and ozone generator must be turned on before mounting the probe onto the FID Head. These membrane switches are mounted on the front of the Sievers module. First press the power button to change status from stand by to on. Then press the labelled switches. Red lights indicate power is applied.

Probe in plastic storage case

FID Head

Module

Note the basic violation of lab policy. Sample, even though in a sealed vial, setting on top of an instrument. If spilled this will destroy paint and attack the electronics.

The probe fits loosely over the top of the FID with a cutout for the signal cable. The Allen wrench has been left in place in this picture to remind that the Allen set screw must be snug to prevent vibrations and thereby reduce signal noise.

The settings for the GC are:

Initial Value

100° C

Initial Time Minutes

2.00

Rate

5.0 Deg./Min

Final Value

270°C

Final Time Minutes

5.00

Injector B

250°C

Detector B

250°C

Equilb Time Minutes

1.00

Supply Gas Settings are:

Head Pressure

15 psi

H2 Top of Ball

15

Air Top of Ball

34

Sample preparation :

Add enough Phenyl Sulfide to a portion of crude acid so that the percent added sulfur from the Phenyl Sulfide is approximately equal to the RSH sulfur previously measured. Each gram of Phenyl Sulfide equals 0.172 grams sulfur. So if a Crude Acid contains 0.3 weight percent Mercaptan Sulfur you would add about 1.74 grams to 100 grams of Crude Acid. Of course lower amounts can be used in the same ratio. After adding the Phenyl Sulfide mix well.

Suggested weight of Phenyl Sulfide

Mercaptan Sulfur by titration Wt%

Grams Phenyl Sulfide to add to 10 grams Crude Acid

Grams Phenyl Sulfide to add to 50 grams Crude Acid

0.02

0.012

0.060

0.05

0.029

0.145

0.10

0.058

0.290

0.15

0.087

0.435

0.20

0.120

0.600

0.25

0.145

0.725

0.30

0.174

0.870

0.35

0.203

1.015

The Phenyl Sulfide and Crude Acid must be weighed on a balance with at least 3 decimal point sensitivity.

Gas Chromatographic Analysis:

Inject 1 uL of the prepared sample into the rear inlet of the Sievers GC. Start the temperature program and the integrator.

When the run is ended, either automatically or manually after last peak has eluted, identify the sulfur peaks according to the chromatogram below.

Divide the area for the Phenyl Sulfide peak by the weight percent sulfur added by the Phenyl Sulfide. This is the KF for SULFUR for this shot. It will vary from shot to shot. The peaks after Phenyl Sulfide are Aromatic Disulfides and can be ignored because they do not contribute to titratable mercaptan values. The peaks before MeSH (Methyl Mercaptan, Methane Thiol) can be ignored because MeSH is the earliest mercaptan so these peaks cannot be mercaptans.

Divide the area for each of the remaining peaks by the KF. Sum those values. If the sum is low compared to the titrated sulur value the Crude Acid contains Poly Sulfide and extra Sweetening will not help. The Poly Sulfides will fall out, normally, in springing or in the flash tower.

Note: Recently it has come to this author’s attention that a dirty inlet can cause “cracking” of Disulfides back to Mercaptans. A series of analyses showed an impossibly high level of mercaptans. Trouble shooting found an incredibly fouled inlet insert.

This is or was an inverted cup type quartz insert. The dark material is tarry and apparently residue from many previous shots. It was replaced and flows re-established through inlet and no evidence of cracking was seen. If “cracking” is suspected or system verification is needed inject a known solution of Phenyl Disulfide in MP99. There should be no peak at the Benzene thiol retention time.

Note added June 19, 2002

Whenever selecting mechanical pump fluids for use in Mass Spectrometers, it is important to select the fluid with the lowest vapor pressure. The vapor pressure of the oil is important because it determines the ultimate vacuum the pump can achieve. Lower vapor pressures will allow the user to minimize backstreaming, significantly reduce pumpdown time and maximize vacuum. The major three mineral based oils used for rough pumps in mass spectrometers include Inland 19, Invoil 20 and Inland 45. Any of these pump fluids can be used in all Edward, Alcatel, Welch and other mechanical rough pumps.
Comparison of Rough Pump Oils
.Inland 19Invoil 20Inland 45Inland TWFomplin Y06/6
Vapor Pressure3 x 10-5 torr3 x 10-61 x 10-71 x 10-63 x 10-6



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