US3338812A - Electrolytic titration apparatus - Google Patents

Description

29. 1967 D. o. DWORAK ETAL 3,333,812

ELECTROLYTIC TITRATION APPARATUS Filed April 22, 1,965 2 Sheets-Sheet 1 FIG. I

INVENTORS. DENNIS D. DWORAK ELWIN N. DAVIS A 29. 1967. D. D. owoRAk ETAL 3,338,812

ELECTROLYTI C TITHATION, APPARATUS Filed April 22, 1963 2 Sheets-Sheet 2 INVENTORS. DENNIS D. DWORAK E LWIN N. DAVIS United States Patent This invention is a novel method for automatic electrolytic titration to determine, for example, mercaptan sulfur content in hydrocarbons, and a novel apparatus which may be employed in this method. The invention enables an operator to maintain a constant check on the impurity content of gases including materials such as liquefied petroleum gas, off-gases from petroleum refineries, etc. A silver ion generation may be employed as the titrant to determine the content of the mercaptan in petroleum gases. In the petroleum industry, for example, there are many processes in which it is imperative to measure quantitatively the proportion of known contaminants. For example, in refineries wherein petroleum stocks are treated to remove mercaptan sulfur, a constant check on the mercaptan content of the feedstock and/ or the eflluent is necessary for eflicient operation.

There have been proposed and used many methods of determining mercaptan content, most of which have been based on titration using known volumes of an aqueous or alcoholic solution, with the determination of the impurity content being accomplished by a mathematical formula. Volumetric analysis, involving the addition of a reagent of known concentration in known quantity, suflicient to produce a stoichiometric reaction with, for example, the

mercaptan, usually is based on bringing the test solution to an end point. Since the operator knows the concentration and the volume of the added reagent, he may readily ascertain the concentration of the mercaptan. Such a method, however, requires the preparation, standardization, storage, controlled introduction and precise measurement of volumetric reagents and usually is a batch, rather than a continuous, operation. Further, volumetric systems are often unreliable andnot adaptable for plant control. First, the operator must prepare a good, reliable standard solution with the expenditure of diflicult, time consuming techniques. Secondly, the characteristics of the solutions may change on standing for a length of time. Thirdly, the solution may be difficult to measure, and it may be diflicult for one to add in the precise amounts involved in the titrations.

Recognizing the above difliculties, the art has developed titration methods based upon electro-chemical reagent production and/ or potentiometric or amperometric endpoint detection, with the flow of fluid to be tested, that is, the fluid containing the impurity, varied to bring about stoichiometric conditions. In such methods, however, the full potential use of electrochemical reagent production is not exploited and the inaccuracies inherent in regulating and measuring small amounts of fluid flow are still present. Also, Where such methods employ electrochemical production of bromine as the reagent, the presence of olefins in the sample can lead to inaccuracies. Even though most natural and liquefied petroleum gases contain only traces of olefins, this still constitutes a problem because normal mercaptan content is in the range of only about 20 to 40 I p.p.m. Also, the instruments devised to perform such methods are often high in cost, not readily adaptable to gaseous samples, and bulky.

In this invention, advantage is taken of the fact that the rate of electromechanical production of a tit-rating agent is proportional to, or may be readily correlated with, the applied electrical current. Accordingly, this invention makes possible continuous operation of systems for determining compositonal characteristics in terms of the applied electrical current.

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In electrochemical titration, according to this invention, the amount of titrating agent present and available for the titration reaction is controlled by in-situ electrolysis in a reaction zone. The amount of the titrating agent consumed in the reaction may thus be determined from the measurement of the current used in the electrolytic production of the titrating agent, if provision is made for determining and maintaining stoichiometric conditions. These latter functions can be performed automatically, the current employed in generation being controlled in response, for example, amperometrically, to the unreacted amount of one of the reactants. Where the concentration of a constituent in a solution is determined amperometrically, i.e. where there is a diffusion current created by the presence of an unreacted amount of one of the reactants, it is desirable that a separate electrical circuit, outside of the cell or reaction zone, be provided for control of the electrolytic current as distinguished from the amperometric detecting circuit. The instant invention generally employs a coulometric method of generating a reagent in-situ in a titration cell and an amperometric method for determining the end-point of the unknown.

In the present invention the fluid to be tested is brought into contact with an electrolyte-solvent for the unknown. Often the material in which the unknown carries itself is not soluble in the electrolyte, so that this procedure serves to scrub the unknown from the test solution, especially a test gas. The contact is brought about at a constant rate of introduction of the test fluid. In the case of a mercaptan-containing gas sample, the titration zone may be provided with an aqueous ammonium nitrate-ammonium hydroxide solution, e.g. 0.005% NH NO and 0.001% NH OH in H O, the concentrations depending upon the distance between the electrodes and the area of conduction, which scrubs the mercaptan from its carrier gas and thus absorbs the mercaptan at a constant rate.

The sample is analyzed by passing, through a source of reagent ions and through the titration zone, a current suificient to generate an amount of titrant, for example, silver ions from a sliver anode, equivalent to the mercaptan introduced. A norm is established whereby a certain quantity of the reagent ions are present in the titration zone, and the electrical characteristics of the solution in the titrating zone are measured by means of a detection circuit. The generation current is adjusted during the titration, to maintain a constant current in the detection circuit and the amount of this generation current is used to calculate the amount of unknown in the sample gas analyzed. The constant current may be detected by an am meter, and the normal reading of the ammeter is a constant endpoint.

The constant flow of sample gas into the titration cell, along with an efficient and regular manner of generation of Ag ions, therefore provides a correspondingly regular manner of endpoint determination; hence, the only adjustment that need be made is in the readily adjustable silver ion-generation current. An instantaneous determination of the amount of mercaptan in the sample is made possible at any time the operator desires such information.

Because of the relatively simple method involved in this invention, the apparatus required to carry it out is likewise relatively simple. Generally, the apparatus involved may consist of a cell, to be used as the titrating zone, an electrolyte within the cell and two relatively independent electric circuits which include the electrolyte solution. These circuits make up the generating system and the detecting system, respectively. Each circuit may be comprised of two electrodes, one positive and one negative, and a current-furnishing means. The detection circuit includes a current indicating means for monitoring the amount of free reagent ions in the electrolyte andhas a current controlling means, generally a potentiometer, for establishing the desired norm, that is, the end point. The generation circuit also has a current controlling means, usually a rheostat for keeping reagent ion generation equivalent to the sample introduced, and a current indicating means for use in determining the quantity of the unknown. The two circuits usually are relatively independent and distinct in the normal operating condition of a device, i.e. the generating circuit has its own power source, electrodes and meter, and the detection circuit, likewise, has its own components.

The electrolyte in the titration cell may be, for example, the aqueous solution of ammonium nitrate and ammonium hydroxide mentioned above. The two pairs of electrodes are made of materials substantially non-corrodible by the electrolyte or the test sample. One pair of electrodes may be connected to the generating circuit, and the other may be incorporated within the detecting circuit. The generating electrode may be, for example, a silver anode used in conjunction with a platinum cathode. The anode or silver electrode serves as the source of silver ions and the cathode or platinum electrode is so chosen, when the anode is silver, because platinum is lower in the electromotive series than is silver. The generating circuit preferably contains a battery of, for example, 22.5 volts, an ammeter, for example, a to 1 milliammeter, and a variable resistor, for example, a 50,000 ohm rheostat. The detection circuit may have, as its electrodes, a platinum cathode and a standard reference cell, e.g. a calomel halfcell, as its anode. The rest of the detection circuit may be comprised of, for example, a battery, a potentiometer, and a current meter. In a preferred apparatus the detection circuit battery supplies 1.5 volts, the potentiometer is rated at 1,000 ohms and the meter is a 0 to microammeter.

In order to provide a compact and even portable apparatus, this invention preferably uses a five-position rotary switch for establishing and changing the electrical circuitry involved. This switch generally has means for effecting the following conditions in the apparatus: (1) In position a the switch opens and thus deactivates all circuits; (2) in position b a battery, microammeter and large resistor are connected in series to allow for a small amount of current to flow through the microammeter to protect the meter during transit by holding its indicator at midpoint while not effecting the generation circuit; (3) in position c the detection circuit voltage may be adjusted to some desired value by connecting the detection circuit battery, a fixed resistance, and the potentiometer in parallel with the milliammeter and adjusting the potentiometer until the desired voltage is attained; (4) in position d the two major circuits, i.e. the generation circuit and the detection circuit, are activated, permitting titration and concurrent detection and monitoring, and (5 in position e, the two major circuits remain activated with the added feature being a parallel shunt across the milliammeter in the generation circuit which has a resistance equal to a fraction, e.g. one-ninth, of the internal resistance of the milliammeter, thereby permitting a current ten times the previously permitted current to pass through the electrolyte and consequently allowing for a faster generation of titrant or adjustment of the norma free ion content. Operating independently of the rotary switch may be, for example, a toggle switch which permits the flow of current around the milliammeter and rheostat, thereby boosting the generation of titrant.

Further details of the invention will be described in connection with the accompanying drawings wherein:

FIGURE 1 is a circuit diagram of a preferred embodiment of the invention showing in detail the electrical components of the apparatus; and

FIGURE 2 is a perspective view of the preferred apparatus incorporated into a carrying case.

Referring to the drawings, the titration cell is illustrated as a 300 ml. cell which contains electrolyte 12 and two pairs of electrodes 13 and 14. Reagent-generating electrode pair 13 comprises the platinum cathode 13a encased preferably, in a micro-porous material, e.g. sintered glass, and the silver anode 13b, while the detection electrode pair 14 comprises the platinum cathode 14a and the standard reference calomel half-cell 14b is the anode.

The electrode pairs 13 and 14 are held in a spaced relationship to each other, for example, by means of the plug 16 which fits into the mouth of the cell 10. Through this plug pass the electrodes.

Referring to FIGURE 1, the apparatus is comprised of at least nine circuits. The circuits are manipulated generally by the switch 20 shown in nine locations in the drawing. The switch 20 is one which can be moved to five positions and has means for providing at least nine circuits. When the switch is turned to each position a to e, it contacts points a to e, respectively, in all nine circuits. These means are designated by the Roman numerals I to IX while each of the five positions is given a letter, i.e., a through e. Thus, contact point Ic is provided for the circuit forming means I in position 0.

The generation circuit, activated when the switch 20 is in position d in all nine circuits, includes, besides the electrodes 13 and the electrolyte 12, the lead 40, the switch 42 in position y, the lead 43, the switch 20 at contact point IX-d, the leads 44, 45 and 46, the milliammeter 48, the leads 49, 50 and 51, the switch 20 at contact point VIId, the lead 52, the rheostat 54, the leads 55 and 56, the ohm resistance 57, the lead 58, the switch 20 at contact point VI-d, the lead 59, the 22.5 volt battery 60, the lead 61, the switch 20 at contact point V-d, and the lead 62.

An alternate generation circuit afforded by an adjustment of switch 20 at the position e, i.e., all nine circuits are switched to position e, provides a shunt designated by lead 63, resistor 64, lead 65, switch 20 at contact point VIII-e and lead 66 which intersects leads 50 and 51. The resistor 64 has a resistance equal to about of the internal resistance of the milliammeter 48. The alternate circuit provided for by switch 42 in position x and lead 68, provides means by which the milliammeter 48 and the rheostat 54 are eliminated from the circuit, thus speeding the rate of reagent generation.

The apparatus is provided with the gas entry tube 78 which may be connected to a source of gas to be tested. From the tube 78, the gas flows through the valve 80 and the tube 81 to the flow meter 82. From the meter, the gas is conducted to the cell 10 by way of the tube 84. Tube 84 is connected, by means of an adapter 85, to a coarse gas dispersion tube 86. The unknown gas to be tested enters the electrolyte 12 through the inner tube 87, and, as pointed out above, this electrolyte dissolves out from the carrier gas the material to be determined. The scrubbed carrier gas leaves the container through the annular passage and out by way of vent 88.

A formula, given below, is used to coordinate the reading on the milliammeter 48 and the flow meter reading to determine the mercaptan content of the test gas. The milliammeter is in series with the titration cell, and is adjusted to read the current flowing through the titration cell. The Ag+ ion generation is affected by adjustment of the rheostat 54 in line with the readings taken from the detection circuit, i.e. if the reading on microammeter 136 is lower than some selected norm, the generation is increased, and, if higher, the generation is decreased.

The detection circuit includes, besides the electrodes 14 and the electrolyte 12, the leads and 122, the potentiometer 124, which may be at about 1000 ohms, the lead 126, the resistance 128, and lead 129, the switch 20 at contact points II-c, d and e, which are electrically joined, the lead 130, the 1.5 volt battery 131, and lead 132, and the lead 133, which joins lead 132 to potentiometer 124, the leads 134 and 135, the microammeter 136, the lead 137, the switch 20 at contact points IV-d and e, and the lead 138.

Other arrangements in the detection circuit are also allowed by switch 20. The Battery Check circuit, wherein the milliammeter 48 is incorporated within the detection circuit as a voltmeter, is comprised of the lead 122, the potentiometer 124, the lead 126, the resistor 128, the lead 129, the switch 20 at contact point II-c, the lead 130, the battery 131, the lead 132 and the lead 133 and a parallel circuit represented by the leads 134 and 139, the switch 20 at contact point III-c, the leads 140 and 49, the milliammeter'48, the leads 46 and 142, the switch 20 at contact point I-c and the lead 143.

The apparatus of this invention also includes a Meter Protect circuit wherein switch 20 is at position b in all nine circuits. This arrangement permits a small amount of current to flow through the meter and is represented by the potentiometer 124, the lead 133, the lead 132, the battery 131, the lead 130, the switch 20 at contact point II-b, the lead 144, the large resistor 145, the lead 146, the microammeter 136, the lead 135, the lead 134, the lead 133 and the potentiometer 124. With switch 20 in operating position a, the proper titration current is determined by obtaining a steady reading on the microammeter 136 by adjustment of the rheostat 54.

A compact and portable embodiment of this invention suitable for field use is shown in FIGURE II. The titration cell 10, the two pairs of electrodes 13 and 14, together with their leads to adapter 150, and the dispersion tube 86 are removable from operative position to be stored in a compartment of the carrying case 152. The major components of the circuit are represented on the instrument board by the dials and meters shown. The microammeter 136 and the milliammeter 48 are shown. The knob 154 controls the rheostat 54; the toggle switch 156 controls the switch 42; the knob 158 controls the five-position rotary switch 20; the knob 160 controls the potentiometer 124.

An example of the procedure which may be followed in the operation of the process of this invention using the apparatus described would be as follows:

The operator may wish to determine the amount of mercaptan sulfur in a petroleum product gas sample. He first makes sure that the flowmeter 82 is level by adjusting the case until the bubble level 164 indicates such a condition. Then, electrolyte is poured into the titration cell 10, and the polyethylene tube 84 is connected to the unknown gas line. The knob 158, which controls switch 20, may then be adjusted to the c or Battery Check position, in which position the milliammeter 48 is placed in the detection circuit as a voltmeter, and the potentiometer 124 is adjusted with the Voltage Adjust knob 160 to regulate the potential to be realized from the battery in the detection circuit. The voltage which is selected corresponds to a reading of, for example, 0.83 ma. on the milliammeter 48. The gaseous sample is then allowed to flow through the titration zone 10 at a constant rate of, for example, approximately 500 ml. per minute as shown by the flowmeter 82. The knob 158 should then be changed to Low Titrate, which corresponds to position d in switch 20, and the rheostat 54 is then adjusted by means of knob 154. The adjustment of the rheostat regulates the current passing through the generation circuit; thus, a low resistance provided by the rheostat corresponds to a high rate of silver ion generation. The generation is continued until a reading of, for example, about 2 microamperes is maintained on the microammeter 136 for about 5 minutes. The reading of 2 microamperes represents the selected endpoint. Any convenient value may be selected since the microammeter reading simply represents the current passing through the detection electrodes or the amount of free silver ions in the titration cell. When this value is constant, all the silver ions which are generated are reacted with the unknown mercaptan; therefore, when the microammeter reading is constant for a reasonable time period, e.g. 5 minutes, the end point is reached. If the titration rate, i.e. the rate of silver ion generation, is too slow as indicated by a reading on the microammeter of less than the selected endpoints, i.e. 2 microamperes, the knob 158 may be changed to High Titrate or position e. In this position a shunt around the milliammeter 48 is provided which, in effect, diverts of the current around the milliammeter. The result of this adjustment is that a greater current, e.g. ten times as much, is allowed to pass through the generation circuit thereby providing for a greater rate of silver ion generation. The Fast Titrate toggle switch 158 which regulates the two-position switch 42 may be activated momentarily to minimize the time between adjustments. By this adjustment, the milliammeter is eliminated from the circuit so that an extremely high rate of silver ion generation is realized for an instant. The switch is conveniently spring-loaded so that the circuit returns to normal when the operator removes his hand. The flow meter 82 reading may then be converted to milliliters per minute from a previously plotted graph and the p.p.m. of mercaptan in the sample may be calculated from the formula:

Where X =p.p.m. mercaptan sulfur in the sample;

A=milliamperes per minute of generating current;

Z=fiow in milliliters per minute of sample through the titration cell;

F=amount of sulfur, in grams,'titrated by 1 milliampere of current for one minute; and

W=weight in grams of 1 milliliter of gaseous sample.

The quantity F is a constant having a specific value for every reagent-unknown combination. It may be calculated on the basis of the electrical current required to produce the amount of the reagent ion stoichiometrically equivalent to one gram of the unknown. In the case of silver and sulfur this quantity is 0.00002, meaning that one milliampere of current for one minute generates enough silver to titrate 0.00002 gram of mercaptan sulfur.

Excellent results may be obtained by using the titrator in this fashion. In a series of 18 analyses, the average deviation was :3 p.p.m., and the average difference between the instrumental and silver nitrate methods discussed previously was :2 p.p.m. Table I, below, gives a comparison of instrumental and silver nitrate analyses of mercaptan blends in terms of p.p.m. mercaptan sulfur.

TABLE I Instrumental Silver Nitrate Sample I 63 71 Average 66 Sample II 57 53 Average 54 Sample III 82 85 Average 86 Sample IV 58 53 Average 53 This invention provides an automatic titrator at a nominal cost which can be easily carried into the field,

enabling an accurate analysis to be performed in approximately 15 minutes. The apparatus may be fully contained in a carrying case about 16 inches high and 13 inches wide and weighing only about 20 pounds. The method which this instrument employs has been proved to be equal to or better than the silver nitrate titration method, and its simplicity of operation and the fact that a number of analyses can be made without changing the electrolyte or disturbing the electrodes leads to a more efiicient operation.

It is claimed:

1. An electrical titrating apparatus for the determination of the mercaptan sulfur content in a gas containing mercaptans comprising a titration cell containing an electrolyte and having means for introducing said gas at a constant rate, a platinum cathode and a silver anode for electrically generating silver ions, and a pair of sensing electrodes; a detection circuit comprising said sensing electrodes, a power source and a current measuring means connected across the said sensing electrodes for amperometrically detecting change in the concentration of silver ion in the electrolyte; and a generating circuit comprising the said platinum cathode, the said silver anode, a power source, a current measuring means, a means for regulating the magnitude of the current in relation to a change of the concentration of said mercaptan sulfur and shunt means for reducing resistance in the generation circuit and allowing rapid silver ion generation.

2. The apparatus of claim 1 wherein the sensing electrodes comprise a platinum cathode and a calomel reference half-cell anode.

3. The apparatus of claim troducing the constituent to cell includes a flow meter.

1 wherein the means for inbe measured in the titration References Cited UNITED STATES PATENTS 2,621,671 12/1952 Eckfeldt 204-195 2,668,097 2/ 1954 Hallikainen et al 23--25 3 2,745,804 5/1956 Shaffer 204-195 2,928,774 3/1960 Leisey 204-l95 2,954,336 4/1960 Grutsch 204-l95 3,131,133 4/1964- Barendrecht 204-195 3,248,309 4/1966 Robinson 204--195 OTHER REFERENCES Landsberg et al., Industrial & Engineering Chemistry, vol. 46, No. 7, July 1954, pp. 1422-1428.

Shaffer et al., Analytical Chemical, vol. 20, No. 11, November 1948, pp. 1008-1014.

JOHN H. MACK, Primary Examiner.

T. TUNG, Assistant Examiner.

Claims (1)

1. AN ELECTRICAL TITRATING APPARATUS FOR THE DETERMINATION OF THE MERCAPTAN SULFUR CONTENT IN A GAS CONTAINING MERCAPTANS COMPRISING A TITRATION CELL CONTAINING AN ELECTROLYTE AND HAVING MEANS FOR INTRODUCING SAID GAS AT A CONSTANT RATE, A PLATINUM CATHODE AND A SILVER ANODE FOR ELECTRICALLY GENERATING SILVER IONS, AND A PAIR OF SENSING ELECTODES; A DETECTION CIRCUIT COMPRISING SAID SENSING ELECTRODES, A POWER SOURCE AND A CURRENT MEASURING MEANS CONNECTED ACROSS THE SAID SENSING ELECTRODES FOR AMPEROMETRICALLY DETECTING CHANGE IN THE CONCENTRATION OF SILVER ION IN THE ELECTROLYTE; AND A GENERATING CIRCUIT COMPRISING THE SAID PLATINUM CATHODE, THE SAID

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