Amperometric Titration
OF
Mercaptans with Silver Nitrate
Using the Rotating Platinum Electrode 1. M. KOLTHOFF AND W. E. HARRIS, School of Chemistry, University of Minnesota, Minneapolis, Minn.
THE
much ammonia, The titration in the presence of ammonia is as simple as that in neutral or acid medium. The reaction equation is:
rotating platinum wire electrode has been introduced as an indicator electrode in amperometric titrations (8) by Laitinen and Kolthoff (3). The performance of an amperometrie titration with the rotating platinum electrode as indicator elcctrode becomes especially simple, when no e.m.f. needs to be applied to the cell consisting of the indicator electrode and the reference electrode. The rotating electrode is placed in the solution to be titrated, electrolytic connection is made with the reference electrode, and the current which flows through the cell during the titration is read on a microammeter. The diffusion current of the substance titrated or of the reagent is measured a t the potential of the rotating electrode. For example, in the titration of a mercaptan with silver nitrate using the saturated calomel electrode as a reference electrode, the current is zero until the end point. After the end point, when there is an excess of silver in the solution, the diffusion current, of silver is measured with the microammeter, this diffusion current being proportional to the concentration of silver ions. When the current readings during the titration are plotted against the volume of reagent added, two straight lines are obtained, which intersect a t the end point (see Figure 2 ) . I n the example under consideration, the current before the end point is practically zero and is equal to the residual current of the medium. If the titration is carried out in ammoniacal medium as in the titration of mercaptans with silver in the presence of chloride, the potential of the saturated calomel electrode is not negative enough to yield the diffusion current of the amino-silver ions. Therefore, another reference electrode, which is more negative than the saturated calomel electrode, but not sufficiently negative to give cathodic currents of oxygen, has been used. I n the present paper the amperometric titration of very small amounts of primary, secondary, and tertiary mercaptans p i t h silver nitrate is described. In the absence of chloride, this titration can be carried out in acid or neutral medium with the saturated calomel electrode as reference electrode. Only the procedure in ammoniacal medium is presented here, since much chloride and little bromide do not interfere in the presence of
+ RSH +RSAg + Hf Ag(NHa): + RSH RSAg + NH: + NHs Ag+
or
---f
RSH denotes a mercaptan. APPARATUS AND MATERIALS
The apparatus and the circuit are shown in Figure 1. A rotating platinum wire electrode, A , ahout, 6 to 8 mm. long and 0.5 mm. in diameter, sealed i n 6-mm. soft glass tubing, serves as indicator electrode. Since the titrations can be completed within a short time, an ordinary motor can be used to rotate the electrode. A synchronous motor is not necessary in titration work. For amperomctric titration a simplc rotating electrode can be made by replacing the shaft of an ordinary cone drive motor, H, with a short lcngth of brass tubing (devdopcd in this laboratory by D. G. Weiblen). The 6-mm. glass tubing with the electrode is fastened inside the brass tubing. Electrical contact is then readily made by dipping a wirc in the mercury inside the glass tubing. A reference clectrode, F. is used which has a pot,ential of -0.23 volt against the saturated calomel electrode. Thc elect,rolyte solution for the rcfercnce half-ccll is preparcd by dissolving 4.2 grams of potassium iodide and 1.3 grams of mercuric iodide in 100 ml. of saturated potassium chloride solution. A layer of mercury serves as the electrode. Electrical conncction between thc refrrence and indicator electrodes is made by mcans of a salt bridge, E , consisting of about 60 cm. (2 feet) of 6-mm. (inside diametrr) soft rubber tubing filled with saturated potassium chloride solution ( I ) . The rubber tubing is connected with a short length of glass tubing, D, filled with a gc.1 of 3% agar and 30% potassium chloridc. A t the end of the glass tube, a coarscly sintered glass disk may be inscrtcd, if desircd. For further protcct,ion of the solution from contnmination with iodide, the glass tube, I?, hnving :in agar or a fine sintered-glass plug at its end, may be intrrposcd. The clectrolyte solution, C , inside L3 can be easily rinscbd out and replaced with fresh electrolyte whencver it has bcen in use long cmough to become contnminated with iodide. I t is rsgontial that all sources of high resistance, such as air bubbles, bc climiiiated from the salt bridge. To complrt,e the circuit the two half-cells are short-circuited through a microammeter, G . A Weston Electrical Instrument Corporat.ion, Newark, microammct.er (Modrl 430) h&q bcen used in the present work. Instead of the microammeter, trthcr currentindicating devices may be used, such as a pointer galvanometer with sensitivity of 0.25 microampere per division on the attached scale. The mercaptans tested were analyzed for mercaptan sulfur content by tbe potentiometric. method of Tamcle and Ryland (6). Sources of the various mercaptans were: n-Dodecyl
Organic Chemical Division, University of Minnesota Commercial primary CIZ U. S.Rubber Co. Conimercial tertiary Cn Sharples Chcmirals. Inc. Cyclopentyl C. S.Marvel, Univevsity of Illinois PROCEDURE
Figure 1.
In a 250-ml. beaker dilute a sample of mercaptan containing about 5 mg. of mercaptan sulfur to 100 ml. with 9.5% ethanol. Make the solution about 0.25 M in ammonia and 0.01 to 0.1 M with some noninterfering electrolyte such as ammonium nitrate. Immerse the end of the salt bridge and the rotating platinum electrode in this solution. Titrate with aqueous 0.005 11.1 silver nitrate. Make 2 or 3 rradings of the microammetcr before the end point. As long as the silver nitrate is not in rx('css t h r current is very small or zero. After the end point the deHection of the ammeter corresponds to the diffusion current of the excess of silver. When
Apparatus for Amperometric Titration
161
162
Vol. 18, No. 3
INDUSTRIAL AND ENGINEERING CHEMISTRY Table
1.
Amperometric Titrations of Mercaptans Mercaptan Sulfur Present
Method of Taking Sample
Mercaptan Used n-Dodecyl
Weighing Dilution of standard solution
Commercial primary
Cu Weighing
Commercial tertiary Ci:
Weighing
Cyclopentyl
Dilution of standard solution
Mercaptan Sulfur Found
Mu
Me.
5.04 5.48 1.942
5.075 5.490 1.945. 1.943
7.77 1.93 5.26 5.01 6.47 6.94 3.047
7.77,7.80 1.89,1.93 5.249 5.006 6.456 6.944 3.051
the ammeter indicates the end point is passed add two or three small increments of silver nitrate; read the current after each addition. If the electrode becomes sluggish, clean, by wiping with a piece of cloth or between two fingers. Plot the readings of the microammeter against the volume of silver nitrate added. Draw two straight lines (4 and B , Figure 2); the point of intersection, C, corresponds to the end point. A titration carried out by an experienced operator requires no more than 2 minutes. RESULTS
OF T I T R A T I O N
Table I shows the results obtained with various mercaptans. The figures of mercaptan sulfur present are based upon the results of potentiometric titrations of large samples. With 5 mg. or more of mercaptan sulfur the amperometric titrations were reproducible within 0.2%. The current readings obtained during the titration of 1.942 mg. of n-dodecyl mercaptan sulfur are shown in Figure 2 . The method has been applied to a large number of other mercaptans with satisfactory end-point determinations in all cases Among mercaptans that have been titrated are: primary C1, CR,CS,CIO, Cia, CIS,and CIS; secondary Csand C12; tertiary (34, CB, C?, CS, C,o, Clc, and Cie as well as thiols with other functional groups.
$;21 q
I,
I
2 VOLUME
4 OF
,
A;,
~
2zgs 1
:i1
3
L
Infrequently the glass in the region of the glass-to-platinum seal may become very slightly cracked. In such case a new electrode must be prepared. I n some cases much suspended material in solution interferes mechanically xith the current readings. The use of an electrode of the design shown in Figure 3 eliminates interferences of this nature. INTERFERENCES
Cyanide in ammoniacal medium forms a stable complex with silver ions and interferes in the titration. Other ions, like iodide and sulfide, which form insoluble silver salts in ammoniacal medium, also interfere. I t is a simple matter to separate the mercaptan from interfering ions by shaking the mercaptan out in ether or some other suitable organic solvent. Large amounts of chloride and small amounts of bromide do not interfere in the procedure (Table 11). SUMMARY
.Isimple, rapid, and accurate amperometric titration method for the routine determination of primary, secondary, and tertiary mercaptans with silver nitrate is described, using the rotating platinum wire electrode as indicator electrode. The apparatus required is simple and is available in most DlRLCTlOH OP ROTATION laboratories. Amounts of mercaptan sulfur VIEW as small as 0.2 mg. in 100 ml. of ethanol can be determined Figure 3. Rotating Platinum with an accuracy of 1 or 2%. Electrode Amounts greater than 2 mg. per 100 ml. can be determined with an accuracy and precision of a t least 0.3%. The time required for performance of the entire titration need not be greater than 2 minutes. In ammonical medium, large amounts of chloride and small amounts of bromide do not interfere. Large amounts of bromide, as well as cyanide and other ions which yield insoluble silver salts in ammoniacal medium, interfere.
n Y-
0
6 8 SILVER NITRATE
IO SOLUTION
12
, ML.
Figure 9. Amperometric Titration of 1.942 M g . of Mercaptan Sulfur with 0.00495 N Silver Nitrate
NOTE. An indicator electrode may become insensitive or erratic after long use or when titrating large amounts of mercaptan. As suggested by May (4) full sensitivity can be restored by wipin the electrode with a piece of cloth or even betneen two fingers. I n the most troublesome cases the following procedure is recommended in determining the end point. After the addition of the first excess of silver nitrate as indicated by the first slight deflection of the ammeter stop the rotating electrode, wipe, and start again. Read the current immediately; add small increment of silver nitrate. Stop the rotating electrode, wipe, and continue as before. Read the current immediately after starting the electrode. Repeat the above operations 3 to 4 times. The end point is obtained from the readings in the manner previously described. Thorough cleaning of the electrode with concentrated nitric acid is usually necessary only after it has been used for several hundred titrations. When a new or freshly cleaned electrode is placed in the ammoniacal mercaptan solution and the cell is phortcircuited, a large current of 20 to 30 microamperes may be observed. This current decreases rapidly and is practically zero after waithg for 5 to 10 minutes.
Table
II. Titration of n-Dodecyl Mercaptan in Presence of Chloride or Bromide
Chlpride or Bromide Added
Mercaptan Sulfur Taken
Mercaptan Sulfur Found
MO.
Mg.
1Mg.
LITERATURE CITED
(1) Hume, D. N., and Harris, W.E., ISD. EKG.CHEN.,Ax.4~. ED.,15,
465 (1943).
( 2 ) Kolthoff, I. >I., and Lingane, J. J., "Polarography", S e w York,
Interscience Publisherb, 1941.
(3) Laitinen, H. A , . and Kolthoff, I. 11.. J . Phys. C h t n ~ . ,45, 1079
(1941). May, D . R., private comniunication. (5) Tamele, M. IT.,and Ryland, L.B , I s n 8, 16 (1936). (4)
EYG.C H E x . , ANAL.ED.,
THISinvestigation was carried out under the sponsorship of the Office of Rubber Reserve, Reconstruction Finance Corporation, in connection with the Government Synthetic Ruhher Program.
