going beyond. going further
N. H. EnINGER George Washington High School Bustleton Ave. and Verree Road Philadelphia, PA 19116
QuaternaryAmmonium Halides Versatile Reagents for Precipitation Titrations Walter S. Selig Lawrence Livermore National Laboratory, University of California. Livermore, CA 94550 For the study of potentiometric titrations only very simple equipment is required: a buret, apH1millivolt meter, arefereuce electrode, and a homemade plastic-coated graphite indicating electrode (I). Quaternary ammonium halides were found very suitable a s titrants for precipitation titrations (2). We recommend cetylpyridinium chloride (CPC), a compound which is relatively nontoxic, inexpensive, and very versatile. It can he used for the determination of many inorganic and organic anions. Many cations can h e determined after conversion to their halide or cyanide complexes. Background Willard and Smith (3) recommended tetravhenvlarson. . ium chloride \e,AKl) for the gravimetric determination of some Iaree anionsand someromvlex halides. In 1968 Haceuk i used s maAsCl for t h e potentiometric titration and ~ u ~ o(4) of perchlorate, using a perchlorate ion-selective electrode (ISE). We have found t h a t quaternary ammonium halides can replace dAsC1 a s t h e titrant in this and many other titrations (5). In earlier work we used cetyltrirnethylammo~~ium I)rumide (CETAR) a s titrnnt ( 6 ) . Recent work has shown thac CI'C is nrefcrahle hecause of its hieher aoluhilitv " in water, which makes i t possible t o use more concentrated titrant solutions (2).We have also replaced the fairly expensive commercial ISE's with inexpensive homemade plasticcoated graphite indicating electrodes (1). ~~
Table 1. Optimum and Feasible pH Ranges for the Determlnatlon of lnorganlc Anlons with CETAB, Uslng the Flvoroborate ISE (2)
~~~~
Experimental The preparation of the polyvinyl(chloride)/dioctylphthalatecoated graphite rod was described previously in this Journal (1). The titrant was a 0.01 M aqueous solution of cetylpyridinium chloride which is available from various suppliers.' It was prepared by dissolving 3.58 g of the monohydrate in approximately 250 mL of warm water, and diluting to volume with cold distilled water. Determinations of 0.025 mmol of analyte were made in a volume of 25 mL. Potentiometric titrations may he performed using an automatic titration apparatus, or by manually adding increments of titrant, letting the emf equilibrate, taking emf reading, etc., and finallv drawine a titration curve. If this latter method is used. it is helnf;l for enz~ointcalculation to add eaual increments of titrant ~~~,~~~~~~ in the endpuint region according to the methud of Lingane r71. The expwimentd conditims,snmple titration curves, and srntirties of recoveries have been described in a review of the analytical
~~.~~ ~~~
Work performed under the auspices of the V. S. Department of Energy by the Lawrence Livermore National Laboratory under Contraci no. W-7405-ENG48. Aldrich catalog no. 85.856-1, Alfa catalog no. 13,651; Eastman catalog no. P5361. The reagent from Alfa was the least expensive material. One liter of a 0.01 M solution costs $0.17.
'
applications of quaternary ammonium halides (2)and some recent A brief summary of the feasible pH ranges is presented work (8,9). in Tables 1,2, and 3. This should help in getting the student started without having to refer to the literature. For the experiments reported in Tables 1 and 2, the titrant was mainly CETAB. As stated previously, this titrant was replaced in later experiments with CPC. A single experiment in Table 2, using CPC, gives an indication of the improvement in tKe magnitude of endpoint breaks possible with this titrant. The student may want to verify this for some of the results reported in Tables 1and 2. In most of the experiments reported in Tables 1 and 2, a fluoroborate ISE was used. As reported elsewhere (I),a coated-graphite sensor can be used for all the experiments mentioned in this paper. Again, a comparison of the coated-graphite sensor with those used previously is invited. Obviously,this electrode, as well as some of the so-called "ion-selective" electrodes, are quite nonselective. More recent work using CPC as titrant and the coated-graphite sensor is summarized in Table 3.
Anion CIO, BF4 MnOl
ReOn PFs AsF. AuCI, TICIA 1-
Mean endpoint break, mV
Optimumb pH range
Feasible' pH range
65 50 125 45 150 175
4.0-8.0 4.0-8.0 2.9-7.0 4.0-8.0 2.1-7.9 4.0-8.0 1.5-2.7 2.3 2.7-10.7 4.0-6.0 2.2-5.7 3.0-6.0 0.65 1.9-9.6 0.65-2.3 0.65 6 N HCI 1 N HCI 7-10 2.2-9.75 1.9-8.4 1.8-2.2 2.7-7.8
1.2-12.8 2.3-10.6 1.8-10.7 1.9-11.0 1.5-10.6 1.8-10.6 52.75 0.2-4.0 1.8-10.7 1.6-7.3
260 200
SbFs'
36 30 15 86
Cr&
70
lo4" PdClr
Fe(CNb HgCI,
RCI, SnCl, SnCls W e PtCle
OsCI, F~(CN)B IrCI,
95 60 35 50 45 70 100 125 65 40
9 . 7
1.8-7.7 0-11.3 1.0-11.3 0-9.4 0-2.0 2.5-11.3 0.35-12.1 0.2-6.4 1.8-10.0 1.7-8.0
' N M malytl~ally U O ~ ~ U I , beoaras endpaint breaks are very gradual. bwimin me optimum pH ranges the steepest endpoim breaks arc obtained; ble" pH ranges include the regions where the breaks are less simp.
Volume 64
Number 2
February 1967
"least-
141
label 2.
Some Analytical Data for the Determination of Organlc Anions with Quaternary Ammonlum Halldes (2)
Anlon Nitrofonn Nitroform Nitroform Nitroform 2.4.5aid110mbenzwnesuIf0nate 2.edinltrOb~nz~nes~IfOMte Picryl~ulfonate Tetraphenylb~~ate Cyanotriphenylbwale Picrate Dodecyl sulfate Bromophenol blue Brom~~resoI purple &thophenathroline dlsultonlc acid disodium san. trihydrate
Electrcde
Tltram
Mean endpoint breh, mV
Optimum pH range
Feasible pH range
BFI BF4 divalent Ca2+ BF4 BF, BF4 BFI BF4 BF4 BF, BFI BF4 BF4
CETAB CPC CETAB CETAB CETAB CETAB CETAB CETAB CETAB CETAB CETAB CETAB CETAB CETAB
150 165 140 130 110 50 110 450 330 180 220 175 200 45
7.0-10.0 7.0-10.0 7.0-10.0 7.0-10.0 4.0-12.0 22 6.0-10.0 28 8.0-12.0 5.0- 7.0 5.5- 9.8 1.0- 2.0 1.8- 2.2 8.6-10.4
1.5-12.2 1.5-12.2 1.5-12.2 1.5-12.2 0.2-12.3 1.4-11.3 1.7-12.2 1.6-12.0 1.8-12.0 2.1- 9.2 2.1-12.2 0.8- 3.1 1.0- 2.5 6.2-12.0
A few conditioning runs should precede each experiment. This is
good practice when working with ISE's. In highly acidic aolutions the coated graphite indicating electrode will deteriorate more quickly than in other media. This requires re-coating of the graphite sensor as outlined in reference I. also
Inorgank Anlona A list of inorganic anions that can he determined with CPC is given in Tahle 4. Many anions can be titrated, with
Table 3. CondMlons for the Potentlomark ntratlon ot Some Anions wMh CPC, using a Coated-graphlte Sensor (8, 9 ) Ion Optimum determined pHlackdity 0.5-1 1.15
