until 1-liter had been collected. Each The large “break” between the last of the amino acids and the beginning of fraction Was taken t o dryness to remove piperidine and then1 to transform the the eluted sodium chlor.de demonstrates inorganic base to a was treated that piperidine displacement is an effiwith excess concentrated hydrochloric fratcient desalting procedure even for the acid and again dried, StTith tions, the residue was weighed, ignited, most basic amino acid, arginine. Amand reweighed. The salt was then dismonk is sufficiently alkaline for dissolved and assayed for sodium and placement desalting of other basic potassium by flame photometry. Calamino acids (3, 4 ) and pyridine can be cium and magnesium were absent by used when only neutral and acidic amino standard qualitative procedures. acids are to be recovered ( 3 ) . The virtual The data obtained are plotted in ~ i ~ - absence of potassium from the soure 1, The 200 ml. of effluent follotvdium peak indicates that this procedure might be efficient for the separation of ing the emergence of the last amino acid these alkalies on a Preparative scale* contained only a trace of nonvolatile material and this vanished on ignition. LITERATURE CITED The next 800 ml. represented a broad skewed peak which proved to be sodium (1) Astrup, T., Stage, A., Olsen, E., Acfa chloride with only a trace of potassium. Chem. Scand. 5, 1343 (1951).
( 2 ) Awapara, J., Arch. Biochem. 19, 172 (1948). (3) Buchanan, D. L., J . Biol. Chem., in
press.
(4) Carsten, hf. E., J. Am. Chem. SOC.74,
5954 (1952). (5) Consden, R.2 Gordon, A* H.1 ’‘a*ti1’, A. J. P., Biochem. J . 41, 590 (1947). ( 6 ) Dr&ze,A., bloore, S.,Bigwood, E. J., Anal. Chim. Acta 11, 554 (1954). (7) Haugaard, K., Haugaard, E. S., Compt. rend. Lab. Carlsberg, Ser. Chim 29 347 (1955). (8) bfuellei, G’, c., B ~ G , , H~ ~ ~ -~ ranen, A., ANAL. CHEX. 27, 1357 (1955). (9) PieZ1 K* ~4.3 Tooper, E. B.9 Fosdick, L. D., J . Bid. Chem. 194, 669 (1952).
RECEIVED for reviem June 12, 1957. Accepted July 25, 1957.
Potentiometric Recorder for Hydrogen Sulfide and Hydrogen ‘Cyanide JOHN P.
STRANGE
Mine Safety Appliances Co., Pittsburgh, Pa. An instrument has been developed for the continuous potentiometric determination of hydrogen sulfide and hydrogen cyanide in air or in plant process streams. The gas i s removed from the sample b y an alkaline liquid absorbent, and the amount i s determined by means of a potentiometric cell containing a vibrating silver electrode and a saturated calomel reference. Constancy of sample and absorbent flow i s achieved by metering pumps. By choice of flow rates the instrument can be adjusted to give full-scale deflection for concentrations of hydrogen sulfide or hydrogen cyanide ranging from 4 p.p.b. to 50 p.p.m. or higher. A plot of the logarithm of the ion concentration vs. the recorder reading yields a straight line, which makes it possible to calibrate the instrument by determining its response to only two known concentrations. The response of the instrument to mixtures of hydrogen sulfide and hydrogen cyanide i s not an additive function of the responses to the separate gases. Use of the instrument to monitor mixtures of these two gases i s therefore not recommended.
creased production has brought with it new and greater needs for a continuous instrument to monitor the air of plants and working or storage areas t o prevent the build-up of toxic concentrations of hydrogen cyanide. Roth has reviewed the various methods used for hydrogen cyanide detection and described an experimental instrument developed a t Dow Chemical Co. which had the advantage of continuous operation (4). This instrument, which is applicable to either hydrogen cyanide or hydrogen sulfide detection, is based on the measurement
CAUSTIC SOLUTION
of the potential of a silver electrode (2) immersed in an electrolyte through which the sampled air has been passed. Figure 1 is a schematic diagram of an instrument based on the Roth design, showing how the air sampIe is passed through a scrubber tower a t A to which B supplies buffered water a t a continuous, uniform rate. The electrolyte then passes to cell C, where the dissolved cyanide or sulfide influences the silver half-cell potential. The instrument is made quantitative by carefully controlling the air and liquid flow rates and measuring the electrode
AIR
SILVER ELECTRODE
A
in annual acrylonitrile production capacity, 14,000,000 pounds in 1950 to 204,000,000 pounds in 1955, has caused a similar rise in the manufacture of the very toxic hydrogen cyanide (1). This inVERY LARGE INCREASE
1878
ANALYTICAL CHEMISTRY
Figure 1. Schematic diagram o f cell and flow system for hydrogen sulfide-hydrogen cyanide recorder
Toteutial under conditions of nearly zero current drain. PRINCIPLE
E
=
E,
+ 0.0591 log C
through the electrolyte. Figure 2 shows how the voltage of an electrode approaches linearity with log concentrations when it is rotated a t 1700 r.p.m., but is nonlinear when stationary.
where
E = silver electrode potential
The silver half-cell potential is dependent upon the concentration of the silver ion in the solution. The silver ion concentration is in turn infiuenced by the presence of cyanide or sulfide ions, hecause the cyanide or sulfide reacts with the silver ions to form relatively insoluble salts. The silver electrode potential is measured against a standard cdomel electrode. The potential is related to the concentration of silver ions in solution in accordance with the Nernst equation:
Eo = standard half-cell potential C = silver ion concentration
DESCRIPTION OF INSTRUMENT
For low concentrations the e.m.f. varies as the logarithm of concentration. I n practice, however, the depressed ionization a t higher concentrations causes some deviation from the ideal relationship. lecause of the nartial denletiou of the
0,
The Dow instrument was duplicated in these laboratories and the nerform-
mal plant operating conditions. Figures 3 and 4 show theinstrnment as constructed for air monitoring or process control. One motor drives the air
method depends upon the reproducibility of the air and liquid flow rates, care is taken to ensure that these remain constant. The air sample flow is controlled by a sensitive differential pressure regulator across a fixed orifice. The scrubbing liquid is pumped from a constant head Mariotte hottle by means of a Teflon gear pump. The relatively low liquid flow rate of 0.5 ml. per minute is held within +3'% by this method. The scrubbed air is vented and the used solutions are allowed to pass out to the drain or a collection container. As the liquid consumption is only about 5 gall Ions per month, servicing of the instrument is kept to a minimum, and reprocessing of the solution and recirculation have heen deemed unnecessary. For convenience and ease in operation, the instrument has been built into a ~
.CELL MILLIWLTS
~
~~
~~~~
~~
~~~
~
Figure 2. Effect oi'silver electrode rotation upon potential of silver-calomel ce,II
.
I.
2.
7 7 ~ wiectrode statii Electrode rotat
a
Figure 3.
Front view of completed instrument without recorder
Figure 4. Front view instrument with recorder
of mounted
VOL. 29, NO. 12, DECEMBER 1957
1879
coboratoryorm suitable for plant or lampact fuse. It may be mounted on a panel, in which case the solution supply would be located on a shelf on the tiark of the piinel (Figure 4). DEVELOPMENT OF INSTRUMENT
Before thc instrument described above n a s constructed, the effect. of electrode condition, scrubber efficiency, p H of absorbing solution, and contaniinants in the air or electrolyte nere further explored. The electrode surface coiidition has a significant effect on the potential and the linearity of the response to cyanide. Figure 5 shons the beneficial effect of aging the electrode, C u i w 1 is the response of a mechanically cleaned and washed electrode before aging in the cell solution. Curre 2 is the response after 21 days of aging in the cell electrolyte. An electrode n hich had been given a sulfide coating by exposure to hydrogen sulfide gave a negatively displaced potential but equal sensitivity to changes in cyanide concentration (curve 3). Figure 6 shows the advantage of buffering the absorbing solution as reported by Roth. The curves show the potentials obtained for three cyanide concentrations in the cell solution as the pH was varied. Above p H 10.5 there is much less dependence of the potential upon pH. \Then the instrument is 0peratc.d on high concentrations of cyanide. the use of sodium carbonate as a buffer gave greater stability to the response than n-as obtained using a sodium hydroxide solution. Among other simplifications it has been found possible to eliminate the salt bridge betaeen the cell and the calomel reference electrode. N o deleterious effect was noted from operating up to 90 days with the reference directly in the cell solution. The response time and recovery time have been shortened by using a small volume and small electrodes. The response time to better than 90% of a step change in concentration is of the order of 1 minute at the usual rates of liquid and air florr. Complete equilibrium after a shift in concentration is attained after 10 to 15 minutes. The range of the instrument can be adjusted by controlling the ratio of liquid to air flow. These flows are selected so that the concentration of cyanide or sulfide in the liquid reaching the cell is about the same for all fullscale readings. The cyanide or sulfide concentrations under these optimum conditions vary from 1 x 10-2 to 1 X 10-4M for the span of the instrument. A study was made of the efficiency of various scrubbers for removing the 1880
ANALYTICAL CHEMISTRY
CELL MILLIVOLTS ( S I L V E R VS SATURATED C A L O M E L )
Figure 5. Effect of surface treatments on silver electrode potential
1. Wire cleaned mechanically and washed with distilled rater 2. Cleaned and aged 3 weeks in sodium carbonate solution 3 . Sulfide coated by immersion in sodium carbonate with 14 p,p.m. of hydrogen sulfide bubbling through
I IO0
0
-2
-.I
I -.3
SCE VOLTAGE Figure 6. Effect of pH on silver electrode potential for three cyanide concentrations SILVER
VI.
I ro!
200
I M*
u)
IW
J IW
HCN CONCENTRATION IN AIR,PP.M.
3
40
Figure 7. Efficiency of common scrubbing methods 1. Petticoat bubbler 2. Simple dip tube bubbler 3 . Glass bead column
m
m
70
80
RECORDER CHART READING
Figure 8. Reproducibility of sponse to hydrogen sulfide
instrument re-
Data taken over 2-n-eek period as follo~~-s: 0 June 28 0 June 23 A June 24 A July 7
hydrogen cyanide or hytlrogen sulfide froni the air sample. Because the instrument reports the concentration of the eontaminant in the scrubbing solution, it is vital that the scrubber function efficiently a t all concentrations within the desired operation range. Figure 7 slion-s a comparison of three coiiimon scrubbing methods a t concent’rations up to 650 p.p.m. of hydrogen cyanide. I n still later \\-ark on concentrations in the parts per billion rangc: tlie solution was pumped to the top of a plastic cylinder containing a closely fitting core on which was machined a lielical groove. The air sample and solution flow concurrently down through the grooye to the cell a t the bottom. Titration analyses made on back-up scmbhers s h o ~no detectable leakage, indicating that this scrubber approaches 1 0 0 ~ ccfficiency for very low concentrations. Becausc. of the current interrst in the detwtion of trace quantities of air contaminants, the response of the silver c>lc,ctrode potentiometric method to iiiuch l o w r concentrations than deemed toxic was investigated. Hydrogen cyanide concentrations in the range betwccn 1 p.p.b. and 1 p.p.ni. of normal laboratory air n-ere obtained by niultiple dilution techniques. The response of the instrument Tr-as found to be linear in this range.
n ith the 10% mean deliation liiiiits indicated. These data ryere taken nith the electrode rotating a t 600 r.p.ni.; hence, the calibration is not linear with concentration as \\-as obtained with faster moving electrodes. For convenience and reliability in making contact to the moving silver electrode, vibratory motion was substituted for rotation. The frequency and amplitude of the vibration r e r e chosen so that the velocity of the silver past the liquid a t the electrode solution interface was equal to that of the electrode rotating a t l T O O r.p.m. Instruments nith this construction gave the desired linearity and operated reliably over the test periods of 90 and 120 days, indicating that the vibrating rlectrode was fully as sensitive and reliable as the rotating electrode. Similar tests are eontinuing xith much longer periods now elapsed.
tit--+I - t -
CELL MILLIVOLTS ( S I L V E R VS. CALOMEL)
Figure 10. air sample
Effect of contaminants in
1. Hydiogen sulfide with no contaminant 2 . 9 p.p.m. of hydrogen cyanide 3. 22 p.p.m. of hydrogen cvanide x 30 p.p.m. of sulfur dioxide 0 0 0
CELL MILLIVOLTS
0.1
cx carbon dioxide
O.-lyccarbon dioxide
120 p.p.m. of sulfur dioxide
INTERFERENCES
( S I L V E R V S S A T U R A T E D CALOMEL)
Figure 9. Effect of contaminants in cell electrolyte 1. 0.0027, hydrochloric acid
2. KOcontaminant 3. 3.5% ammonium hydroxide
Reproducibility of the method was studied by recording the response to known concentrations for several days of continuous operation without adjustnient. Figure 8 shows the variation in response during a 14-day period,
The silver electrode potential is substantially free from effects of atmospheric constituents other than hydrogen sulfide and hydrogen cyanide. The method is, therefore, fairly specific for these air contaminants. The reproducibility tests discussed above n-ere made using mixtures of hydrogen sulfide in compressed laboratory air. No scrubbing or conditioning of the air sample n-as required to achieve reproducibility within & l o % in the range of 2 to 50 p . p m . of hydrogen sulfide in air. Similarly) no special precautions or treatment of the sample nere needed
to ohtain thc, mc:i>urcnicnts in thc parts per billion range of hydrogen cyanide in air. However, certain contaminants do shift the silver potential, especially if they are pre>ent in relatively high concentrations. Figure 9 shous the shift in the calibration for hydrogen sulfide caused by the presence of hydrochloric acid or ammonium hydroxide in the cell solution. The greater relative effect of the acid coiitaiiiinant follon-s the p H influence described above. Because tlie liquid and air flow rates are selected so that full-scale readings are obtained a t very low concentrations of cyanide or sulfide in solution, it is more significant to look a t the effects of contaminants in the air stream. Figure 10 is a plot of the response of the cell to hydrogen sulfide in the presence of mrious concentrations of contaminants. T h e effects of 0.1 or 0.4y0 carbon dioxidr or of 30 p.p.m. of sulfur dioxide are iiegligihle. Hon-ever, a high concentration of sulfur dioxide, 120 p.p.ni., has the effect of reducing the p H of the solution and decreases the negative potential as shoir-n a t 1 and 2 p.p.ni. of hydrogen sulfide. The p H with 120 p.p.m. of sulfur dioxide present was found to be 9.6 instead of the normal 10.5. As would lie expected, there is cross sensitivity to hydrogen cyanide. Curves 2 and 3 of Figure 10 show the result of superimposing fixed concentrations of hydrogm cyanide, 9 and 22 p.p.m., upon a varl-ing hydrogen sulfide concentration. The effects are not additive, but instead no indication of the hydrogen sulfide is seen below the minimum potential produced by the cyanide. The mechanism of the effect has not been studied. ACKNOWLEDGMENT
The development of this instrument has been the work of many people, including HT H. Roth, Dovv Chemical Co. and H. S.Cotabish, R. G. Hudson, and K. W. Charlton, Mine Safety .4ppliances Co. REFERENCES
(1) Chem. W e e k 77, S o . 14, 69-70 (1955). (2) “Handbook of Chemistry and Yhyeics,” 38th ed., p. 1594, Chemical Rubber Publishing Co., Cleveland, 1956. (3) Kolthoff, I. AI., Furman, S. H., “Potentiometric Titrations,” p. 101, Kiley, Yew York, 1926. ( 4 ) Roth, H. H., “Hydrogen Cyanide Detector with the Silver Electrode,” 99th Annual Neeting, Electrochemical Society, Washington, D. C., -4pril 10, 1951. RECEIVED for review October 23, 1956. Accepted July 17, 1987. Division of Analytical Chemistry, 130th Meeting, ACS, Atlantic City, N. J., September 1956. VOL 29, NO. 12, DECEMBER 1957
1881
