Portable Analyzer for Determination of Dissolved Oxygen in Water Application of Rapid-Dropping Mercury Electrode CARL
P.
TYLER and J.
H. KARCHMER
Humble Oil & Refining Co., Baytown, Tex.
b A portable polarographic apparatus has been developed for the determination of dissolved oxygen in natural and effluent waters. Its unique feature is the use of a mercury drop time of about 0.25 second per drop, which produces a current relatively insensitive to agitation or to sample flow. The low drop time is achieved by mounting the dropping mercury electrode horizontally in the electrolytic cell and by judicious selection of the parameters of the capillary. Detailed instructions for carrying out the procedure, assembling the apparatus, and selecting the capillary are given.
T
HE determination of dissolved oxygen in industrial and natural water is preferably done a t the sampling site, because this eliminates the sometimes misleading effects of chemical oxygen demand, physical handling, and temperature changes. Quantitative chemical methods are usually not adaptable to field use, and polarographic methods (2, 7, 9, 10) are generally for fixed laboratory installations. The rotating electrode of a portable polarographic apparatus (6) can be poisoned by certain constituents of effluent waters, and thus necessitate frequent cleaning and calibration. Portable instruments with conventional dropping mercury electrodes are not practical because motion causes an unsteady current, a drawback if sampling is carried out from a boat. The portable analyzer described here eliminates most of these objections. It is inexpensive and simple enough to be operated from a small boat. The circuit is arranged so the microammeter can also serve as the voltmeter. The instrument can determine 0.1 p.p.m. of oxygen in the time needed to change the sample in the cell and take a single reading on a meter after approximate calibration. The method is applicable to water samples in the p H range between 4.5 and 11, and a t salt concentrations between 0.001 and 0.05M. Below 0.001M there is not sufficient electrolyte;
.above 0.05111 the diffusion current increases as the salt concentration increases. The addition of a small quantity of a special reagent containing gelatin, phenol, and sodium chloride mill eliminate difficulties caused by salt concentrations lying outside the indicated range. The method is based upon polarographic reduction of dissolved oxygen (4, 5 ) a t a dropping mercury electrode (with a mercury pool as anode) to produce two reduction waves of equal height. The early wave is said to be due to the reduction of oxygen to hydrogen peroxide +rZH+ + 2e + 2H20 + 2e
0 2
O2
-
-+
H202
(acidic)
+
(1)
H202
2 0 H - (neutral or basic)
(2)
This is a fairly steep wave reducing a t a potential of about -0.2 volt. HOTever, it possesses a huge maximum and has a short and shifting plateau. The second wave results from the reduction of hydrogen peroxide to m-ater or the hydroxyl ion:
+ +
+
H202 2Hf 2e 2H20 (acidic) (3) H202 2e + 2 0 H - (neutral or basic) (3') -+
This wave is long and drawn out going from about -0.5 to -1.3 volts, but is well formed, has no maximum, and has a broad flat plateau. EXPERIMENTAL
The apparatus is shown in Figure 1. The sample (plus a small amount of special reagent) is allowed to flow by gravity through a cell fitted with a special, rapid-dropping mercury cathode and a mercury pool anode. The cathode is inserted into the cell horizontally and the solution flows perpendicular to the orifice opening. A voltage of -1.6 volts us. mercury is applied to the cell by a simple arrangement using two small dry cell batteries, a meter which serves as both ammeter and voltmeter, and a variable resistance for adjusting the voltage. The resulting current, due chiefly to the reduction of oxygen in the sample, is read on a (0- to 30-pa.) microammeter. This current, corrected
Table 1. Dissolved Oxygen in Water at yarious Temperatures (Solution contains 10 ml. of gelatin-phenolaalt solution per pint) O
TemPerittUre c. F. O
10 11 12 13 14
15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
Dissolved Oxygen, P.P.R.I. by Wt. 11.2 10.9 10.7 10.4 10.2 10.0 9.8 9.6 9.4 9.2 9.0 8.9 8.7 8.6 8.4 8.3 8.1 8.0 7.8 7.7 7.5
50 51.8 53.6 55.4 57.2 59 .O 60.8 62.8 64.4 66.2 68.0 69.8 71.6 73.4 75.2 77.0 78.8 80.6 82.4 84.2 8G.O
for temperature variation and for residual current, is proportional to the dissolved oxygen content of the water. The actual oxygen concentration is obtained frcm this current and a previously determined calibration factor calculated from the analysis of a known sample. Flow of sample past the electrode is not essential. Ordinary stationary sample cells may be employed or the solution n a y be stirred. The flowing sample cell is described because of its usefulness in field operations. Reagents. Gelatin-Phenol-Sodium Chloride $Solution. Dissolve 5.0 grams of powdered gelatin in about 75 ml. of hot water. Cool, add 2.0 grams of phenol and 0.3 gram of sodium chloride, and idjust to 100 ml. with water, Air-Saturated Calibration Solution. To aboul, 1 pint of distilled water add 10.0 ml. (or about 2 ml. per 100 ml. of water) of gelatin-phenol-salt solution. Saturate with air by bubbling 1 hour a t a constant temperature and allowing to stand 0.5 hour before use. Thi,3solution contains 8.10 p.p.m. of dissolved oxvcren a t 26" C. (78.8" F.). Table :[ gives oxygen solubility at other temperatures. These values were " U
VOL. 31, NO. 4, APRIL 1959
499
horizontal electrode J . Flow may be adjusted with stopcock so that, in conjunction with piece of small bore tubing at E, a sample flow of 80 ml. per minute is maintained while current readings are taken. This need not be exact C. Glass siphoning tube (6 mm.) D. Stopcock t o sample cell E. Tubing (approximately 2-mm. id.) long enough t o produce a sample flow of 80 ml. per niinute with the sample head described under B P. Leveling bulb and mercury reservoir G. Mercury level, kept 28 em. (or some other constant level to give proper drop time) above J H . Electrical leads I . Rubber or plastic tubing J . Dropping mercury electrode. A 2-inch piece of ASTM aniline point thermometer, No. 217050, range -38" to 42' C., sealed into cell Q with sealing wax K. Platinum wire sealed in soft glass, and sealed at top to Q with sealing wax L. Mercury pool anode M Excess mercury take-off N . SamDle outlet (rubber tubing- and pinch clamp) 0. Venf tube P . Glass tubing (6 mm.) approximately 14 inches long with 2-inch side arm Glass cell with oDeninas as indicated: total volume Q. should be 15 to 26 ml. R. Stopcock for stopping mercury flow MA. D. c. microammeter, 0 to 30 pa., about 2000 ohms (similar to Model 741. Weston) DPST 1. Double-pole switch for disconnecting battery and simultaneously shorting ammeter DPST 2. Double-pole switch for converting ammeter t o voltmeter. Serves as microammeter in position shown; voltmeter in other position. Obtain voltage by multiplying dial reading by 0.1. Variable voltage is obtained by use of a 100-ohm, 2-watt, wire-wound adjustable resistance (similar to R.lOOL, R. R. $l?llory & Co., Indianapolis, Ind.). With very sensitive microammeters, a 100-mfd. condenser, placed in parallel with the meter, may be used to dampen the swing
GP-
I
I l l
R -,
I-
K
Figure
1.
Portable
apparatus
for
determining
dissolved oxygen in water
A . Sample container (pint bottle or B.O.D. bottle) B. Sample level at which current readings should be made if flowng system is used; approximately 48 cm. above
obtained from a standard reference (1) but slightly adjusted to allow for the presence of the special reagent, assuming that the gelatin and phenol added to the mater lowered the solubility of the oxygen in the water to the same extent as would an equal weight of salt. Values for the lowered solubility of oxygen in the presence of salt were obtained from the American Public Health manual (1). Truesdale, Downing, and Lawden (11) give more accurate data. Although the results reported here are based upon the APHA data, values of Truesdale, Downing, and Lawden should be used whenever possible. Preparation of Apparatus. Set u p the apparatus as shown in Figure 1. Open D and close M. Apply suction a t N until & is filled with a representative sample. Stop or continue the flow as desired. Open R and allow the mercury to drop 1 or 2 minutes to reach its normal state. CALIBRATION.For best results the instrument should be calibrated under conditions under which the samples are to be run. Calibration curves should be made for either flowing or stationary samples. Use air-saturated calibration solution (of known oxygen content) in container A and allow it to flow through cell Q. Turn on switch 1, set switch 2 to voltmeter, and adjust the voltage t o -1.6 volts. When the sample level has reached B, read the current on the 500
ANALYTICAL CHEMISTRY
microammeter and record as i t k . Measure the sample temperature and record as T,. Takc another sample of calibration solution, add about 1 gram of sodium sulfite, shrke well, and allow to stand for 15 mimtes. Record the current reading on this sample as i r k . (Another method of preparing a sample to obtain residual current is to purge the sample for 20 minutes with an oxygen-free inert g3.s.) Calculate the calibration factor, j : f, p.p.m. per pa. =
p.p.m. D.O. in stanzlard solution a t T, (itk
-'
irk)
(4)
where
irk
= observed
total current on known sample, pa. irk = residual c-irrent on known sample, pa. D.O. = dissolved olygen T, = temperature of calibration, 'F. Factor f is valid only for measurements made a t the temperature of calibration. At other temperatui tes, the net current should be corrected to the calibrating temperature before is applied. PROCEDURE. Catch the sample in container A , avoiding entrainment of atmospheric oxygen or loss of dissolved oxygen. Pipet into the bottom of the bottle 2.0 ml. of gelatin-phenol-salt solution for every ' 0 0 ml. of sample. Allow the displaced sample to rise in the top of the botlle. Insert a glass
B.O.D. stopper and mix by upending the bottle several times. Insert the rubber stopper containing C and start the sample flowing through cell &. Proceed as directed under Calibration to obtain a current reading ( i t s ,total sample current). Record the temperature of the sample, T,, as it passes through the cell. Add 1 gram of sodium sulfite to another portion of sample, and after 15 minutes record the residual current, ars. If the recorded sample temperature is within 2" F. of the calibrating temperature, calculate dissolved oxygen directly : D.O. in sample, p.p.m. = (it8 - ir8)f (5)
If the temperature deviates more than 2" F. from the calibrating temperature, apply the following correction to convert the current to that which mould be obtained a t the calibrating temperature: net current, corr. to T , = (it, - i d at T , 0.012 (T. - T,) 1
+
where
tts zrs
= current at T,,in pa. = residual current after sulfite
T,
= sample temperature,
addition a t T,,in
p?.
F.
0,012 = approximate factor for making
temperature corrections over a short range for capillaries of this type. n7hen the salt content of the sample
is above 0.00166 (about 35 p.p.m. of chloride) and remains constant (*10%), omit the gelatin-phenol-salt solution. Make the calibration solution by airsaturating a synthetic water of the same salt content. Obtain the oxygen content of the calibration solution from the APHA data (1) instead of Table I. If the residual current of the sample water remains the same for a long period, the value need not be determined each time. Sample dilution with the air-saturated gelatin-phenol-salt solution introduces a small error, which is ordinarily below the precision of the method. However, if the true oxygen concentration of the sample is extremely low, this error may have significance. If necessary, this error can be corrected by calculation, knowing the oxygen concentration of the gelatin-phenol-salt solution.
t =
Table 11.
where (T
= interfacial tension between mer-
cury and aqueous solution, dynes per em. 1 = length of capillary, cm. r , = radius of capillary, em. p = net pressure, em. of mercury
It mas shown empirically that the drop time of a capillary placed horizontally instead of vertically is 0.285 times as great, and as the interfacial tension between mercury and water is about 375 dynes per cm. (S), this equation becomes:
DISCUSSION
1.488 X r&
lo-’
for the capillary in a horizontal position. The factors governing M , the flow of mercury per second, are 1
(7)
where
Id
of mercury second, grams
= mass
5.2 3.1 6.1 5.9 6.3
4.9 2.9 6.2 6.1 6.0 1 3 4 5
Stream 2 3.0
3.1 2.5
2.7 14.3 7.9
14.9 8.5
Table 111. Reproducibility of Values on Consecutive Samples Withdrawn from Same 3-Gallon Bottle
1
111 = 4.627 X 106prc4
Determination of Dissolved Oxygen
Dissolved Oxygen, P.P.N. Laboratorv Portable polarograFh analyzer Stream 1
t = drop time, seconds
t =
Application to EWuent Waters. Table I1 compares values obtained on effluent waters on several successive days to values obtained with a laboratory polarograph by a method similar to that of Busch and Sawyer (2). Table I11 illustrates the reproducibility of the method. Principles of Operation. The unique feature of this method is the utilization of a rapid drop rate, obtained by a horizontally mounted capillary. This capillary has a fairly large diameter, which issues relatively large quantities of mercury per second and produces a high current. Its horizontal mounting causes the drop to fall into the solution a t right angles to the capillary orifice. Single measurements are made on a simple and inexpensive microammeter a t a fixed potential of -1.6 volts us. a mercury pool. The horizontal mounting of the capillary promotes a short drop time, and this makes the current less sensitive to external vibration. The life of each drop is so short that even the relatively fast microammeter does not follow the drop oscillations; hence, steady currents can be obtained. A rapid flow of mercury contributes to a high current, which makes i t possible to use an inexpensive and rugged microammeter. I n a subsequent investigation the critical dimensions of this capillary mere studied. It is important to have the right drop time with a given rate of mercury flow. Short drop times produce systems that are insensitive to motion if the mercury flom is sufficiently great. A drop time of 0.20 to 0.33 second per drop is recommended for a minimum flom of 9 mg. of mercury per second. Selecting Capillary Conditions, The factor affecting the drop time of a vertical (conventional) dropping mercury electrode is given by
1.393 X lO-9uZ re3p
Sample Nc.
Dissolved Oxygen, P.P.M.
1 2 3 4 5 6 7
7.58 7.56 7.56 7.53 7.56 7.56 7.53
flo-iving per
Multiplying Equations 6’ and 7 gives Mt
=
0.688r,
(8)
This shows that the drop weight, Mt, is directly proportional to the radius of the capillary and independent of pressure and length. I n practice the limiting dimension will be the radius of the capillary, as it is difficult t o procure capillary tubing of sufficient bore to permit a large flow of mercury with relatively little pressure. Thermometer tubing could be used, but because i t generally has a n elliptical cross section, it would have to be oriented in the same way each time-i.e., if the major axis is oriented up and down, subsequent measurements should be made with this same orientation. Theoretically, the elliptical cross section makes it difficult to calculate drop time or rate of mercury flow, because the radius of the capillary is not definite. If such a capillary is used, optimum capillary lengths and mercury pressures must be determined by experimental means, using the given equations as rough guides. However, if a round capillary is available and its diameter can be determined, the following equation can be used to select the remaining parameters, Substitution of the measured value of the capillary radius (in centimeters) and 0.25 second as the drop time gives
- ,0.1486 E tr2 X 106
1
Std. dev. 0.046
This will yield the ratio of the net pressure to the length of the capillary. If 211 is lonw than 0.009 (as determined by Equation 8), it may be desirable to select another capillary of a slightly larger d i m eter. The relationship shown in Equation 9 is applicable only to the horizontal dropping m(?rcuryelectrode in an aqueous solution. For the conventional dropping n- ercury electrode Equation 10 is applicable.
The merc iry pressure recommended, 28 cm., was selected to yield optimum current values and drop times with the particular crtpillary used, which was a 2-inch section of a thermometer whose cross section measured 0.12 mm. across the major axis and 0.08 mm. across the minor axis. The mercury pool is desirable for field use because of its convenience and because it is not easy to contaminate. With a mercury pool anode the reference voltage mill be largely determined by the chloride concentration of the water. This can cause a voltage shift as mucn as 0.2 volt. The lower the chloride concentration, the more negative the oxygen waves will appear VOL. 31, NO. 4, APRIL 1959
501
to be. Consequently, a voltage of - 1.6 was selected to be sure that measurements are made a t the top of the second oxygen plateau. Large quantities of salt can cause a much more serious difficulty. Orlemann and Kolthoff (8) reported that, in the presence of salt concentrations >0.5N, an anomalous current is produced which is much greater than accounted for by the diffusion current of oxygen alone. This anomalous current was enhanced by short, high drop times and high currents. Since the present technique employs extremely small drop times (about 0.25 second per drop) and high current (about 10 to 20 pa.), the effect described by Orlemann and IZolthoff is greatly accentuated.
ACKNO WLE1:lGMENT
The authors thank ’IValter A. Morgan for suggesting a circuit, which eliminated the voltmeter and the Humble Oil & Refining Co. for permission to publish this paper.
“Polarography,” 2nd ed., p. 552, Interscience, New York, 1952. (5) Kolthoff, I. M., Miller, C. S., J . Am. Chern. Soc. 63, 1013 (1941). ( 6 ) Marsh, G. A., ANAL. CHEM.23, 1427 (1951). (7) Moore, E. W.,Morris, J. C., Okun, D. A,, Sewage Works J. 20, 1041 (19481. - --, . (8) Orlemann, E. F., Ilolthoff, I. &I., J. Am. Chem. SOC.64,833 (1942). (9) Rand, RI. C., Heukelekian, H., Sewage and Znd. Wastes 23, 1141 (1951). (10) Seaman, W.,Allen, W.,Zbid., 22, \-
LlTERATURi CITED
(1) American Public Health Association,
New York 19, N. Y ”, “Standard Methods for the Examination of Water, Sewage, and Indusi,rial Wastes,” 10th ed., p. 254 1955. (2) Busch, A. W., Sawyer, C. N., ANAL. CHEM.24, 1887 (1952). (3) “Handbook of Chemistry and Physics,” 39th ed., p. 2022, Chemical Rubber Publishing Co., Cltrveland, Ohio, 19575s.
(4) Kolthoff, I. M., Lingane, J. J.,
912 (1950).
(11) Truesdale, G. A., Downing, A. L., Lawden, G. F., J . A p p l . Chem. 5 , 53
(1955).
RECEIVEDfor review June 9, 1958. Accepted December 1, 1958. Regional Meeting, ACS, San Antonio, Tex., December 1958.
Po arographic Determination of Dissolved Oxygen Study of Drop Time with Rapid-Dropping Mercury Electrode J. H. KARCHMER Humble Oil & Refining Co., Bayfown, Tex.
F A single capillary was used to study the effect of very small drop times at a constant mercury flow rate upon the polarographic current produced by dissolved oxygen in water. At first the capillary was mounted vertically, then horizontally, and finally its orifice was scratched and it was mounted vertically. This system provided for essentially the same mercury flow rate, but the horizontal electrode had a drop time 0.285 times as great as the vertical, and the scratched capillary’s drop time was 0.0263 times as great. Currents produced in the very low drop time regions were considerably less than predicted from the Ilkovic‘ equation. When the solutions are stirred, the effect on the current is less as the drop time decreases and the mercury flow rate becomes greater. The fundamental equations governing the drop times and mercury flow rate were studied and reduced to simple forms so that predictions involving mercury pressure, capillary length, capillary radius, and interfacial tension could be made in selecting capillaries of desired characteristics. The very rapid-dropping mercury capillary (
