Determination of Residual Chlorine in Metal Finishing Wastes HENRY C. MARKS
AND NOEL S. CHAMBERLIN Wallace & Tiernan Co., Inc., Belleville, iV. J.
Methods for determining residual chlorine in water and sewage are frequently not suitable for industrial wastes. In each case it is necessary to examine the applicability of known methods and perhaps to devise modifications. The present discussion concerns experiences with several types of plating wastes, showing how the usual methods can be modified to meet particular situations. When it is unnecessary to distinguish between free available chlorine and combined available chlorine in the treatment of cyanide wastes, any of the usual meth-
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S THK expanding progiam of industrial waste treatment,
chlorine plays its important part. Frequently it is used as a stinitary mcasure on the final effluent from a waste treated with another chmiicnl agent or a biological or physical process. I n other cases chlorine itself may be the actual destructive or modifying agent. As usual, reliable analytical control is necessary to obtain the desired result and the most economic use of chemicals. This frequently means that the residual be controlled to a given concentration of free available chlorine as contrasted to chloramine. The methods for determining residual c-hlorine in water or se\+age are not necessarily satisfactory for this purpose. There may be :tdvprse conditions and interfering substances utterly unlike thosca normally encountered. \Vhere such conditions are known or suspected, it is necessary to examine the applicability of the usual methods carefully and to devise proper modifications. The object of this discussion is to present typical examples of special coriditions and the possibilities of modification of :wailable nwthods to meet these conditions. CYANIDE WASTES
The treatment of cyanide wastes from plating or metal-treating operations is an example wherein chlorine is used as the actual destroying or modifying agent. The highly toxic and objectionable nature of the waste product makes it very important to carry the chlorination to the proper point. Careful control and good caontrol methods are necessary to ensure complete treatment of the toxic material a t all times and to make the most economical use of the chemicals used. The problem of detrrmining residual chlorine in this waste varies with the exact composition of thr waste being tieated and the desired result. \Vhether or not it is necessarv to determine free availal)le chlorine separately depends on TI hrther heavy metsls are present and on whether the cyanidr is being completely destroyed or merely converted to cyanate. \Vhen the waste is free of heavy metal cyanides and where it is required only to convert cyanide to cyanate, only a chloramine residual of sufficient magnitude is necessary a t the end of the treatment. I n this case, control of chlorination consists merely in determination of total residual chlorine with no attempt to determine free available chlorine separately. Any of the usual residual chlorine tests may be applied without particular complication. The o-tolidine method, the amperometric titration ( d - 4 ) , or the ordinary iodometric titration can be used. If the waste contains heavy metal cyanides, or even if these are absent but the cyanide has to he completely dcetroyed, more
ods for residual chlorine in water are suitable. For distinguishing between the two forms of available chlorine, only an amperometric titration procedure properly modified is satisfactory. In wastes containing chromate it is necessary to resort to an amperometric titration procedure which is carried out within certain definite experimental restrictions. Of the other heavy metals likely to be encountered in the plating wastes, cuprous ion, silver ion, and high concentrations of cupric ion interfere with the amperometric titration.
careful control is required. To ensure the desired end it is necrssary to have a definite residual of free available chlorine after the treatment is completed. Othern-ise, there is no certainty that a11 the cyanate will be destroyed and that all the insoluble cyanides will be 0xidizc.d. This means that a method suitable for distinguishing free available chlorine from combined chlorine must be used. Tho two methods noTv in general use for this purpose are the OTA method ( 1 ) and the amperometric titration with phenylarsene oxide (4). There is a serious limitation on the use of the OTA test for thip purpose. A sizable fraction of the combined chlorine reacts with o-tolidine as rapidly as free available chlorine. It is not feasible to apply a correction factor, as Moore suggests for water ( 6 ) . For one thing, t,he speed of reaction of combined chlorine with the o-tolidine reagent depends on the relative proportions of monochloramine anti dichloramine. Table I shows the results of tests with these two substances a t several temperatures. I n each case the particulitr chloramine was made in solution a t a concentration of 100 p.p.m. The proper amount to give concentrations of the order of 1 to 3 1i.p.m. \vas t,hen added to distilled water at the right tempcwturt: just ilefore starting the OTA procedure. Colors were wad in n caomparator with 26-ml. tubps, using :I 15ml. sample and 0.6 nil. of o-tolidine solution. The arsenite w i s added as quicskly as possible, n-hich is estimated to be 5 to 7 seconds. The values given are averages of several determinatioiis. There is easrnti:il agreement with Moore's data which appl>- to periods of trictitration with phrnylarsene oxide is free of
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ANALYTICAL CHEMISTRY
this fault. High concentratioris of chloramine in either form give no t’itrntion in the absence of iodide ion and in the vicinity of pH 7 . Under these conditions the free nvai1:ahle chlorine or hypochlorous acid can be titrated n-ithout interference. hlodificntion of the usual procedure is required because the totnl residual often is estremely high. Since the electrodes are usually designed for sensitivity :it IOK concentrations, rsposure to vcry high concmtrat.ions of availahle chlorine causes a film-type po1:trizntion which reverses v n y sIo~1-1y.This is avoided by dilub ing with demand-free m t e r until the solution titrated contains no more than 10 p.p.m. of available chlorine. Too much dilution naturally reduces the precision of the free chlorine determination. 4 free chlorine content, of as low as 0.2 p.p.m. can be determined satisfactorily. .It the end of chlorination the total chlorine may be as high as 500 p.p.m. A fiftyfold dilution means that chlorination would have to be carried far enough to give 10 p.p.ni. of free available chlorine in the waste. \There the total chlorine is less, the analytical method will permit adequat,e control with correspondingly less chlorine. This is important, because to obtain 10 p.p.ni. of free available chlorine requires the addition of a much greater quantity. Another modification is necessary \vhen the ratio of combined to free chlorine is as high as this. Even after dilution, the current produced by the titrating cell may be so great that the microammeter will not read on scnle wen a t the end point in the free chlorine titration. Recent experiments show that dichlorsniiiie is particularly active, producing four to five times as muc,h current as nionochloramine. I t is possible to titrate free chlorine in the presence of up to 25 p.p.m. of monochloramine, but the dichlornmine must be less than 5 p.p.m. The remedy is to apply opposing voltage to make the proper change in the zero point of the meter scale. Because the usual amperometric titrator circuit does not permit the use of sufficient opposing voltage, an out,side voltage source must be inserted. In the usual circuit, voltage obtained from a suitable winding in tlie agitator motor is passed through a rectifier and then impressed on the indicating microammeter in opposition to the voltage of the cell. This opposing voltage can be increased to any desired value by disconnecting the leads from the rectifier and connecting them to another source, such as a 1.5-volt dry cell. K i t h these modifications the amperometric titration procedure is suitable for determination of free chlorine to ensure complete destruction of cyanides. The method has been proved in practice a n d is in common use for this purpose. CHROME WASTES
Problems in the determination of residual chlorine in wastes arise through contamination with chromate-bearing wastes from the plating industry. Chromate may be present in a cysnide waste and complicate the select,ion of the proper degree of chlorination. I n other instances chromate may be .found in a mhed waste which is being chlorinated for sanitary or other purposes. Chromate ion interferes with the o-tolidine test not only in an optical sense but also by reaction to produce the same colored oxidation product as chlorine. The color of the chromate ion itself can be readily compensated in the usual way by using as a blank a sample without o-tolidine. The second type of interference is the more serious. The magnitudes of the possible errors are shown in Table 11. I n these tests chromium &-aspresent as potassium dichromate dissolved in distilled water. In the table the column headed “Color Equivalent” shows the effect of the chromate color itself with no o-tolidine added. In determining the flash and maximum o-tolidine readings, the chromate color was compensated for by placing some of the acidified sample in the proper position in the comparator. Even 20 p.p.m. of chromium as chromate produces appreciable error. This error will be important in any case where the chromium concentration is a t least two to three times that of the residual chlorine.
Table 11. Effect of Chromate on o-Tolidine Test (Parts per million) Color Eqiiirnlent Flash Reading 0.50 0 50 0.30 0 1;
Chromate 100 50 20
RIax. Reading 2
2
0 Oi
0.12
O.G
Talde 111. Amperometric Titration in Presence of Chromate pH 4 4 4
Chromate, P.P.11. 0 100 0
100 0 100 0 100
P.iO, 111. 0 97
0.97
0 98 0 97
0.9B
0.93 0 92
0.91
The amperometric titration method as applied to residual chlorine might be expected to include other oxidizing agents as well. Since chromate is a fairly powerful oxidizing agent, it was necessary to conduct a series of experiments to determine whether proper conditions could be found to determine residual chlorine in the presence of chromate. It was immediately found that helow pH 4 chromate ion poisoned the platinum electrode with l o s ~ of sensitivity. At higher pH values the electrode is not affected in this way nor is there any change in the current flowing through the cell with moderate concentrations of chromate. Other m periments showed that chromate does not liberate iodine at the concentrations of potassium iodide used in the chlorine determination as long as the pH is 4 or above. I n a typical experiment 5 1111 of 5% potassium iodide solution was added to 1 liter of nnter buffered a t pH 4. At various intervals 200-ml. solutions ere titrated amperometrically with phenylarsene ouide. Even aftw 15 minutes no titratable iodine Ras found. In the same way it \v:iq demonstrated that chromate ion is not titrated with phenvlarsene oxide in the absence of potassium iodide in this pH range. Conforming to these requirements, a series of titrations of low concentrations of free iodine with and without added chromate was performed. The propw amount of stock iodine solution R as added to a quantity of distilled water buffered a t the desired pH, and 200-ml. aliquots of these solutions were then titrated with 0.00564 X phenylarsene oyide solution amperometrically, 100 p.p.ni. of chromium as chromate being added to alternate d i quots. The results a t pH 4 and i are given in Table 111. The absence of any effect means that the amperometric titration for hypochlorous acid, monochloranline, and dichloramine separately according to the usual procedure can be performed \\ ith no difficulty in the presence of as much as 100 p.p.m. of chromium. I t is safe to conclude that cahrDmate will have no effect either on the electrode or on the phenylarsene oxide during the titration for hypochlorous acid n t pII 7 in the absence of iodide LikeNise, there will be no interference in the determination of monochloramine and dichloramine in the preRence of potassium iodide a t pH values of 7 and 4, respectively. The amperometric titration procedure as modified for sewage cannot be used in the presence of chromate. While electrode poisoning may be avoided by niaintaining the pH above 4, serious error arises from another source. The excess phenylarsene oxide reacts with chromate to an appreciable extent before it can be titrated with standard iodine. When in accordance with the regular procedure 5 ml. of 0.00564 it’ phenylarsene oxide and I ml. of 5% potassium iodide solution \!ere added to 200 ml. of water containing 100 p.p.m. of chromium a t pH 4, there was a loss of 20% of the phenylarsene oxide in 4 minutes, 35% in 10 minutes. and 67% in 30 minutes. Since several minutes may elapse between addition of phenylarsene oxide and titration of tlie excesq, the method cannot be relied upon even as an approximation. I n general, any procedure where an appreciable concentration
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V O L U M E 2 4 , NO. 1 2 , D E C E M B E R 1 9 5 2 of phenylarsene oxide does not have to remain unchanged in the presence of chromate will give satisfactory results, provided the necessary conditions are fulfilled. This means that the amperometric titration nil1 be satisfactory in the presence of chromate wherever there is no interference from organic matter. It is believed that most wastes containing chromate will meet this requirement and can be analyzed by the direct titration. There may be instances where chromate gets into selvage and remains in the presence of organic matter for sufficient length of time to cause difficulty. As these instances are probably rare, the amp’romrtric titration procedure is the method of choice.
as 5 p.p.m. gave an enormous increase in current a t p H 4 and 7 and almost as much increase a t pH 2. There was complete poisoning of the electrode a t all p H values. It is concluded that the amperometric titration cannot give reliable results in the presence of cuprous ion and silver ion. Fortunately, neither is likely to be present in sufficient concentration once available chlorine persists in the solution. Cupric ion increases the current, so that a t high concentrations measures have to be taken to compensate for this current. The poisoning effect of cupric ion probably would not interfere too seriously unless there was continuous exposure to high concentrations SUMMARY
HEAVY SlETAL IOKS
The effect of various metallic ions on the amperometric titration procedure is of particular importance because they frequently are present where this procedure is to be used. ,4 series of the most common metallic ions was tested for their effect in two w-a>s. First, the relation between concentration of metal ion and current through the polarized cell was determined a t pH values of 2, 4, and 7 . In addition, elcctrode poisoning as detected by measuring the increase in current through the cell per unit concentration of iodine a t pH i both before and after exposure of the cell to the metal ion. Cadmium, trivalent chromium, divalent nickel, and zinc in concentrations up to 1000 p.p.m. had no immediate effect on the current flowing nor did they Fhovi any tendency to poison the electrode. Cuprous copper as the complex cyanide increased the current at pH 2, 500 p.p.ni. of copper giving an increase approximatply equal to that given by 1 p.p.m. of iodine. This concentration of copper shon-ed a definite poisoning effect on the electrode a t all three pH values. Cupric ion increased the current flowing a t all p H values, 500 p.p.m, of copper being equivalent to somexhat less than 1 p.p.m. of iodine. -it pH 4 and 7, 500 p.p.m. of caopper showed a slight poisoning tendency on the electrode. Silver as the complex cyanide did not give an increase in cell current a t p H i nor did it shox any poisoning effect a t this pH. .4t pH 2 the complex ion was apparently decomposed, since there were both an increase in current and a poisoning effect. This conclusion is based partly on the result with silver ion, where as little
The applicability of a given method for determination of residual chlorine in cyanide n astes depends on the detailed compoeition of the n-aste and the desired result. Khen heavy metal cyanides are absent and cyanide is to be converted to cyanate it is unnecessary to distinguish betxveen free available chlorine and combined chlorine. The o-tolidine method, amperometric titration, and iodometric titration are all satisfactory. When heavy metal cyanides are present or simple cyanide is to be completely destroyed, a definite concentration of free available chlorine is neccssary. The amptrometric titration with certain modification< is the method of choice. Chromate ion interferes 11-ith the 0-tolidine method, so that in chromate-bearing v astes the amperometric titration is used for determination of i esidual chlorine. Of the heavy metal ions present in plating wastes, cuprous ion, silver ion, and high concentrations of cupric ion interferc n ith the amperomrtric titration of residual chlorine. LITERATURE CITED
(1)
Gilcreas, F. IT., and Hallinan, F. J., J . A m . W a t e r W o r k s Assoc.,
36, 1343-8 (1944). ( 2 ) Mahan, R. -4., Water & Seuage W o r k s , 96, l i l - 4 (1949).
13) Marks, H. C . , and Glass, J. R., J. Am. Water W o r k s Assoc.. 34. 1227-40 (1942). (4)
Marks, H. C., TTilliams, D. B., and Glasgow, G. U., Ibid., 43,2017 (1951).
( 5 ) Moore, E. IT., R’ater & Seuage W o r k s , 98, 130-6 (1951). RECEIVED for review May 14, 1952. Accepted October 30, 1952.
Determination of the B.O.D. of Sewage and lndust rial Wastes with the Polarograph ARTHUR W. BUSCH AND CLAIR N. SAWYER Sedgwick Laboratories of Sanitary Science, Massachusetts Institute of Technology, Cambridge, Mass.
T
HIS paper reports the results of an investigation to determine the practicability of using the dropping mercury electrode (polarograph) for dissolved oxygen measurements in B.O.D. (biochemical oxygen demand) studies. Recent studies by Ruchhoft and his colleagues (13-16), Gotaas ( d ) , and the Committee on Sanitary Engineering of the Kational Research Council ( 2 ) have shown great variations in the velocity constant of deoxygenation rates in polluted waters, thus indicating the need for the actual determination of deoxygenation rates for proper evaluation of B.O.D. data and stream conditions. Such determinations are usually based on daily B.O.D. values obtained over a 7-day period, and, therefore, multiply considerably the number of dissolved oxygen measurements required especially n hen replicate samples are set to obtain statistically reliable data. Under such conditions, use of the Winkler ( 1 ) method for measuring dissolved oxygen represents a laborious and time-
consuming procedure. Furthermore, analysis of some industrial wastes requires special modifications of the TVinkler procedure rrhich add to the labor and time involved. Application of the polarograph to dissolved oxygen m4asurements in B.O.D. studies requires control of two factors of prime importance : 1. Elimination of residual current determinations which require much more time than does the Winkler test. 2. Elimination of the addition of chemical reagents which preclude subsequent use of the Winkler test on the same samples to obtain calibration and/or correlation data.
The polarographic method of chemical analysis, developed by Heyrovsk? (8),is based on an interpretation of the current-voltage curves that are obtained when solutions of electroreducible or electro-oxidizable substances are electrolyzed, with one elec-
