April 15, 1932
INDUSTRIAL AND ENGINEERING CHEMISTRY
(15) Rubens and Wood, Verhandl. deut. physik. Ges., 9, 88-100 61911). (le) &hack, Z.tech. Physik, 6,530-40 (1925). (17) Schmidt, E.,Beih. z. Ges.-Ing., [I]No. 20 (1927). (18) Schmidt and f i r t h a n n , Mitt, Kaiser-Wilhelm ~ ~,qiseW ~ jorsch. msseldorf, 109,225(1928). (19) Stark, Ann. Physik, 62, 353-67 (1897).
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(20) Worthing, Phys. Reo., 10, 377-94 (1917): Z. Physik, 22, 9 11924).
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September 28, 1931. Presented before the Division of Gas snd t RECEIVED , Fuel Chemistry at the S2nd Meeting of the Ameriaan Chemioal Sooiety. Buffalo, N. Y., August 31 t o September 4, 1931.
Continuous Measurement of pH with Quinhydrone Electrodes-I1 C. C. COONS,Leeds and NorthrupCompany, 4901 Stenton Ave., Philadelphia, Pa. H E q u i n % y d r o n e elecAn improved apparatus for applying the is sufficiently high to clog the quinhy&one electrode to continuousp~ measure- electrode, it has been necessary trode has proved itself to to filter the test sample. Such be an excellent means for ments is described. The error in the reading liquids may be filtered through obtaining continuous pH measurements or pH controlin many Obtained not elcceed *Oao5 p H between a s i m p l e s c r e e n filter. The i n d u s t r i a l installations. For and 7.5 PH. Above 7.5 p H the error usually whitewater isallowed toimpinge increases, becoming about tO.l at 8 p H and on an inclined screen set a t such this purpose it must be installed a pitch that the screen is cleaned in a special apparatus which emabout 0.2 at g p ~ . continuously, the filtered water bodies features The application of the improved apparatus to being passed through the quinfor the successful application of several industrial processes and the various eshydrone apparatus. the electrode. It is the purpose of this paper sentid features contributing to its successful To avoid red water, the p H of potable water is maintained at to describe briefly a few typical performance are discussed. The time lag of the apparatus, when operating under various conthe optimum value by liming. installations of this i m p r o v e d A precise adjustment and an quinhydrone apparatus, to disditions, is also discussed and pointed out as being economical dosage are obtained cuss factors essential for its sucin most applications, but by feeding the lime in accordance cessful installation, to outline the as causing an error Of * 0.2 PH in a few inwith the measurements resulting operation of the apparatus, and from the use of the special quinto give particular attention to the stallations. hydrone apparatus. design,of the important parts of the apparatus. Various factors such as accuracy, pH range of Many boiler-plant operators are employing pH control in electrode, limitations of the apparatus, and the time lag of the the treatment of feed water with alum. For a given water the optimum flock occurs a t a definite pH value. It is a apparatus in following changes in pH will be discussed. well-recognized fact that corrosion in boilers and pipe lines is FIELDOF APPLICATION accentuated by incorrect pH conditions (3). Some states require industrial waste waters to be above a For continuous, industrial pH measurements the quinhydrone electrode is being successfully employed in the range specified pH value (usually the methyl orange end point) of 0 to 8.0 or sometimes 9.0 pH. The electrode may be before being dumped into the waterways. The pH of such applied to any solution within this range provided oxidizing or reducing agents which cause a too serious error in the quinhydrone potential are absent. Whether the electrode can be applied in the range from 8.0 to 9.0 pH depends upon the accura,cy desired. I n most dilute industrial solutions the electrode may be used profitably for the continuous control or improved quinhydrone electrode used for continuous pH measurement of pH (9) are to be found in textile industries, measurements is accurate within *0.05 up to 7.5 pH. Above dye works, color motion-picture film industries, molasses this pH value the error usually increases gradually, becoming fermentation plants, oil refineries, settling and flotation, and *O.l a t 8.0 pH, and approximately *0.2 at 9.0 pH. An work involving neutralization reactions. individual quinhydrone pH measurement, of course, can be IMPROVED APPARATUS made with much greater accuracy in the range 0 to 7.5 pH. However, for most industrial processes the continuous quinI n a previous paper (1) three important characteristics of hydrone electrode has more than sufficient accuracy. the quinhydrone electrode employed for continuous pH The improved quinhydrone electrode apparatus, pictured in measurements were discussed. It was shown (1) that in Figure 1, is limited to solutions to which the quinhydrone order to obtain reliable quinhydrone potentials it is essential electrode itself is applicable and to solutions which will pass to maintain, continuously, a quinhydrone concentration of a t freely through the apparatus. Solutions containing sus- least 7 mg. per 100 cc. in the flowing test solution; (2) that by pended materials that retard or stop the flow or those that have giving proper consideration to the solubility of quinhydrone considerable foam cannot be measured satisfactorily with this and its rate of solution under various conditions, provision apparatus. I n many cases suspended matter may be filtered could be made for the continued maintenance of the correct from the solution easily. For example, in the paper industry quinhydrone concentrations; and (3) that a platinum electrode the continuous quinhydrone electrode is being applied success- is superior to a gold electrode for continuous, quinhydrone, pH fully to white water, but in those cases where the fiber content measurements.
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The improved quinhydrone electrode apparatus includes in its construction and operation not only these important features but also several other unique features in design which
Vol. 4, No. 2
variation in capillary sizes and viscosity changes in the quinhydrone solutions, the pressure .head may be adjusted by moving tube A-6 in or out of tube B-4, as the case may require. Once made, this adjustment for a particular solution is permanent. The glass capillary tube has an inverted, flared head to prevent small solid particles from entering the bore of the capillary. The glass tube, B-2, which surrounds the capillary tip, not only serves as a protection for it, but also retards the evaporation of the solvent from the quinhydrone solution. For proper operation of the unit, the flow is adjusted so that about 300 cc. of quinhydrone solution enter the test solution in 24 hours, or approximately 18 drops per minute Since the solution of quinhydrone contains 30 grams per liter of quinhydrone, and since the test solution is maintained at a flow of 80 to 100 cc. through the flow channel, a simple calculation will show that a concentration of quinhydrone sufficient for a pH measurement exists in the test solution a t all times.
FIGURE1. IMPROVED QUINHYDAONE ELECTRODE APPARATUS
are discussed below. I n general, the apparatus, as diagrammed in Figure 2, is operated as follows: The solution of which the pH is t o be continuously measured is led into the constant head apparatus, D-1, by tube D-2, and a constant flow of test solution is conducted from the apparatus by the curved rubber tube to the side arm of the flow channel, F-I. At this point, a solution of quinhydrone is added, a t a constant rate, to the test solution as it passes into the flow channel, F-I, flowing downward and then upward through the curved overflow tube, F-2 and F-3. The potential difference between the platinum electrode, E-8, and the calomel cell, E-7, which are suspended in the flow channel as a single unit, is a direct measure of the pH since the voltage is automatically corrected for temperature variations by a temperature compensator (shown in Figure I), which is suspended in the flowing solution in the constant head apparatus. QUINHYDRONE SOLUTION FEED APPARATUS.One of the important parts of the improved quinhydrone apparatus is the unit for introducing a quinhydrone solution into the flowing test solution. The quinhydrone solution which is contained in the reservoir, A-I, with a tight stopper, A-6, is led into the glass tube B-4, in which is situated an alundum filter cup, B-3, through pinchcock A-2. Immediately below the alundum cup a capillary tube, B-I, is cemented into an extended portion of tube B-4. The rubber stopper, A-4, situated in the top of tube B-4 bears a tube, A-S, which is open to atmospheric pressure, and also the glass tube A-6, which conducts the quinhydrone solution from the reservoir to tube B-4. The level of quinhydrone solution in B-4 is fixed by the position of the lower orifice of tube A-5. When the quinhydrone solution level falls below the orifice of tube A-6, air enters the reservoir displacing some of the quinhydrone solution which flows into B-4, reestablishing the former level. The rate a t which the quinhydrone solution passes from the capillary tube into the test solution is governed by the pressure head on the capillary, all other influencing factors being maintained constant. However, in order to allow for slight
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FIGURE2. D I A G A A OF~ IMPROVED QUINHYDRONE ELECTRODE APPARATUS The solvent for quinhydrone may be acetone, ethyl alcohol and acetone, or methyl alcohol. In any case, the viscosity of the solution is adjusted by the addition of distilled water. In Table I are given the data for various quinhydrone solutions which may be used with the apparatus. The temperature range refers to the temperature of the quinhydrone solution and not to the test solution. If the temperature of the quinhydrone solution falls much below the lower temperature limit, not only will quinhydrone be crystallized from the solution, but the increased viscosity will prevent a sufficient flow of solution through the capillary. A heating unit (for example, an electric lamp) placed close to the quinhydrone unit will maintain the temperature above the lower limit,
April 15, 1932
INDUSTRIAL AND ENGINEERING CHEMISTRY
TABLE I. QUINHYDRONE SOLUTIONSFOR CONTINUOUS PH MEASUREMENTS
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(see Figure 3). At b the city water was exchanged for the acid water, the pH dropping rapidly and then more slowly (Quinhydrone concentration, 30 to 31 grams per liter of solvent) until constant at c. At d the original water was allowed to TEMPERATURE enter the flow channel in place of the acid water. The pH SOLVENT RANQE increased rapidly and then more slowly until constant at e. a c. 10-40 800 cc. of 95 methanol + 200 cc. of distilled water It will be noted that the lag in the direction of a decreasing 900 cc of 95 methanol + 100 cc. of diatilled water 0-20 pH i s about a fourth of the lag in the direction of an increasing 900 cc.' of 95 ,"ethanol + 100 cc. of acetone (government de13-40 natured alcohol No. 23A) pH. For the explanation of this difference, consider the 10-40 700 cc. of acetone + 300 cc. of distilled water 0-20 800 CC. of acetone + 200 cc. of distilled water following example involving two 100 cc. volumes of distilled mater, one with a pH of 7.0 and the other previously having THEELECTRODE UNIT. The saturated calomel electrode and the platinum electrode are held suspended in the flowing test solution by a rubber stopper. The rubber stopper is split vertically on one side, permitting the platinum electrode to be removed easily for occasional cleaning. A semi-circular strip of platinum foil serves as the electrode proper, by which the quinhydrone potentials are measured. The platinum foil is welded to a platinum wire which is sealed into a glass stem. Within the glass stem a copper lead wire is welded to the platinum wire. Sealing wax within the glass stem is fused in intimate contact with the platinum-glass seal. The wax serves a dual purpose in that it first almost entirely eliminates the cracking of the seal, which otherwise occurs frequently, during the cleaning of the electrode, and second, if the seal does crack slightly the sealing wax prevents solutions from coming into contact with the copper wire. In some instances this occurrence has been the cause of serious QUINHYDRONE APPARATUS FIGURE 3. ABILITY OF IMPROVED discrepancies in pH measurements. TO FOLLOW CHANGING PH The platinum electrode proper is adjusted so as to be situated entirely above the peripheral opening of the ground had its pH lowered to 3.0 by the addition of sulfuric acid. A joint of the salt bridge, This precaution, as well as a down- simple calculation or a pH measurement will show that if 1 cc. ward flow of test solution, prevents excessive concentrations of the acidified water is added to the distilled water a large of potassium chloride from coming into contact with the pH change will occur. However, if 1 cc. of distilled water is platinum electrode. It was found that in designs which did added to the acidified water, the pH of the solution is practinot observe this precaution, erratic potentialswould result from cally unchanged. the contact of potassium chloride solution with the metal Essentially the same conditions prevail in the interchange of electrode. Erroneous potentials were especially prevalent in solutions as described above. The time required for the poorly buffered solutions. acidified water to reach the metal electrode is but a few The platinum electrode of the calomel cell which dips into seconds, and with its ability to effect a rapid pH change the the mercury-calomel paste is held in position by a rubber lag is small. However, in order that the pH be returned to its tube, permitting the electrode to be removed easily and to be former value of 6.6 all of the sulfuric acid must be washed from flexible to shock. The saturated potassium chloride solution the flow channel. Hence, the lag in this direction is much in the vessel is maintained a t a level higher than that of test larger and is actually a measure of the time required to remove solution in the flow channel. Thus, since a one-hole rubber Ifuric acid from the flow channel. stopper is used in the calomel vessel, there results a slow seepfference in pH value of the two solutions is made less age of the saturated potassium chloride solution through the omes correspondingly smaller. Thus, if the lag is ground joint. This seepage, together with potassium chloride measured between two solutions similar to the above except diffusion from the salt bridge, maintains the liauid . junction " that their pH values are 6.6 and 5.6, the lags are respectiveiy potential at a small and p r a c k h l y constant value. those given in Figure 3 . buffered solution flowing through the flow TIMELAG ced by a strongly buffered solution of a differe lag is small even though the pH of the bufThe time lag of the apparatus in following a changing pH is higher than that of the unbuffered solution arbitrarily defined as the time required for the pH of the test solution to attain its new value in the neighborhood of the In reversing the conditions a longer lag, of course, will result. platinum electrode, the time being measured from the instant The amount of lag will depend upon the degree of buffering the test solution with a new pH value enters the side arm of the and the magnitude of the p H change. From this discussion it appears that serious operating flow channel. The amount of lag is dependent largely upon three factors: first, the direction in which the pH is changing; difficulties might arise because of the-inability of the quinsecond, the magnitude of the pH change; and third, the hydrone electrode apparatus to follow a rapidly changing pH. However, such is not the case, because not only are large, degree of buffering of the solution. Considering first the direction of pH change, attention is rapid pH changes rare in industrial installations, but also it directed to an extreme example in Figure 3. This curve was will be noted that even in the extreme example of Figure 3, copied directly from the recorder paper and was the result of the major portion of the lag occurs after the pH has changed interchanging quickly the flow of city water entering the flow to a value within 0.2 pH of the correct one. The actual time channel with a sulfuric acid solution in city water. The flow required in this extreme case for the pH measurement to in each case mas 100 cc. per minute, the volume of water in the approach within 0.2 pH of its time value is about 2 minutes for the decreasing pH change and about 10 minutes for the flow channel being about 120 cc. To be more explicit, the quinhydrone apparatus had been increasing pH. I n the most severe industrial test of the operating on city water having a pH of about 6.6 from a to b quinhydrone apparatus studied to date, the pH as measured
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by the quinhydrone apparatus was never more than 0.1 pH different from that of the solution entering the flow thannel. Another important fact to be recognized in this connection is that in the majority of industrial installations an endeavor is made to maintain the pH value of a solution within narrow limits. I n such cases, the time lag becomes negligible.
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LITERATURE CITED (1) Coons, c. c.,IND. ENG.CEEM.,Anal. Ed.,3,402 (1931). (2) Greer, W. N., Canadian Chem. Met., 15,239(1931). (3)Joos, C.E.,Combustion,3 (4),25 (1931). (4) Richardson, K.8 Chem. Met. EW.9 37,293 (1930). RECEIVBID November la, 1931.
Examination of Electrodeposited Metals and Alloys with X-Rays H. KERSTEN, University of Cincinnati, Cincinnati, Ohio
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HE methods of x-ray crystrll structure analysis may be
used for examining electrodeposited metals and alloys to determine their structure, approximate chemical composition, approximate thickness, and relative grain size.
FIQURE1. X-RAYCAMERAFOR EXAMININQ ELECTRODEPOSITED METALS The theory of the methods has been described in several books on the subject @,S, 6,9,IO) and need not be repeated here. The special apparatus required is illustrated in Figure 1 where the x-rays from a slit at B strike the sample clamped in such a way a t D that its surface is on a diameter of the hoop A . The x-rays strike the sample at the
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taken with the radiation from a special gas-type x-ray tube (6)having an iron iarge-t; operated a t 50 milliamperes and 20,000 volts. At this voltage and current each picture required an exposure of onehalf hour. All the photographs shown are full size, but much of the detail observable in the original negatives cannot be seen in the printed pictures. The strong white line which can be seen a t the left of each picture was made by the main beam comh g from the slit before the sample was clamped in place. If an object is electroplated with an alloy solely to give it a desired color, as is usually the case, a comparison of colors is the onlv examination needed to determine whether.& deposit is satisfactory. I n the cases of alloys which have a constant
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color for a wide range of compositions, it is sometimes possible to determine their composition with a sufficient degree of accuracy by means of the methods of crystal structure analysis without stripping the deposit or damaging the electroplated object in any way. With this method it is the crystal structure of the surface layer and not its color of chemical composition which is examined. The relation between structure and chemical composition for many alloys has been investigated. A collection of these results has been published in two volumes by Neuburger (7, 8). Figure 2 shows photographs of zinc, two kinds of brass, and copper taken with the camera described above. A comparison of these with those given by Westgren and Pragmen (11) indicates that one of the brasses belongs to the alpha phase and the other to the alpha-plus-beta phase. These samples were plated from baths having the following compositions: ALPHA (1): Copper cyanide (CuCN), grama.. Zinc cyanide Zn CN I) grama... Sodium cyani6e (hadN\ grama.. Sodium oarbonate (NazCbs), grama.. Water, liter Current density, 0 . 6 ampJdrn.2 Temperature, 23O C.
............. 27 ............. 9 ............. 54 .......... 30 ................................. 1
FIQIJRE 2. ELECTROPLATED BRASS.Top, zinc; upper center, alpha brass; lower center, alpha-plus-beta brass; bottom, copper
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