Modified Photoelectric Photometer For Colorimetric Determinations in Water and Sewage Laboratories WILLIAM D. HATFIELD AND GEORGE E. PHILLIPS Sanitary District, Deeatur, 111.
A vertical-type photometer, utilizing; 1 or 2 photronic-type eells and low-form 100-ml. Nessler tubes for absorption cells, is described. The singlecell unit can be built for $26 and the balanced-cell unit for $60, including four light filters with liiht transmissions of 390 to 510, 430 to 540, 490 to 590, and greater than 500 milliiiorons. Other light filters are described for special work, tbe filters costing less than 50 cents each. The two units give praotically the same standardization curves. T h e single-cell unit is tedious in operation, because of 0uctuations in light intensity. The balanced circuit is free from suob fluctuations and costs less than voltage regulators. The low-form 100-ml. Nessler tubes were d e d with inside depths of 25, 50, 75,100, and 150 mm. The
DURING
1939 Miiller and many others ( I , 6 , 7) published excellent summaries, including complete bibliographies, on photoelectric methods and circuits for use in analytical chemistry. Many laboratory supply houses offer excellent photoelectric photometers, costing between $150 and $300 and even $500 or more. These instnunents are of a horizontal-beam type and only the more expensive ones will accommodate absorption cells ~
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optical properties of these tubes (Exax) have been similar, a b u t 50 per cent giving identical readings, and thus the tubes can be used interchangeably. Colorimetric pH can be determined with greater aeouraey than by visual methods, using only 0.25 ml. of indicator in 25 to 100 ml. of ssmple. Turbidi t y is easily determined and compensated for in the same sample. After standardizing the instrument, nitrates, nitrites, ammonia, residual eblorine, iron, m a n ganese, and even iodine in the D. 0.and B. 0.D. determination may be determined from the standardization curves. thus doing away w i t h eolor standards or permanent standards which often do not have the same absorption spectrum as the unknown color.
concentrates and straightens out the light beam sufficiently, 80 that lenses have been found unnecessary. The galvanometer is sensitive to 1 micro-amnere ner mm. scale division. A mater sensitivity would make ihe sitting of the apparatus too &lieate for practical work. As Brice pointed oit, the resistance in the galvanometer and the circuit should be very low compared to the internal resistance in the nhotocells. Therefore the resistance of this galvanometer is 52 ohms and the damping resistance is 15 ohms. The slide-wire rheostat is verniercontrolled and has an approximate resistance of 28 ohms. (This does not have to be exact, so long as the operating resistance is above the damping resistance of the galvanometer.) This rheostat was easy to scale into 100 per cent divisions and the
of the colorimetric det&minatiog in wat& and
the large water &d sewage laboratories.‘ For this reason the authors have developed ‘‘vertcal”-beam photoelectric photometers using one unbalanced and two balanced circuits, and a teehnioue for usine modified short-form 100-d.
Photoelectric Circuits Three photoelectric circuits have been built.
A is the unbalanced simple type described by Hurwita (3)which was built for $26, B is the balanced series photranic cell circuit described by Brice (2), and C is the balanced parallel hotronic cell circuit described by Winthrop, phrewsbury, and Kraybill (9). Circuits B and C are twice as expensive to build but have the distinct advantage of being independent of changes in light intensity due to voltage fluctuations when laboratory heating units OP automatic pumps o on or off the power circuit. However, wit% patience, excellent results may be obtained with A by always checking the blank both before and after each unknown reading and calculating the per cent transmissions, rather than tryiug to keep the machine set at 100 for the blank and reading the transmissions direct from the scale. Photographs of A and B and the wiring diagrams of A, B, and Care shown in Figure 1. The light sowce used in these photometers is a 150-watt Mazds. projector spot bulb which
FIGWE 1. PHOTOMETER^
430
AND
WIEINQ DIAQRAMS
ANALYTICAL EDITION
lune 15, 1941
LAMP PAR-38
114 IN. ASBESTOS 4 IN. AIR SPACE
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FIGURE2 . WIRINGDIAGRAM scale has been found very accurate by calibration against Leeds & Northrup resistance boxes. The photronic cells used are No. 594, Type 1, disk 69,902, which are sold particularly to match with microammeter 301. These cells are about three times as sensitive as those usually furnished when No. 594, Type 1, cells are ordered without specifying the disk number and not purchased with the microammeter 301. The specifications and cost of materials after discount are presented below. 1 1
1 1
4 4
Circuit A Weston photronic cell No. 594, Type 1, D69 902 WeAon microammeter No. 301, 360 ohms resistance iris diaphragm 1.25-inch diameter, Chicago Apparatus Co. 150-watt Maada projector spot bulb PAR-38 Wratten gelatin filters, Eastman Kodak Co., Nos. 12, 45H. 47A, and 61N, a t 40 cents each, Eastman thin cover glassesforlantern slides, 3.25 X 4 and 2 X 2 inches Miscellaneous lumber, screws, switches. etc. Total
Circuit B (preferred) 2 Weston photronic cells KO.594, Type 1, D69 902 1 G - M Laboratories, Inc., No. 2561-D galvanometer (without case) 1 G-M Laboratories, Inc., No. L-28-4.0 vernier action rheostat 2 iris diaphragms, 1.25-inch diameter opening 1 150-watt Mazda projector spot bulb PAR-38 8 Wratten filters, Eastman Kodak Co.. 2 each of Nos. 12 45H,47A, and 61N 8 each Eastma: thin cover glasses for lantirn slides, 3.25 X 4 and, 2 X 2 inches 1 Philco refrigerator door switch, as galvanometer shunt Miscellaneous lumber, screws, switches, eta.
431
100-ohm fixed resistance ($1.25) and a rheostat of 100-ohm resistance which should be balanced against the 100-ohm fixed resistance. I n circuit B the rheostat is not balanced against another resistance and may be 20 t o 30 ohms or less, or just so it does not damp the galvanometer. For these reasons B is preferred t o C. Identical results have been obtained with all three instruments, but because of the inconvenience of voltage variations with A , and the more complicated resistance balance in C, B is recommended,for the operator wishing to build his own apparatus. Details of construction are given in Figure 2. The light source is attached to a 0.63-cm. (0.25inch) pipe (0.94-cm., 0.375-inch, outside diameter) which is firmly attached to the base and frame of the apparatus. The bulb socket is held in a buret clamp and clamp holder, thus affording easy adjustment of the light bulb so that it focuses equally on both photo cells. Switch DSis a Philco refrigerator door switch which shunts the galvanometer whenever the door is opened and absorption cells are being changed. Switch AS and the plus and minus outlets at the rear of the apparatus are not necessary to circuit B. With AS closed the apparatus functions as the Brice balanced cell circuit but with AS open and microammeter attached to the plus and minus outlets as shown, photocell PI acts alone so circuit A , thus affording an easy way to switch from one circuit to the other. In building B AS can be eliminated and the circuit closed a t this point. FILTERS.The Wratten gelatin light filters made by the Eastman Kodak Co. have proved satisfactory and most economical. Although they are not so permanent as glass filters, they are so cheap that they can be renewed frequently. They were placed between two Eastman thin glass lantern slides 8.1 X 10 and 5 X 5 cm. (3.25 X 4 and 2 X 2 inches), and sealed from air and moisture with Scotch cellulose tape used for mounting color films. Each mounted filter cost about 48 cents complete. These slip into the apparatus just under the Nessler-tube absorption cell
WRATTEN LIGHT FILTERS
0
$ 6.30a
I
9.41
5.28
10
1.05 1.60 0.31 2.05 $26.00
$18.900 16.00 6.38 10.54
IO0
z o 0 U J v , I
3i5
IO
a
I-
1.05 3.20 0.62 0.77 3.00
Total $60.46 0 These cells when purchased with Weston microammeter No. 301 list a t $10 less 37%; without microammeter, a t $15 less 37% discount.
CIRCUITC. The same materials are used as listed above for B except for one additional
I 10 100
-
MILL1 MICRONS OF WRATTENFILTEIW FIGURE 3. TRANSMISSIONS
INDUSTRIAL AND ENGINEERING CHEMISTRY
432
TABLE
I.
WAVE
Filter u o .
OF WRAwEN LIGHT FILTERS (Data from Eastman Kodak Co.) Wave Transmission
Millimicrons 2A 12 27A 89 35D 47A 45H 61N 76 75 74 73
Filter Combinations 2A and 35D 47A and 35D 45H and 35D 61N and 47A 61N and 45H 12 and45H 12 and 75 27A and 61N
4104500 560 650 310-480 and 650f 390-510 320-390, 430-540 490-590 320-470 460-520 510-560 560-600 410-480 390-480 430-480 490-510 490-530 500-530 500-520 560-590
Vol. 13, No. 6
100microamperes and with B and C the scale setting is 100. These adjustments are made with the single iris diaphra m when using A , and by adjusting both diaphragms so that the afvanometer reads zero (center) when using B or C circuits. I#’ith the latter circuits identical results are obtained with light intensities which produce 50 to 150 or 200 microamperes current in the photronic cells. However, the larger currents make the zero-blank settings so tedious that most tests are made at the 50-microampere level. The authors have roughly calibrated each filter so that they know the diaphragm opening which gives about 50 microamperes with distilled water blanks 150 mm. deep. With the apparatus, as constructed, the following diaphragm openings give 50-milliampere current from the cells: Wratten Filter No.
Diameter Opening
Per Cent of Area
Mm. 12 45H 47A 61N Wide open
3.5 12. 15. 10. 32.
1.25 14.05
22.
9.55 100.
The adjusting levers of the iris diaphragms should have a friction brake, so that they are not too easily moved for adjustment and so that after adjustment they will not change when the door is opened or closed. The water level in the Nessler tube above Pp is unimportant, since this tube is used only as a light condenser for Pz, but the liquid levels in the blank and unknown must be carefully measured to the desired depth which was used in the calibration of the instrument for the particular determination.
and above the photronic cell and are therefore not easily harmed by heat from the light bulb. These filters have been used for 2 years without deterioration. Although for water and sewage work four filters have served all purposes (Nos. 12,45H, 47A, and 61N) it is convenient to have a rather complete set of filters giving Procedure narrower transmission bands. The transmissions of 12 Wratten filters and combinations of these filters are given in Table I and With instrument A the proper filter is inserted between the Figure 3. Nessler tube and the photronic cell, the tube containing the zero ABSORPTION CELLS. Low-form 100-ml. Exax Nessler tubes blank is inserted, and the iris diaphragm is adjusted until the miwere marked for the authors by the Kimble Glass Co. for inside croammeter reads 100 (the meter specified reads 200, so that this depths of 25, 50, 75, 100, and 150 mm. at a cost of less than $1 reading is in the middle of the scale). The unknown tube is now each, but without the 100-ml. volume calibration. In the fuinserted in place of the blank and the reading of the meter is ture the authors would purchase tubes with both the depth noted. Immediately the blank is reinserted and if it still reads markings and the 100-mLvolume calibration, so that the same 100 the per cent transmission of the unknown is indicated directly. cell may be used for both measuring out the volume and then If the blank reading is more or less than 100 the unknown must removing the amount necessary to leave a depth of 150 mm. for again be inserted and the process repeated until it is certain that absorption cell testing. Between 40 and 50 per cent of the the blank and unknown readings are taken on constant voltage. tubes furnished give identical transmissions when filled with Then the per cent transmission is calculated from the final blank distilled water, and may therefore be used interchangeably. and unknown readings. This greatly faciiitates routine work. However, the cells that PROCEDURE WHENUSINGB A N D C. Put proper “filter pair” differ from the average by a few scale divisions on the galvanomin place, insert tubes of distilled water above PI and P p , adjust eter may also be used, provided the blank and unknown absorpiris diaphragms approximately for filters being used, and turn on tions are made in the same cell. light, allowing 15 minutes for equilibrium to be reached. Replace cell above PI with zero blank of exact depth and adjust iris SCALE. The per cent of light transmitted by the unknown is determined by balancing the current produced by cell PI with diaphragm on the left until the galvanometer reads zero. If the that from Pp, the latter being adjusted by introducing resistance in the P, part of the circuit. The scale for the rheostat is made by dividing the length of the resistance coil (which was 254 mm. on the G-M rheostat used) into 100 equal parts representing per cent current from Pp, the 100 per cent being obtained when there is no resistance in the P) side of the 9000 circuit. The scale is drawn on I a strip of frosted glass and is v) mounted on the rheost’at, so that the 100 per cent is indicated by ,2000 the first deflection of the galvaw nometer as the vernier adjustment 2 is moved to the left. Likewise the zero on the scale may be checked by blocking all the light from PL and adjusting the lower end accordingly. ZERO BLANKSETTINGS. All three circuits are so designed that they read directly in per cent transmission, the apparatus being adjusted to read 100 per cent for the zero blank. This blank should contain none of the unknown substance, and should be treated in the same way and with the same Y. TRANSMISYON reagents as the unknown sample. FIGURE 4. TURBIDITY TRANSMISSIONS With circuit A the blank setting is
June 15, 1941
ANALYTICAL EDITION
433
two filters were found necessary for the entire p H range from 1 to 10; filter 12 was used for thymol blue (alkaline), bromothymol blue, bromocresol purple, and 90 bromocresol green; filter 61N was used for cresol red, phenol red, chlorophenol red, bromophenol blue, and thymol blue 80 (acid). The curves shown in Figure 5 are plotted on standard cross-section paper from data obtained with carefully prez 7o pared Clark buffer solutions, using the exact 0 quantity of indicator and the filter indi3! cated a t the right end of each curve. The 3z 6 0 depth was 25 mm. of buffer, to which 4 was added the amount of indicator F shown. The exact depth of solution may rt 50 vary, so long as the blank has the same depth. I n determining p H the absorption is 40 due to the color and the concentration of the indicator, so that care must be taken to add the exact amount of indicator used 30 in the calibration. One-milliliter pipets graduated in 0.01 ml. were satisfactory in this respect. The concentrations of PH freshly prepared indicator solutions must FIGURE5. PH INDICATOR TRANSMISSIONS be checked because they often vary slightly in intensity. This is easily done bv choosing a buffer a t about mid-point galvanometer is too sensitive cut the light to the cells by reducing on the curve and knding how much indicator is necessary the opening in both diaphragms a little. Having obtained exact to produce the transmission given by the calibration curve. balance, remove the blank and insert the unknown of same depth. For example the new indicator solution may require 0.27 If the color is deep, move the rheostat t o 30 or 50 per cent before ml. instead of 0.25 ml. to produce the transmission reclosing the door to protect the galvanometer. With the vernier quired, particularly when a new supply of indicator salt has control, adjust the rheostat t o exact balance and read the transmission of the sample from the scale. Determine the amounts of been used. unknown from calibration curves. Since it is easy to read the transmissions to within *0.5 per cent, the calibration curves on cross-section paper may CALIBRATIOX.The instruments are calibrated for each easily be read to the second decimal place. However to justify determination by using color standards prepared according to such accuracy one must be sure that the calibration buffers are Standard Methods of Water Analysis (4), except that in genequally accurate. I n general, accuracy greater than 0.1 pH is eral 100 ml. of the color standards are prepared. After the not justified. proper time interval for color formation the standards are COMPENSATION FOR TURBIDITY AND COLOR. The phomeasured into the absorption tubes, usually to the 150-mm. tometer technique easily compensates for turbidity and color depth line, and the transmissions determined. The transin the water or sewage. For example, after determining the mission curves obtained when the results are plotted on semitransmission on a sample 25 mm. deep for turbidity and/or log paper replace all color standards. color, the iris diaphragm is opened, with the rheostat again Determinations set a t 100, until the zero galvanometer reading is again obtained, then the indicator is added with the pipet extending TURBIDITIES. The determination of raw water and sewage almost to the liquid surface, and the transmission is again turbidities is very easily accomplished with the photometer, determined. [The authors have not had the occasion nor using a 25-mm. depth for turbidities of 20 to 3500 p. p. m. facilities to study natural color in water. By the proper and a 50- or 150-mm. depth for the lower ranges from 10 to choice of filters, turbidity and color might easily be separated 400 p. p. m. No attempt has been made to develop a techand determined (6, 8).] With the aid of the calibration nique for effluents below 10 p. p. m. The curves shown in curves the p H is easily determined. With unsettled sewage Figure 4 represent calibration curves made with bentonite turbidities greater than 300 p. p. m. were found to absorb the clay suspensions, the turbidities of which were determined indicator and give false readings, but this difficulty was not with a Jackson turbidimeter. These curves were made using encountered when the sewage was settled to remove the a Yo. 12 filter. Experiments have shown that unfiltered coarser particles. light and filters 2A, 12, and 27A give the same turbidity SLIGHTLY BUFFEREDWATERS. I n order to determine the curves, while the narrower band filters give 1 to 2 per cent p H of slightly buffered waters, isohydric indicators must be greater transmissions. Because the authors’ routine proused or the volume of water must be large compared to the vided for subsequent determination of pH with bromothymol amount of indicator used. The technique using a 25-mm. blue, and because for this indicator they use filter 12, they depth has been satisfactory on the buffered sewage used in used KO. 12 for their turbidity calibration. The samples the authors’ routine; however, 150-mm. depths would do should be introduced into the Nessler tubes with a pipet, SO equally well, thus using only about 0.25 ml. of indicator in that drops are not splashed and do not remain on the side slightly less than 100 ml. of water. I n fact, for water analywalls of the tube. sis, 150-mm. depths would probably be the best for turbidity HYDROGEN-IOKCONCENTRATIOX. The Clark sulfoand p H determination. Some of the curves will be the same phthalein indicators give beautiful calibration curves using for 150 mm. as for 25 mm., but most of them will be changed 25-mm. (or greater) depths i n the absorption tubes. Only
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 13, No. 6
of ammonia and for the nitrite and nitrate determinations (4). However, a fine floc which does not settle easily should be filtered. Experience with the photoelectric cell has made the authors lose confidence in visually estimating color intensities which are masked by slight turbidities or are slightly off color. When necessary, turbidities may be compensated for by determining the absorption due to the turbidity when using a a t e r which transmits light waves t.hat are not absorbed by the colored solution and then correcting the transmission obtained with the filter used for the unknown substance. For example, the nitrite color gives very good absorption with filter 61N, but shows no absorption with filter 35D. Therefore the absorption due to turbidity may be determined with 35D and that amount added to the transmission when using filter 61N. This correction has been found valid for slight turbidities which occur in routine work. However, with proper coagulation the need for this correction may be eliminated. I n Figure 6 the calibration curves with the respective filters are given, the transmissions being d o t t e d against the milliliters of standard solutions ;sed to obtain the colors. In this way all three curves may NITRITE, AND NITR.4TE NITROGEN FIGURE 6. AMMOKIA, be plotted on the same scale, although in each case the number of milligrams of nitrogen per milliliter of standard is different. about 2 per cent. I n p H work the exact volume of water is not important, but the amount of indicator in the tube cell PROCEDURE. A sample of sewage, effluent, or turbid water is must be accurately measured. poured into a 250-ml. bottle to which is added 1 ml. of the coPH OF THICKSLUDGES. It is often difficult, with sewage agulant: 10 er cent copper sulfate, 3.6 per cent ferric chloride, 3.6 per cent Zrrous sulfate, or 3.6 per cent alum. The sample is sludges, to obtain a sufficiently clear supernatant liquor for mixed by inverting 3 or 4 times, 1 ml. of 30 per cent sodium hycolorimetric work, and the dilution procedure (4)is often droxide solution is added, and the sample is mixed by inversion unsatisfactory because the buffer value of the sludge will not and allowed to settle. stand dilution. The authors have found that a clear sample Ammonia Nitrogen. One to 10 ml. of the supernatant liquor are diluted to 100 ml. with ammonia-free water, and nesslerized for colorimetric p H may be easily and accurately obtained with 2 ml. of Nessler reagent. Color formation is rapid in conby diffusion through dialyzer parchment paper or cellophane. centrations greater than 0.01 mg. of nitrogen in 100 ml. and may be read in the photometer after 30 minutes, but with concentraDiffusion cells were made by cutting off the bottoms of 120-ml. tions of 0.001 to 0.005 mg. in 100 ml., 45 to 60 minutes are (4-ounce) glass bottles and covering the opening with parchment required for complete color formation. The 10-minute period paper by means of a rubber band. The inside of the cell was for color formation recommended in “Standard Methods” (4) then filled with aerated distilled water (pH 6.9 to 7.0) and susmay be satisfactory for visual comparison, but the photronic cell pended in a large beaker of sludge, the two liquid levels being is so much more sensitive that in most determinations a lon er about equal. Repeated tests proved that within 15 minutes the period for color formation has been found necessary, especiaty distilled water had reached equilibrium with the sludge, and a with the lower concentrations. After complete color formation perfectly clear solution was available on which to run colorimetric the solution is poured into the transmission tube to the depth of pH determinations. Dialyzer tubing may be used instead of the 1.50 mm. and the transmission determined (having already carecell, although the cell is more easily handled than the tubing. fully set the photometer a t 100 with a zero blank) with light filter This diffusion method is the most satisfactory method the authors have found for Decatur sludges with p H from 5.0 to 7.0 and is recommended as superior to dilution, centrifuging, settling, or filtering.
5
Ammonia, Nitrite, and Nitrate Nitrogens
4
The determination of ammonia, nitrite, and nitrate nitrogens has been accomplished with the photometer with equal and usually greater accuracy than by visual comparisons of color (4) using 50-ml. tall-form Nessler tubes. Slight turbidities interfere with the transmissions of these colors, but by COagulating a 200-ml. sample of the sewage, effluent, or river water with copper sulfate, ferric chloride, ferrous sulfate, or alum, and sodium hydroxide those elements causing turbidity and off colors are precipitated. The clear supernatant liquor without filtration may often be used for direct nesslerization
6
3
9$
2
I
% TRANSMISSION
FIQURE
AND TOTAL IRON 7. FERROUS
June 15, 1941
45H. From the calibration curve, the number of milliliters of standard ammonia solution corresponding to the per cent transmission is obtained and the p. p. m. of ammonia nitrogen calculated as follows, when V equals milliliters of sample used for test: P. p. m. of N H ~-
s
=
-
x 0.01 x ml. of standard solution 10 X ml. of standard solution
Nitrite Xitrogen. Similarly 1 to 10 nil. of the clarified supernatant liquor are diluted to 100 ml. with nitrite-free water to which 2 ml. each of the two reagents are added. The transmissions are determined on 150-mm. depths, using filter 61N after setting the photometer with a zero blank made with nitrite-free water and the reagents. With the aid of the calibration curve the quantities may be calculated. A period of 30 minutes is aIlowed for color formation. Nitrate Nitrogen. The nitrate nitrogen may be determined by either the reduction or phenoldisulfonic acid method. By the latter method 10 ml. or less of the supernatant liquor are treated according t o “Standard Methods” ( 4 ) and the resulting color is diluted to 100 ml. The transmission of the colored solution is determined using a 150-mm. depth and filter 47A. For accuracy quantities of the original supernatant liquor should be taken, so that readings not greater than 4 ml. of standard solution-i. e., transmissions of less than 60 per cent-are obtained. The above photometric methods have checked repeatedly against the reduction method, which the authors have always used for the routine determination of nitrite plus nitrate nitrogen in the final trickling filter effluent-i. e., the sum of the photometric nitrite and nitrate determinations checked the reduction method which was run in the standard visual way. Ferrous and Total Iron. These have been determined by the technique described above using the procedures of “Standard Methods” (4)to produce the color. With ferrous iron the absorptions are so high that the transmissions are determined on 25-mm. depths and with filter 12, but the transmission of the ferric iron color is made at the usual 150-mm. depth and with filter 61N. The calibration curve is shown in Figure 7. Dissolved Oxygen. It was a happy surprise to find that dissolved oxygen could be determined almost as accurately in the photometer as by titration with standard 0.025 N sodium thiosulfate, by measuring the transmission of the iodine color at 150-mm. depths with filter 61N. I n biochemical oxygen demand work no correction for turbidity has been found necessary if the bottles are allowed to settle 30 minutes after acidification and the clear iodine solution is pipetted from the upper portion of the bottle. However, if there is a foreign turbidity, the absorption due to the turbidity is easily determined by adding a few drops (avoiding an excess) of 10 per cent sodium thiosulfate to destroy the iodine color and then determining the absorption of the colorless but turbid solution. The transmission of the turbid iodine sample is then corrected for the absorption due to the turbidity. The calibration curve, Figure 8, was made by preparing a series of dissolved oxygen samples using well water, determinin the transmissions, and titratin 200-ml. samples with 0.025 sodium thiosulfate. Routine 8eterminations of the B. 0. D. dissolved oxygens have also been plotted. This is a double curve, the left-hand portion being read from the scale on the left of the figure.
Residual Chlorine. Also on Figure 8 are two calibration curves for residual chlorine, using o-tolidine and standard chlorine solutions which were made from BK hypochlorite so that 1 ml. contained 0.1 mg. of chlorine. These two curves illustrate the advantage of two different light filters. With No. 47A, residual chlorine may be very accurately determined between 0.1 to 0.5 p. p. m. b u t above 0.5 the curve rapidly becomes of no value. If one is interested in residuals from 1 to 5 p. p. m . filter 61N can be used to advantage.
For routine control of water and sewage chlorination, both day and night the photometer offers a solution.
Kinks in Operation Operation of the photometer is easy and accurate. The light bulb should be directed so that with equd openings in the two diaphragms and equal depths of water in both Nessler tubes, the galvanometer reads approximately zero. The photometer must have reached equilibrium before the zero setting. At least 15 minutes are allowed for the photocells to come to equilibrium. For accurate work care must be taken that the zero blank is representative of the samples and the calibration curve. If the photometer is not properly set at 100 with a good zero blank the determinations are worthless. During the winter months, when the laboratory had a zero humidity, no trouble was experienced from finger prints on the Nessler transmission tubes, but with warm, humid summer weather such fingerprints will throw the results completely off. Taking a tip from the safe blower, the tubes are f i s t wiped off with alcohol and then handled with a pair of old silk gloves. This has eliminated all trouble with finger prints. The insides of the tubes are cleaned with cleaning solution and occasionally with strong lye. It pays to have the tubes scrupulously clean. If the photometer is in continuous use for 2 or 3 hours vapor rising from the surface of the tube above cell P, may collect on the glass above the water and cause a drift in the galvanometer. The tube is taken out and refilled with cool water as full as possible to reduce the area on which the vapor will collect. If others than the parts specified are to be used in the construction, it is important that the resistances in the cells, the rheostat, the galvanometer coil, and the galvanometer damping resistance should be properly correlated.
Literature Cited (1) Ashley, S. E. Q., IKD.EXG.CHEM.,Anal. Ed., 11, 72-9 (1939). (2) Brice, B. A., Rev. Sci. Instruments, 8, 279-83 (1937). (3) Hurwita, E., Sewage W o r k s J . , 11, 134-5 (1939). (4) J o r d a n , H. E., “ S t a n d a r d Methods of W a t e r Analysis”, 8th ed., New York, American Public H e a l t h Association, 1936. ( 5 ) K e a n e , J. C., a n d Brice, B. 4.,ISD. EKG.CHEM.,Anal. Ed., 9, 258-63 (1937). (6) iMellon, M. G., Ibid., 11, 50-5 (1939). (7) Muller, R. H., Ibid., 11, 1-17 (1939). (8) Nees, A. R., Ibid., 11, 142-5 (1939). (9) Winthrop, R. B., Shrewsbury, C. L., and Kraybill, H. R., Ibid., 8, 214-19 (1936). PREBENTED before the Division of Water, Sewage, and Sanitation Chemistry a t the 100th Meeting of the American Chemical Society, Detroit, Mich.
