Automatic Sulfate Ion Analyzer H. N. CLAUDY,l F. W. KARASEK, B. 0. AYERS, and J. G. SKINNER Phillips Petroleum
Co., Bartlesville,
Okla.
b The construction and operation of an automatic sulfate ion analyzer for use in the wet process for the manufacture of phosphoric acid are described. The analyzer operates b y continuous turbidimetric measurement of a colloidal sol resulting from the precipitation of sulfate ion as barium sulfate. It i s capable of stable and accurate operation, affording a greatly improved means of plant process control, and shows promise for application to other continuous analytical processing probloms not possible b y conventional instrumentation.
normal 2.5% excess may result in severe diEcLlties in plant operation, the excess sulfuric acid concentration must be constantly checked. Intermittent analysis for excess sulfuric acid has been carried out by precipitation of sulfate ion (both from sulfuric acid and from soluble calcium sulfate) by the formation of insoluble barium sulfate, volumetric measurement of the quantity of precipitate, and use of an empirical correlation betiyeen total sulfate ion and excess sulfuric acid concentrations. Although this analysis is satisfactory as a laboratory procedure,
I
wet process for phosphoric acid manirf;icture, phosphate rock reacts R ith sulfuric acid, N THE
+
it is not adequate for plant use, because of the need for a continuous.check on the sulfuric acid concentration. Therefore, development of a continuous sulfate ion analyzer which would be stable and accurate was undertaken. TURBIDIMETRIC SULFATE ION ANALYSIS
A turbidimetric measurement of precipitated barium sulfate concentration was used in the automatic sulfate ion analyzer. This technique has perhaps been less frequently employed than other methods as a result of the diffi-
HOT NEUTRAL GRO
+ 10 H20 + 5CaSOa.2H20+ HF
Ca5F(P04)? 5H2S04
+
3HJP04
to form phosphoric acid, calcium sulfate, and hydrofluoric acid (3). The reaction is so rarried out that calcium sulfate cryst:illizes out of solution as the dihydrate, or gypsum. Maintaining a small, closcly controlled excess of sulfuric acid is essential to the formation of gypsum crj stnls n hich can be easily filtered from the reaction slurry, and also to the progress of the reaction (4). Because even small variations of the sulfuric acid concentration from the
1
Present address, Perkin-Elmer Corp.,
Xorwalk, Conn.
SAMPLE STREAM
Figure 2.
0 N
ii
I
Figure 1 . A. B. C.
D. E. F.
]I
J
Flow diagram of automatic sulfate ion analyzer
Reagent supply bottle Circulating pump Constant head overflow Reagent metering pump Reference cell Reaction vessel
G. H. J.
K. 1.
M.
Magnetic stirrer Measuring cell Flushing pump Overflow Sample dropping assembly Sample controller
Control panel and photometer measuring circuits
6'. Circulating gear pump motor C9 - C 1 0 = 1 0 0 mfd. D'. Reogent metering gear pump motor D1. G. E. IN91 rectifier D2. G. E. IN91 rectifier D3. T. I. 651 C4 silicon reference diode E. Photocell, Weston Model 8 5 6 Type 1 G Stirrer motor 1 0 r.p.m. H. fiotocell, Weston Model 8 5 6 Type 1 J'. Rapid flush pump motor 11. Photo diode lamp 12. Photocell lamp M . Servo-amplifier motor N. Needle valve drive motor PD. Photo diode (CDS) PL3. Pilot lamp, power P14. Pilot lamp, reagent P15. Pilot lamp, sample
.
P16. Pilot lamp, flush RE, R9. Span pot 500-ohm 1 OT dual R10, R 1 1. Zero p a t 50-ohm 1 0 T dual R12. Indicator p o t 50-ohm 1 OT R13. Telemetering pot 50-ohm 1 OT R14. Telemeterlng voltage a d just pot 1,000-ohm R15. 45,030-ohm R16. 2,000-ohm S3. Power switch DPST S4. Reagent swi!ch DPST S5. Sample switch DPST S6. Flush switch DPDT SA. Servo amplifier SL1. Funnel solenoid T2. Sola transformer 3 0 4 9 8 r 3 . Photocell lamp transformer 2 1 . Power line fuse
VOL. 31, NO. 7 , JULY 1 9 5 9
1255
B
Figure 3.
-
95-
Dropper assembly
7 I I I
- 50
I
90-
9
I
-40
,
I
80;p, 85-
0
I
'
.... CVC, CO
x
i-lu"
-C CO
75
. u '
-- ---
0
50
'lk ' '
'ILO
' &O
0
-
-?IUO -
9 x
I y "
- 20 Figure 4.
- 10 -
-______ ' ' 2k ' ' 3;;
--e--
70
-
C I - C 2 - C 6 - C 7 = 2 0 mfd. C3-C4-C5 = 40 mfd. C 8 = 1 .O mfd. motor cop. R1 = 1500-ohm 5 wan R2 = 0.1 meo R3 = i 80,000-ohm R4 = I-megohm pot R5 = 25,000-ohm pot R6 = 50-ohm 1 0 won R 7 = io-ohm R L l = 12,000-ohm SPOT plate relay R12 = 11 5 V. ds. SPDT relay Rl3-Rl4 = I 1 5 v. d.c. DPDT
-
Figure 5. System response as function of reaction vessel volume
culties of reproducibility preparing stable colloidal suspensions of barium sulfate (6),because the turbidity of a colloidal sol varies with the size of the colloidal particles as vel1 as with their conecutration. Any variation in the preparation of the sol may resrilt in lack of particle size uniformity from one determination to thc next. I n addition, the barium sulfate sol is unstable, in that the individual particles tend on standing to or recrpstallize, introducing a further error. showed the sulfate SO1 Could be satisfactorily stabilized by including a small quantity of gelatin (about 0.25 weight %) in the reagent. The gelatin, through ita action as a protective lyophilic colloid (6, 7) gives the sol of barium sulfate excellent long-term stability. Lack of reproducihility in sol preparation mas not a problem with the analyzer, which is capable of much more exacting reprndncibility than a human analyst. I n addition t o gelatin and barium chloride, the reagent contains sufficient hydrochloric acid t o bring its concentration t o 0.05N and a small quantity of sodium benzoate to inhihit decomposition of the gelatin. ANALYZER CONSTRUCTION
The analyzer is built in a dust-tight 1256
ANALYTICAL CHEMISTRY
Sample addition controller circuit 51. 52.
DPJT spring return switch 5-Po$ition rotary shorting witch SRI. 250-mo. selenium recmer
SS1.
Leder 2-gong 1 ?-pori-
tion stepping
sw
ss2. C10.e .+-gong 20-porition stepping sw T1. 500 V. CT 6.3 v. CT 5.2 Y. transformer V1. 5Y3-6T V2. 6C4
.&Y
cabinet 34 inches !vide, 18 inches deep, and 42 inches high. The uppcr portion of the cabinet contains the clectricnl and electronic assemblies, ineluding the sample addition controller, the photomcter measuring circuit and balancing servo system, a constantvoltage transformer for the photometer light source, and the controls for scale span, aero setting, and analyzer power circuits. These components are mounted in a slidine drawer for ease of access. The controls are protected from dust by a hinged covcr with a transparent window. The lower port,ion of the cabinet is divided into two sections. At the left is a dust-tight compartment for a 5gallon bottle of reagent. The righthand section, also dust-tight and isolated from the reagent bottle compartment, contains the pumps, reaction and overflow vessels, sample dropping assembly, and photometer. OPERATION
OF
ANALYZER
Figure 1 shows a flow diagram of the analvzer. Reagent is metered to the reaction vessel, F, by a small gear pump, D, driven a t constant speed to
give a flow rate of 15 ml. per minute, Before discharge to the reaction vessel, reagent passes through the reference cell of the photometer, E. A second gear pump, 8,circulates reagent from supply bott.le A to overflow reservoir C, thus maintaining a constant head of reagent for the metering pump. Pump J is used for rapid system flushing and zero reading checking. Clarified sample from the plant vhosnhoric acid reactor is metered to the jnalyzer reaction vessel by sample dropping assembly L. Reagent and sample are mixed by magnetic stirrer G . From the reaction vessel the colloidal sol flows through the photometer measuring cell, H , to an overflow, K , which discharges into a drain. The overflow maintains the volume of solution in the system constant; the rate of flow through the measuring cell is equal to the sum of reagent and sample flow rates. In the photometer assembly, shown in Figure 2, light from source L2 is split into two collimated beams which pass through the reference and measuring cells and fall on two barrier-layer photoelectric cells, E and H . These are connerted in a servo-balanced bridge
5.01 0.30
X A
-
0.20 a44
0.10
-
5
IO
15
25
20
30
VOLUME OF SAMPLE CELL,CC.
Figure 6.
Dynamic response of measuring cell as function
of its volume 2
1
circuit. The meclianical rutatioii of the balancing potentiometer, R12, is the mea.urP of the barium sulfate concentration, and a tandem-coupled telemetering potentiometer, R13. permits use of a n auxiliary recorder or controller. The bridge circuit includes controls for adjusting scale span and zero. The dropper assembij used for metering sample to the reaction vessel is shown in detail in Figure 3. I n this assembly a ceramic dropper tip is mounted in a valve block through m-hich a streain of sample flows continuously. A motor-operated needle valve adjusts the flow through the dropper tip. As drops of sample fall from the dropper tip they intercept a light beain betn-een a source and a photocell. The signal from the photocell synchrunizes the addition of sample to the reaction vessel and maintains the dropping rate a t 30 drops per minute by a simple control circuit illustrated in Figure 4. This adjusts the motor-operated valve IT hen an error in the dropping rate occurs. Once a minute the pivoted waste funnel into which the drops normally fall is moved aside by a solenoid just long enough for one drop of sample to fall into the reaction vcssd, nhich is mounted directly beneath the dropper. This addition rate corresponds t o a total sample flow to the reaction \-esse1 of about 2 ml. per hour. The dropwise sampling was chosen because of its simplicity and accuracy for delivering small volumes. The number of drops per minute that fall into the reaction vessel may be controlled by switch S 3 which may also be used for calibration of the instrument. ANALYZER PERFORMANCE
Because the flow of reagent to the reaction vessel is continuous and the addition of sample occurs only once per minute, the concentration of barium sulfate sol in the reaction vessel undergoes a regular change which, if plotted against a time axis, yields a sawtooth wave form exhibiting an instantaneous increase upon addition of sample followed by a gradual exponential de-
Figure 7. analysis
EO
"
zt
22
Comparison of plant tests with instrumental X 0
crease until the next addition. Under steady-state conditions, dCT' _dt_ -- -CF
where C is the instantaneous concentration of barium sulfate sol in the reaction vessel, V is the volume of the reaction vessel, and F is reagent f l o ~rate. Hence C =
3 4 TIME - HOURS
Coe-Ftiv
where Co is the concentration in the reaction vessel a t t = 0, immediately after addition of sample. The average concentration in the reaction vessel is given by
and
for sample addition a t 1-minute intervals. The amplitude of the sawtooth function is given by
I n Figure 5 , the ratio of average concentration in the reaction vessel to maximum concentration and the undamped amplitude of the saw-tooth function are plotted against the volume retention of the reaction vessel. It is desirable to obtain as low an amplitude of the saw-tooth wave as possible. Holvever, increasing the retention volume results in a corresponding increase in the instrumental response time. I n practice, a retention volume of 100 ml. represents the maxi-
Plant analyses Results from analyzer
muiii desirable. Because this volume corresponds to a sawtooth amplitude of 13%, an additional means of damping the saw-tooth wave must be employed. Because of some lag effects in the reaction vessel and in the tubing connecting it and the measuring cell, the concentration variation observed with a measuring cell of small volume and rapid response is somewhat distorted from the theoretical sawtooth wave and approaches a sine wave in configuration. This makes possible an approximate analysis of the effect of measuring cell volume on dynamic response using the equation
_e il -
1
dl
+ (.T)2
il!
for dynamic response of a system to a sinusoidal change, in which e is the amplitude indicated by the measuring cell, A the amplitude of change of concentration in the reaction vessel, w the frequency of cycle, and T the time constant of the measuring cell (time required for 63.2% response to a step change) ( I ) . I n Figure 6 the ratio of e to A is plotted against the volume of the measuring cell. A significant degree of damping of the sawtooth wave OCCUIS nith a cell of moderate volume. A cell 1.25 inches in diameter and 0.5 inch in depth is used in the analyzer; its volume 10 ml., corresponds to a ratio B/A of 0.22. The sawtooth amplitude indicated by the analyzer a t full scale reading is the product of the amplitude in the reaction vessel and the ratio e/A. 0.13 X 0.22 = 0.029
or 2.9 scale divisions on an indicator VOL. 31, NO. 7,JULY 1959
1257
having 100 scale divisions. The amplitude is directly proportional to the analyzer scale reading and is therefore smaller with less concentrated samples. Sample supplied to the analyzer is diluted with twice its volume of water in a proportioning system to prevent precipitation of gypsum. The instrument is calibrated by use of standard solutions, or alternatively, by comparison of the instrument reading with the plant analysis obtained simultaneously (Figure 7 ) . The different readings obtained for the same sample are due to the different methods of analysis. The Kno-zles method (6) was used for the plant analyses. Furthermore, the instrument’s sampling system removes most of the calcium sulfate from the sample before delivering it to the instrument and this results in a lower reading. Spot checks made on the calibration over a 4-month period showed that it
Table 1.
Std. Solns.
Calibration Checks
of the analyzer to colorimetric analyses also appears attractive.
so4
Analyzer Readings, yo
LITERATURE CITED
3.33 1.67
9-7-55 11-10-55 1-16-56 3.25 3.45 3.42 1.67 1.72 1.72
(1) Eckman, D. P., “Industrial Instrumentation,” p. 16, Wiley, Yew York, 1950. ( 2 ) Knowles .bsociates, 19 Reactor St., S e w York, private communication.
=
Concn., yo
(3) Kobe, K. A., “Inorganic Process
remains essentially constant as indicated in Table I. The analyzer is readily applicable to other analytical and control problems. The concentration of barium sulfate sol in the reaction vessel for full scale instrument reading is less than 100 p.p.m., and the analyzer could be made more sensitive by increasing the depth of the measuring cell. If reagent and sample streams were reversed in flow ratio, the analyzer would easily perform turbidimetric analyses in the parts per million range Application
Industries,” p. 323, Macmillan, XeF
York, 1958. ( 4 ) Ibid., p. 324. (5) Sandell, E. R., “Colorimetri:, Determination of Traces of Metals, p. 69 Interscience, New York, 1944. ( G ) Snell, F. D., Snell,.C;,T., “Colorimetric Xethods of Analysis, p. 88, Van Sostrand, Sew York; 1936( 7 ) Yoe, J. H., “Photometric Chemical .\nalysis,” p 91. Wiley, New York, 1928. RECEIVED foi review November 9, 1956. .-iccepted February 25, 1959. Pittsburgh Conference on dnalytical Chemistry and hpplied Spectroscopy, Pittsburgh, Pa , Febriiary 27, 1956.
Simple Ultraviolet Photometer RALPH E. THIERS, MARVIN MARGOSHESI1 and BERT L. VALLEE Biophysics Research laboratory, Harvard Medical School and Pefer Bent Brigham Hospital, 72 7 Huntington Ave., Boston, Mass.
F A simple photometer for use in the near-ultraviolet region is described, and considerations of its design and performance are discussed. A monochromatic beam of light with wave length maximum a t 355 mp and a half-band width of 40 mp is obtained from a lamp made commercially for fluorescent displays. Photovoltaic cells are used as detectors, to operate a direct current microammeter. The instrument, because of its small size and sturdy, simple construction, is well suited to routine work, including clinical determinations of serum enzymes, which may even be performed outside the laboratory.
C
and photometers have been the instruments of choice for routine measurement of the absorbance of colored solutions. Spectrophotometers, because of their greater complexity and expense, have been used less for routine work at fixed wave lengths and more for research or development. However, in the ultraviolet region of the spectrum the absence of suitable photometers of reasonable simplicity and monochromatic properties has made i t necessary to use spectrophotometers for routine work. Many analysts have been unable to take
advantage of methods which would be well suited to simple ultraviolet photometry, if a n appropriate but inexpensive instrument were available. I n addition, certain enzymatic determinations have recently become important in clinical medicine, and these are best performed by photometry in the nearultraviolet, This paper describes a new ultraviolet photometer which allows measurements of absorbance in the neighborhood of 355 mp. Because it is simple, small, and sturdy, it is well suited to routine work, including clinical determinations of serum enzymes which may even be performed outside the laboratory.
OLORIMETERS
1 Present address, National Bureau of Standards, Washington, D. C.
1258
ANALYTICAL CHEMISTRY
DESIGN OF INSTRUMENT
Source and Monochromator. Ultraviolet radiation is provided by a 4watt B L B fluorescent lamp (General Electric Co. or Sylvania Electric Products Co.), which is a stock item used in fluorescent advertising. The envelope of the lamp is a special Corning Filter Glass, and the resulta n t spectral output of this source is a single band centered a t 355 m,u and with half-band width of about 40 mp (Figure 1). Detector. Barrier layer or “photovoltaic” photocells (General Electric Co. KO.PV-1, ultraviolet sensitive) form the light-sensitive element of t h e instrument. B y using a cylindrical
lens parallel t o the source, and a cylindrical absoi ption cell, light levels ale reached nhich allow the photocell t o operate a direct current microammeter rather than a galvanometer, and in a simple, nonamplifying circuit. Absorption Cell. Ordinary 13-mm. borosilicate glass culture tubes are used as absorption cells. Optical and Photoelectric Design. Figure 2 s h o w a diagram of the photoelectrical circuit and optical arrangement of the instrument Light from the fluorescent tube, T , passes to the right through a waterfilled cylindrical lens, L , 13 mm. in diameter, then through the solution to be measured, S,in the cuvette, C, through a variable-aperture diaphragm, A , and strikes the photocell, P. Light from the other side of tube T passes to the left, through a Variable-aperture diaphragm, Ai, and strikes photocell PI. The two photocells are connected in series and are bridged by a 20-pa. Ion resistance, direct-current meter, JI so that current from the left-hand photocell deflects the needle downscale, and current from the right-hand photocell moves it upscale. The lens, L , is an essential feature of the design of this instrument and may be used to fill a number of roles. It focuses the light from T for maximum flux through cuvette C and on to photocell P . It physically separates the cuvette and sample from the lamp, thus minimizing variations in enzymatic reaction rates due to excessive heat
