New pH Indicators for Determination of Total Alkalinity in Water Disodium 4,4‘-bis(p-dimethylaminophenylaz;o)~~,2‘-stilbenedisulfonate and Disodium P74‘-bis(o-to1ylt riaz;eno)-2,Zt-stilbenedisulfonate MICHIEL T.IR.kS, Department of Water Supply, Detroit, Mich. Two new indicators of the disazostilbeneaminedisulfonate series are described and a simple method for their preparation is disclosed. One of the indicators undergoes an orange-red to bluish-violet transition, making it suitable for the titration of colored solutions. The other changes from a deep yellow to a muddy hue and is valuable for titrations performed in the absence of color. The operational range of both indicators embraces pH 5.0 to 4.0. The indicators have been successfully applied to the determination of total alkalinities up to 500 p.p.m. From a theoretical standpoint, these indicators are more admissible for total alkalinity determination than traditional methyl orange.
SDER favorable esperimental conditions methyl orange is far from an ideal indicator. .It the end point, of t,he total alkalinity determination of water the indicator changes from yellow to faint orange ( 6 ) . I n the color spect,rum yellow neighbors on orange. For this reason, the buffer effect of the dilute birarbonate-acid system often makes the methyl orange end point difficult to reproduce. To ameliorate this unsat,isfactory condition, modified indicators employing methyl orange (2) and mixed indicators based on methyl red (3) have been proposed for the total alkalinity determination. Used discretely, with an awareness of their limitations, these indicators possess nunierous advantages o w r t,he traditional methyl orange solut,ion. Despite the obvious iniprol-ements, the p H range of 4.0 to 5.0 is still characterized by a dearth of individual indicators with striking color changes. For esample, a pressing need prevails for an indicator which can function in the titration of colored solutions. Such colored solutions are typified by marshy waters. To he .suitable for the titration of colored solutions, an indicator must be capable of a dramatic transition in order to master thr, hues already present in the solution. A color change of this d in the disazostilbeneaminedisulfonate series of dyes. S o record of the previous use of these dyes in analytical work could br found in the literature. Indeed, recourse to Scliult,z’s Farbstofftnbellcn ( 5 ) failed t’o disclose a listing of the d in this paper, notn-ithst,anding the fact that a number of related dyes of the stilbene group were classified and dcscribed. Subsequent papers will suggest the special applications lvhich can be made of the arresting color changes of this st’ries of dyes. For convenience the color changes of the two dyes investigated in this work are sunlmarized in Table I. One dye is adaptable to the titration of colored solutions; the other is applicable to titrations performed in the absence of color. h n outstanding virtue of both indicators is their ability to undergo color responses in a pH range where methyl orange is ineffective. As Cooper (3) has reported, carbonate concentrations below 250 p.p.m. exhibit equivalence points in a pH interval of 5.0 to 4.5, whereas the first visible indication of methyl orange transition occurs a t a pH of 4.5, Analysis of dilute synthetic sodium carbonate solutions, together with titrations of actual samples drawn from the Detroit distribution system, justifies the consideration of these disazostilheneaminedisulfonate dyes for indicator purposes.
These dyes can readily be prepared from available materials and require no special apparatus for synthesis. Only a few simple modifications, providing for the characteristic insolubility of the stilbene molecule, must be observed in the preparation. I1IDIC4TOR PROPERTIES OF DISAZOSTILBENEAMINEDISULFONATES
Prior to ievien ing the general chemical properties, the equations involved in the preparation of the disazostilbeneaminedisulfonate indicators will be presented, with the double objective of picturing the reactions grapically and of facilitiating future reference. The interests of brevity are more conveniently served by identifying an indicator through its amine derivation than resorting repeatedly to the full technical nomenclature. The sole variable is the coupling amine, which alone governs the type and individuality of the indicator produced. All other factcrs remain constant. The equations conform to the typical tetraazotization pattern (see top of next page). Considered as a class the disazostilbeneaminedisulfonate indicators give a yellow to orange alkaline reaction and a purple to blue acid color. The tiansition points invariably lie on the acid side of neutrality, between the p H extremes of 6.0 and 3.5. The Kjeldahl ammonia and the carbonic acid equivalence points fall between this pH range. Disazostilbeneaminedisulfonate indicators have been prepared 11hich are suitable for these titrations. I n the course of this investigation representative amines of the aniline and naphthylamine series were coupled with the parent acid, with a view compound, 4,4’-diaminostilbene-2,2’-disulfonic to discovering the effect of substitution. As a rule, tertiary aniines provided the most acceptable indicators, although three valu-
Table I. PH
Indicator Transformations in Colorless Solutions Indicator I
Indicator I1
5.5 Orange-red Deep yellow Pale purple Deep yellow 5.0 Faint violet First trace of muddiness 4.7 ’ Blue w i t h T-iolet tinge .\Inximum muddiness 4.0 Indicator I . Disodium 4,4‘-bis(p-dimethylaminophenylazo)-2,2’-stilbenedisulfonate. T w o drops of 0.1% indicator used per 100-ml. sample. Indicator 11. Disodium 4,4‘-bis(o-tolyltriaseno)-2,?’-stilbenedisulfonate. Five drops of 0 . 5 % indicator used per 100-ml. sample.
339
ANALYTICAL CHEMISTRY
340
able indicators w ~ r vpreparcd from primary amines. 1Ietliyl:ttion of the ring or thc amino group also conferred supcrior indicator properties on the resulting dye. The inference that, t h r unsuhatitutcd derivatives of aniline and or-naphthylamine are devoid of indicator c1iaractc.ristic.s is invalid. The aniline and a-naphthylamine derivativcss both possess indicator properties. Thc tiran-back t o their use i'cposcs in othcr reasons. For esaniple, the a-riaplitliylamine derivatives requires a greatci, quantity of dye to accentuate the color changes; and even under these conditions the transitions are lacking in distinctness. A palc red color at a pH of 8.0, tho indicator turns a pale mauve at. a pH of 7.0. At a pII of 6.0 and 5.0 a pale and intermediate purple phase sets in, modulating to a hluc at a pH of 3.0. Under such circumstaiirt~athe indicator is unsuitat)lc for tit riincxt,ry, The unsubstitutcd aniline dcrivat ivr, on ttitb other hand, is iiiore decisive in it,s transition but the changes occur too far in the acid region to be of practical benefit. 1-cllon-a t a pH of 4.0, the intlicator shifts t o a faint, orange at a pH of 3.0, assuming a brilliant rctl at a pH of 2.2. The conversion to t,lie red is abrupt. The phenylenediamine derivative proved equally disappointing as an indicator. Opcning n-ith a yellowish orange color at a pII of 7.0, this indicator nic~rgos11)- tl(1gl into :in orange at, a pH of 5.0, and an orange-rct! at a pH of 3.0. Ttic rrmaining dyes $tudicd in this group \\-ill be dincu g i w t w length in future pap(>~'n. Disodium 4,4'-bis(p-Dimethylaminophenylazo~-2,2'--stilbenedisulfonate. A notable property of this indicat,or is its progressive insolubility in the pH range of 5.4 to 4.0. This propert,y engages attention when the titration is pci.formed in a casserole. The gradual addition of acid, coupled with the customary stirring, causes blue rings of adsorbed dye t o a,ppcw along the walls of the rasscrolo. The rings cmlarge as the pH is reduced to 4.0. Only a small fraction of the total dye is thus salted out during t,he entire titration process; nonethc~less,t,hc amount is sufficient t o assist in the detection of the end point. These changes are not so spectacular in an Erlenmeyvrr or a beaker, despite the he ohscr\'ed. fact that a precipitate is formed and Tlicl color changes of thc solution ithP1f alniost8 parallcl tht, salting-out process. The inopt pronounctd color changes take place in the pH rangc of 5.0 to 4.0, enabling the analyst to titrat,e a solution of approsiniatc~lykno\\-ri cart)onat,e concentration to the desired equivalence point.. The palo purple, apparent a t a pH of 5.0, turns hy stagcs t o it blue intc~sperseclivith a shade of violet at a pH of 4.0. These changes are, valuable in thc titration of gellon-colored solutions \\-here the, hluish-violet is rc+lvctct! back as a deep green. The exact, rang(, ov('r which the indicator acts \\-as established by a series of liuffr~rsolutions ( 4 ) p r c p a ~ from ~~l 0.2 .I1sodium dibasic phosphatc and 0.1 .If citric acid. -4 0.1 yo solution of the indicator possesses a derp rcd color. Thv solution retains its strength indilfinittsly, sampl(>sx u f f e i k no loss in color or ov(51~-alleffvctiv(~ncwa year ;iftc.r pnxparation. (3an
' t h o drops of the 0 . 1 5 inttimttor i n it 100-1nl.volume
itrtl
amplr for
t itriinetric purposes.
Disodium 4,4'-bis(o-Tolyltriazeno)-2,2 '-stilbenedisulfonate. This indicator is surprisingly soluhk c*omp:iredwith its dimethylanilincs i.elativc\. Thcs tclltalt~ prc.c*ipitate and ring formed by the dimt:thylanilinc derivative are negligihle in thc; case of this compound. Grnerallp speaking, the dye hehave.; like a standard indicator, and is perhaps more suited t o the titration of colorle-, solutions than the prewding derivative. Thi. ~ a i i i csodium dibasic I)tii)sphati,-c,itric acid buffers r.5 I were einployrd to define the p l l operational limits of this intlicator. The first, trace of niutldincw (the color defies description appears at a plI of 4.7 and dwolops rapidly as thc p H is d(xprc L o 4.0, tht- point of niaxiniuni mutltliness .i 0.5po solution of thc indicator has a brick-red c o k ~ r . The stntility of tho indicator is charactcaristically cscellent, no deterioration in indicator propertim hving csvidmt after a ye oliit ir)n :it'(\ i,cquirc~lfur I I'ive d r o p of the 0.5 .:1111~11(' takc.11 for anal> l'Kb;t'.AR4'~10V O F U l S A Z O S T I L B E N E A ~ l I N E D l S ~ ' L ~ O ~ . A S E R I E S O F DYES
\\'ark up into a pasbe 9.25 grams (0.025 Jl) of 4,4'-tliarnino~t,ilI)one-2,2'-disulfonic acid, Eastman Kodak T4613, with 50 nil. of distilled water and dissolve by addition of a solution of 2.5 grams of sodium hydroxide pellet n 10 nil. of distilled water. S e s t stir in 3.45grama (0.05 Jf) of dium nitrit.e, already dissolved in 10 ml. of distillcd \vitt,c,r, and t ndcr the resulting solution t'o a sepnratory funnel. In a 2.50-nil. bc~ikerinis 25 c ranis of crushed ict. and 25 nil. of concentrated hydrochloric acid, cooling the exterior of the beaker in an ice bsth. When the ttsniperature of the acid-ice mixture has fallen t o 0", run in slo\dy the, stilbene-nitritc solution, n-ith vigorous stirring. If necussary, add ice from tiiw to t,ime t,o inaintain reaction tcmpcrat8urebelow 5' C'. Rernovv the twaker, still in the ice bath, to it refrigerator anti allow t h c t yc~llowdiazonium salt t,o set,tle for a11 hour. Siphon off t,he supernatant liquid and rapidly add the diazonium slurrl- to the coupling solution which has been prepared in t,he meantinie. The coupling solution consists of 7.3 nil. (0.06 M ) of dimethylaniline, Eastman Kodak 97, in 25 ml. of glacial acetic acid. Allow the reaction to take placr at rnoni teniperat,urcs for one hour, with continuous stirring. Filtrr thc precipitate and dry. llecrystallization is unnecessary. Triturate t,he dye a-it,h t h r h calculated volume of 0.05 S sodium hydroside and dilute suffivicntly t,o f o m ~a 0.1$; solution. In the prepitration of disodium i,~'-bis(o-t,olylt,riazeno)-2,2'ntilhcnedisulfonatc thc prcmding procedurt: n-as adhrred to with one exception: t,he coupling \\-as vffected with 6.0 nil. (0.06 31)of o-toluidine, Eastman Koclak 2.53. The intliwtor was prepared in 0.5'';, solution. EXPEKI\IE\I'A I. PROCEDURE
.I 0.01 .\- solution of a n a l y t i d reagvnt grade sodium carbonate :iduptc.ti as the rc~frrc~nce st:indard throughout this investigat i o n . \'ai.ious dilutions of t hi, solution in carhon diouidc-free
\\-its
V O L U M E 19, NO. 5, M A Y 1 9 4 7 Table 11.
Titration of 100 311. of Synthetic Sodium Carbonate Solutions”
Total .\lkalinity Total Found dlkaliniry with Indi- Devia Added cator I tion P.l~.rn.
50 100 200 $00 so0
341
‘ r o tHI .ilkalinity Found with Indicator I1 P.p.ni. P.p.m. P.p.m. 50 0 31 100 0 100 202 201 400 403 499 -1 502
+;
Devia. tion
P.p.m. +1 0 +I +3 +2
Total Alkalinity Found with Indicator I Plus 1311. of Artificial Color Solution
P.p.m. 50 101 202
398 495
Deriation
P.p.m. 0
+1 +2
-2 -5
I n d i c a t n r I . Disodiuni4,4’-bis(p-diinethylam~nophenylazo)-2,2’-stilbenedisulfonate. Indicator 11. Disodium 4,4’-bis(o-tolyltriaseno)-2,2’-stilbenedisulfonat e , a fli
AI1 Txlues represent average of live determinations. Results expressed
i>.p,in.of calcium carbonate.
tli,tillcd n-attr were used for titrations with the indicators. The 0.02 S sulfuric acid eniploped was standardized electrometricallp against the 0.01 A’ sodium carbonate by the differential mcthod. hll indicator titrations w r e wrified by the electronictric titration of :I parallel solution. .-is an additional check on the indicator?, saniples of the watcsr entt,ring the Detroit distribution systc’m wore talccri for anal! Parallel samples were suhjtcted to . Final alkalinity valuc~s n-erc all elrrtroiiietriv detrrinina rouiidcd off to the ntwcsi part pcr million. The titrations wvr(~csarried niit in a white porrclain caswrolt>, a n d in :m Edcnmeyci, fl:isl; wsting on a Ir-hite background. In virtually all instance< t. was clov. although color conle. Titrations were begun only tmzt n ’ n ~*harpcLr in t 1 to a final volume of 100 nil. It’ aftcr thc ;mnplc had h wa.. unnc>ec.ssaryio dilute Dctroit n-atc’r hcmiuse of tho re1:ttivcly lo\v hicarhnnte ronccntration. The ?odium carbonat(, conci,ntrations takin for anal) conipnsstd the span 50 to j 0 0 p.p,ni., e‘iproqsed as calcium carhonatc. The pH nt the rquivalcnrc points varied from 1.0 at thi, higher carbonaic’ conccntrations to 4.8 for thc lower concc’ntrations. This was in 1ic.rping nith the findings of rooper (3’. TITRATIOS OF COLORED SOLUTIONS
,
centration of carbonic acid produced. As the system grows progressively dilute, the p H obtaining at the end point mounts correspondingly. Cooper (3) has worked out the relationship between p H and conrentration prcvailing in the bicarbonate-acid system. riccording to Cooper’s calculations, the equivalence pII soars to a maximum of 5.0 in very dilute solutions. Experimental data corroborate the calculations. LIanifestly, an indicator such a i methyl orange, v-hich records its first faint color transformation at a p H of 4 5 , loses soine of its validity and relevance in the face of these facts. Accuracy decrees that a more admissible indicator be substituted. Examination of the data in Tables I1 and I11 establishes the reliability of disodiuin 4,4’-bis(p-dimethylaminophenylaao)-2,2’stilbmcdisulfonate as an indicator for the titration of dilute bicarbonate solutions, whether occurring naturally or in synthetic solutions, in the presence or absence of color. The behavior of the indicator in a colored medium suggests a supplementary application in colorless solutions. Modified with inert yellow dyes or combined with standard indicators (like phenol red), whose acid color is an unvarying yellow, the color change can be tailored to the personal \\ hiin. For instance, some individuals prefer an orange to blue violet change, while others favor an orange to green tranqition. \Thatever the preference, titrations with this indicator can be conducted with equal effectiveneqa in glassware and jii a poicclain c‘as
Table 111. Titration of 100 311. of Detroit Watern
‘r
t a~
.\lkalinity Total Found by .\lkalinity ElertroFound nietric with IndiTitration cntor I P.p.m. P.p.m.
Deviation P.p.m.
-_
--
IS
0
i B
78 81 83 83
-1 0 +1
io
81 82 84 87
87
-1 0
Total .ilkalinity Found with Indicator I1 P.p.m. 75 80 81’ 83 83 8i
Total Alkalinitv Found ” with Indicator I Plus 1311. of h r t i Ilevia- ficial Color tion Solution P.p.m. P . p m . 0
+1 ‘1
t l
-1 0
76 81 80 82 83 87
Deviation P.p.m.
: ; -1 0 -1 0
Indicator I . Disodium 4,4’-bis(p-diniethylaniinophenyiazo)-?,2’-stilbenedisulfonate. Indicator 11. Disodium 4,4’-bis(o-tol~~ltriaaeno)-2,2’-stilbenedisulfonate
For ostensible reasons, the accurate titration of colorcjd solua .I11 values represent average of fire determinations. Results expressed R S I ) . ~ . I I I . of calcium carbonate. tions is difficult if not inipossible with methyl orange indicator, and, up to the present, no adequate suhstitutr has been proposed. The indicator, disodium ~ , i ’ - ~ ) i s ( p - d i m e t h y l a m i n o p h e n ~ ~ l a ~ o ~ 2,2’-stilbcncdisulfonate. provides a visible cnd point in colored iolutions. a result of the coinplcmentary spectral relations I n the case of disodium 4,1’-bis(o-tolyltriazeno~-2,2‘-stilbcnetht, normal change of orange-red to bluish-violet is supplanted disulfonate, hon.ever, it is advisable to restrict all work to a white lip an orange to c1ec.p green changc, in yellow-colored solutions. porcelain dish where the slightest variation in shade is enhanced This transition drpends o n thr color of the original solution, an by the unrelieved background. Looking down upon a white surintense y l l o ~ vIrnditig itself inoqt favorably to titration. The face intensifies the transition and makes for improved reproduciend point is easily rcprotluccd i n solutions having a color of 70, th(, bility and accuracy. Titration is not impcssible in glassware, but color being determincstl i n accordance with directions prescribed greater caution must be esercised in approaching the end point if hy the American Puhlic Hralth Association ( 1 ) . glass is selected. The analyst is frer t o choose the method most For the purposes of this m r k , a standard color solution was presuitable to his needs. pared by caranielizing pure sucrose and then adjusting the neutral solution so that 1 ml. on dilution to 101 ml. would yield a color LITERATURE CITED of 70. The same saniples taken for titration in the colorless determinations were also used in this phase of the investigation, (1) -%m.Public Health Assoc., “Standard Methods for Examination of Water and Sewage”, pp. 12-14, New York, 1936. with the exception that 1 ml. of artificial color solution was added (2) Ibid., pp. 94-5. to impart a total color of 70. The resulting solutions were then (3) Cooper, S. S., IND.ESG.CHEY..- 1 N i L . ED.,13, 466-70 (1941). titrated in the normal manner to the proper shade of green. (4) McIlvaine, T. C., J.Biol. Chem., 49, 183-6 (1921). (5) Schultz, Gustav, “Farbstofftabellen”, pp. 284-97, Berlin, WeidDISCUSSION mannsche Buchhandlung, 1929. (6) Theroux, F. R., Eldridge, E., and Mallmann, W. L., “Laboratory The equivalence point of a strong carbonate-acid system norManual for Chemical and Bacterial Analysis of Water and mally falls within the pH range below 4.0, because of the high conSewage”, p. 8, New York, McGraw-Hill Book Co., 1936.
