INDUSTRIAL A N D ENGINEERING CHEMISTRY
804
Vol. 16, No. 8
Turbidimetric Estimation of Precipitates' By P. L. Ribbard PLANTNVT~STION Lnnoa~roeu.U N W B R S OP ~~C YI I I I P O B N BEBXBLS?, II. CALIPI
T
URBIDIMETRIC A new j m m of turbidimeter faestimating the amcuntoj calcium. clear, Polished bottom. estimationof calcium, aut/ote, ,,, .,the* ion which A giue granular When t,he observation is to he made, a black rubbermagnesium, s u l f a t e , precipitate. is desoibcd. The instrument is simple. eflecti"eie,and and perhaps other ions is inexpen3iue. successjuful turbidimetry that an ion which is cloth jacket is slipped over the tube so that side light is manner that a giwn quite accurate enough for to bc estimated shall be in amount pr,,duce the o p ~ i m apppeorance. f Con& excluded. KO light should some industrial or technical lions oj concentration. ratio. monner sf mixing, temperature, and reach the turbid suspension purposes and permits 8.great time rate oj mixing the ions which ore to produce the precipitate must except from the white resaving in time. be controlled. A suitable procedure j o t estimation oj calcium. flector at the M t o m of The instrument and Proc e d m here described magnesium. and sulfate is described. the instrument. been found more rapid than Any desired depth of the t,urbid liquid may be used in any other known to the writer. When once the instrument has been calibrated in the observation tuhe, but when the scale has been calibrated terms of known amounts of the ions to be estimated, there for a certain depth, that depth must be approximately mainis no further need of standard or compa.rison suspensions. tained in all observations in which that, calibration is used. The instrument is, perhaps, no more accurate than some Any great variation of the volume in which a given amount of precipithte is suspended will vary the reading. Thus, if a. others. The figures in tlie table show about what accuracy is t u he madiug of 16 is found for a depth of 20 em. of a certain expected with ordinary care and wit.hout duplication of de- turbid Anid, a depth of 10 em. of t.he same fluid will usually terminations. give a reading of somen,hat more than half of 16-that is, over 8. It might be supposed that if the suspemion is too turhid CoMP*luSoN 0s GRAYIMBIPIIC ANY T U K l l r D l M E l R r C RESULTS UP WA7Y.R t,o read st the full depth, the reading at, one-half dept,11might, ANALYLBS be doubled to give a correct result. This would not be true, ( A = giavimefri~, B = turbidimetric) ---.I'I(nTS PER h.IILLI"N----------. however, so some arbitrary standard di:pt,h should be used. Calcium M*gl,eSi"m su1iate In the instrument here described this depth is 5:)nnn., which Sample A B A 13 A B 2603 Iya 104 105 60 630 520 gives a volume of 50 cc. in the er t,ube used. The 2605 1260 1140 510 540 82 102 at without clinnging diameter of the tube may vary so 2609 54 57 30 10 31 21 2626 76 80 44 3s 298 125 340 88 tlie reading. 2632 156 145 33 37 2633 182 1x5 76 66 524 520 On looking into 00 43 40 19 12 63 70 the eyepiece of the T H E TURBIDIXETER instrument tlmre is seen an illuniinated The instrument (Fig. 1) liere described for measuring the circle divided by a turbidity of suspensions is not ideal; yet it gives fairly sat- vertical line. When isfactory results. It is capable of about the same degree of thediaphragmisfully accuracy as ordinary colorimeters, having a probable error open, hothsidesof the of plus or minus 5 to 10 per cent. It was made by modify- circleare equally illuing a Campbell-Hurley colorimeterZ by substituting for the minated. If a turbid glass tube and compensating piston with the standardknown solution, such as a colored solution an iris diaphragm such as is used on the sub- suspension of barium stage of a microscope. The cootrolling lever of the dia- sulfate i n water, is phragm was lengthened to about 10 em. (4 inches) by solder- piaced in the observaing on a piece of brass wire. The outer end of this wire tion tube, thelefthalf moves over the graduated arc of a circle, the plane of ivhicli of the circle becomes is perpendicular to the direction of the beam of light. Thus darker. To compenthe amount of opening of the diaphragm is indicated on the sate this darkening scale. The graduated arc, about one-fifth of a circle, is di- t h e d i a p h r a g m is vided into twenty-five equal parts. The zero point is at full partly closed until opening, and 25 at nea.rly closed. In the bottom of the both sides of the circle box containing the reflecting mirrors at the top of the instru- a p p e a r e q u a 11y mcnt is a sheet of ground glass which diffuses the light pass- lighted, then the depic. i-~rruxninIMiirIra ing up from the reflector at the bottom. This rcfiector is gree of opening is a white card. For illuminat.ing the reflector daylight is read off on the cir~~~~~~~~~~~~~~~~~~~~~~~1~~~~~~ preferable, hut any good whitelight may be used. Theread- c u l a r scale. The the graduated UrC a,,d tl,e W i ~ r hand,e for ings on the samc suspension, however, mill vary a little in operation is compar- Eootroi~inathe Openilig the aiaphragm different qualities of light, so a different light source may re- able to the reading of a polariscope, such as is used for the determination quire a recalibration of the scale. The solution of which the turbidity is to be measured is of sugar. The average of several settings should be taken contained inaglass tube, suchasanordinary Nessler tube, with as the correct reading for the solution in question. In order to translate this reading into weight of a precipitate in 1 Raceivcd April 12. 1SZ4. the suspension, the instrument is calibrated by reading oa 2 Camgbell and Hurley. J. Am. Chcm. Sor., 33, 1112 (1911).
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INDUSTRIAL AND GNGINEERIiYG CHEMISTRY
August, 1924
several differentknown concentrations of each substance to he estimated. The readings are plotted and thus the amount corresponding to any scale reading may he known. Since the opacity produced by the same weight of different substances varies, the instrument must be calibrated for snnnrat,elv. it. ..... . .- ~ ~ -is usually necessary to interpose some slightly opaque substance, such as a celluloid film, on the unknown side of the instrument in order to bring both sides of the light circle to the same degree of illumination when a t the zero point. Succcssive readings on the same suspension may vary one or two divisions either way from the correct point. Some practice and familiarity with the instrument are necessary to secure accoracy. Several readings may be made in a After an minute. approximately corFZC 2 S T I . Y A T E Miran rect setting has been A device for miring the proper proportion made, the pointer is 01 barium chloride with the salfate solution moved one or two scale divisions to one side and another ohser&ion made . It is then moved an equal distance on the other side, and the observation repeated. If one is too dark and the other equally too light, the correct setting is taken as half way between the two. ~
~
COXDITIONS
FOR PR0DCCI.I.G
PnECIPITATEs SCITABLE FOR
CnxpARisoN
Successful turhidirnet.ry depends not only on a satisfactory instrument, but equally as much on a satisfactory method nf producing the precipitate to be measured. This must he such that a giveii veight of the substance to be measured will always produce a precipitate of the same optical quality, so that it will have the same opacity and give the same reading in the t~rhidimeter.~One who has not tried this will hardly realize the difficulty of the problem. By varying the concentration and the manner of mixing the reagents, it is possible to produce a precipitate of barium sulfate having two or three times the opacity of another produced from the same amount of sulfate. Other precipitates aresubject to similar variations. For turbidimetric measurement, the precipitate should he very fine so as not to settle rapidly; yet it must not be of colloidal dimensions. Colloidal precipitates are not uniform in appearance and are not easily compared in the turbidimet,er. Moreover, their suspensions are liable to appear colored by transmitted light. Suspcnsions of large crystals have relatively low opacity and settle very rapidly; therefore, they are not suitable for turbidimetric estimation. I n order to produce a precipitate of uniform physical character, the following conditions must be controlled within somewhat narrow limits: (1) concentration of the two ions a
Cheneveau and Boussu, Cornpi. rend., 111, 1290 (1923).
805
which combine to produce the precipitate, (2) ratio of one to the other in the solutions mixed, (3) manner of mixing the two, (4) time rate of mixing, ( 5 ) amount of other salts in the solutions, and (6)temperature at time Of mixing. Ordinarily, the temperature need not be regulated, but if either of the solutions or hot, ~ is~cold~ ~it should ~ .he brought to room temperature before mixing. Each ion requires a different method of treatment, hut it vi11 prohahly be sufficient to describe the procedure for calcium, magnesium, and sulfate. A large number of experiments were made in the endeavor to find the most suitable conditions. Possibly others would he as good, or better, hut the procedure given is quite fat,isfactory for calcium and sulfate, not so good for magnesium though fairly acceptable. The solution to be precipitated is conveniently held in an ordinary 100-cc. bottle having a medium wide mouth which can he easily stoppered for shaking hy holding the thumb over the mouth. After the reagents are added, the precipitating bottle is at once shaken to secure a uniform mixture, then allowed to stand 10 to 15 minutes to allow the precipitate to form to the fullest extent possible, Longer time seems to cause no perceptible increase of opacity. In general, it is probable that after half an hour there is some increase in size of the aggregates with consequent decrease in opacity. Therefore, the reading should not he long delayed. Precipitatos that have stood for a day may lose 5 to 20 per cent in opacity. If the solution precipitated is too concentrated, the susptlnsion may he diluted afterward to permit a satisfactory reading, though the result is much less reliable than if diluted properly before precipitating. CArcIuM-The concentration should not be over 30, preferably hetween 10 and 25 p. p. m. of calcium. Higher concentrations arc too opaque to read successfully, and the opacity is not proportional t o concentration. To the propcrly diluted solution, add lor each 50 cc., 1cc. of 10 per cent solution of OxdliC acid. followed by 0.4 t o 0.6 gram of powdered potassium oxalate. Immediately shake the Bottle ior a few seconds till the potassium oxalate dissolves, let stand 10 t o 1,s minutes, shake again, poor into the turbidimeter tube, and read a t once as described above. MAmssIoM--The solution from which calcium has been precipitated as previously described is not suitable for determination of magnesium on account of the largc amount of oxalate added t o produce the desired character of calcium precipitate. Another portion of the original solution is precipitated by adding a little excess of ammonium oxalate while boiling. It is filtered and cooled, and is then ready to precipitate the magnesium. To 50 cc. of the cool solution, add 2 cc. of phosphate reagent, then 2 cc. of strong ammonia, mix at once. Let stand 10 t o 15 minutes, mix, and read in the turbidimeter. J[ t h e soliition was too concentrated when precipitated, it may be diluted for reading, hut tlic result is more liahlc to be erroneous than in case of similar treatment of calcium or sulfate precipitates. The phosphate reagent contains 25 grams of disodium phosphate, enough hydrochloric acid t o make the solution acid t o methyl orange, and water to make 100 cc. SULFATE-It was impossible to secure the desired quality of barium sulfate precipitate without controlling the conditions much more closely than for calcium or magnesium. The rate a t which the barium and sulfate are mined is important. Uniform results are obtained by placing the two solutions in separate reservoirs of a special mixing device (Fip. 2), then running the solutions simultaneously into the precipitating bottle so that the reaction takes place inslantancously with always about the same is, about 500:l. Shake the ratio of barium t o sulfate-that mixture a t once, allow l o stand 10 to 15 minutes, shakc again, transfer t o the turbidimetric tube and make the reading. The barium reservoir has an empirically graduated scale, indicating the height at which it should stand in order t o deliver the proper amount of barium t o a solution containing 10, 20, 50, or 100 p. p. m. of sulfate. When once the apparatus and reagent are ready, thc proper precipitation of a suliate solution takes but a few seconds.
Other suhstanees may be estimated in a similar mannef. The proper conditions for each must he worked out expenmentally.
