INDb-STRIAL A K D ENG1NEERI.VG CHEAWISTRY
676
It is clear that the synthetic varnish has an advantage over ester gum varnishes of the same length in this respect. Effect of Successive Coats
To determine whether or not successive coats have any effect on the film, exposure tests were made on iron panels, which were recoated after intervals varying from 4 to 72 hours. All panels received at least one coat of the same varnish, one panel receiving one coat only to serve as check panel. No cracking, checking, or blistering was noted on any panels. Panels recoated in 4 hours were as good as those coated after 72 hours, and as free from blemish as the panel which had only one coat. This would indicate that a second coat may be applied in 4 hours with safety. Panels have also been recoated in 2 hours and are equally as good in appearance as those recoated after 72 hours. Panels have since been exposed that have been given as many as five coats in a day-30 or 40 minutes elapsing between coats-and no defects have shown up yet, although panels have been exposed for BO days. Use of Synthetic Varnish in Paint-Mixing
As might be expected, such a liquid is of immense value to the paint grinder, being neutral t o almost all pigments except zinc and umber. Its use in the paint factory is almost unlimited. Primers, surfacers for wood and metal, undercoaters, and enamels are possible with the same liquid. Satinfinish enamels can also be made with the same form of varnish with slightly different kettle manipulation. A set of panels was prepared on wood and steel as shown in the accompanying table. Panels were prepared so as t o leave exposed about 2 inches of each coat to the weather. Only one coat of each material was applied. Absolutely no failure on any one of the exposed undercoats was noticed. After 9 months of exposure facing south a t an angle of 45 degrees, the lacquered panels, 3 and 4, showed a better state of preservation than the clear
Vol. 20, No. 7
varnish coated panels, 1 and 2, and slightly superior to enameled panels, 5 and 6, although these last two were by no means poor in appearance. PANEL
1 and 2
i1
MATERIAL Primer Surfacer Surfacer Japan Rubbmg enamel Clegywearing
i Primer
1 Surfacer
5 and 6
1
Lacquer enamel Lacquer enamel Primer Surfacer Surfacer Enamel Enamel
NATURE TIMEBETWEEN COATS Synthetic 4 hours Synthetic 4 hours Synthetic Overnight Not synthetic 1 hour Synthetic rubbing Overniaht and rubbed Synthetic Svnthetic Synthetic Synthetic Automobile Automobile Synthetic Synthetic Synthetic Synthetic Synthetic
Exposed after 6 hours 4.~ hours . . . ~
Overnight 4 hours 2 hours Exposed next day 4 hours 4 hours Overnight 4 hours Exposed next day
The undercoaters stood up without showing any signs of deterioration, and lacquer did not lift film or have any damaging effect on the surfacers, which had only dried 4 hours before lacquer was applied. This fact indicates the practicability of fast-drying undercoats of this variety for use under lacquer coatings as well as under varnishes and enamels. Conclusions
Turpentine, the natural solvent for almost anything in the paint and varnish industry, is not conducive of good results in the manufacture of this type of varnish. The use of certain types of driers and thinners has a great effect upon the subsequent manufacture, skinning, and drying of this varnish. The age-old theory of not recoating before the previous coat is entirely dry does not seem to apply with this type of varnish. The adaptability and efficiency of this one type of synthetic resin varnish is an indication of the possibilities that such liquids have when used for straight varnish coatings or as a paint grinding vehicle.
New Indicating Equipment for Industrial pH Measurements’ Henry C. Parker LEEDS & NORTHRUPCOMPANY, PHILADELPHXA, PA
ITHIN the last few Three new potentiometers are described which are required. These impressions suitable for making industrial H-ion measurements. are far from the truth and it years potentiornetTwo of these have scales which are direct-reading in is now possible for an average ric H-ion measureoperator to read over a set of pH. The technic required for making measurements Dents have been 80 far simwith industrial types of quinhydrone and hydrogen directions, make up a calomel plified as to be practical for many industrial uses, This electrodes is described. The limitations regarding the cell from commercially puriuse of the quinhydrone electrode and the relative fied chemicals, and make a simplification has r e s u l t e d suitability of H-ion and conductivity measurements H-ion measurement t o a high primarily from the introducdegree of precision in a retion of portable direct-readare di5cussed. markably short time. ing potentiometers, improvements in the design of electrodes and calomel cells, and from the In the following paragraphs there will be described some of development of the quinhydrone electrode. The industries the new potentiometers which have been developed, towhich make use of H-ion measurements are only just beginning gether with the simplified technic with which it has been to appreciate the advantage of potentiometric measurements, found possible to obtain H-ion measurements with an ease and it still seems to be the general impression that elaborate and accuracy consistent with industrial requirements. purification of chemicals and complicated calculations are ReIative Suitability of Conductivity and H-Ion
.~
1 Received March 26. 1928. Presented under the title “Auulication -..~~. .. of New Indicating Equipment in Making Industrial Measurements Of €3 ~~
~
~
~~
Ion Concentration” before the Division of Industrial and Engineering Chemistry a t the 75th Meeting of the American Chemical Society, St. Louis, Mo., April 18 to 19, 1928.
Measurements
The distinction between H-ion and conductivity measureacids Or bases is not for determining clear. In some cases either measurement would prove
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
July, 1928
suitable and it is then always advisable to use conductivity since these measurements are somewhat more “fool-proof” than H-ion measurements. In general, when the solution is practically neutral H-ion measurements are applicable, but when the concentration of acid or base amounts to 0.1 per cent or over, conductivity measurements are likely t o be satisfactory. I n applying conductivity measurements it must be remembered that no distinction is made between a slightly acid and a slightly alkaline solution, since the conductivity usually passes through a minimum a t the neutral point. It must also be determined whether the concentration of salts will interfere. If this concentration remains fairly constant, conductivity will usually give a sufficiently accurate measurement of the concentration of the acid or base for industrial applications. This is due to the predominating mobility of the hydrogen and hydroxyl ions. In measuring the total salt concentration or the total dissolved solids in a solution, conductivity measurements will give accurate results as long as the salts remain in the same ratio. Transformation of Voltage Readings to pH
On account of the universal use of the pH value to represent H-ion concentrations and the linear relation between p H and voltage, it has been possible to design simple potentiometric equipment which is direct-reading. However, direct-reading instruments are not essential to making industrial use of H-ion measurements, since the voltage has just as much significance as the pH. If the voltage is once determined for the conditions which produce maximum clarification of an effluent or maximum stability of a colloid, for instance, all that is required subsequently is so to control conditions that this voltage is duplicated. Table I-Voltages 0
of Saturated Calomel Cell and Hydrogen Electrode for Various p H Valuesa 5°C. 1 0 ° C . 15’C. 2 0 ° C . 2 5 ’ C . 30’C. 35’C. 0 . 2 5 8 0.255 0.252 0.249 0.246 0.244 0.241
1 2 3
0.313 0.368 0.423
0.311 0.367 0.423
0 309 0.366 0.423
0.307 0.365 0.424
0.306 0.365 0.424
0.304 0.364 0.424
0.302 0.363 0.424
4 5 6
0.478 0.533 0.589
0.479 0.536 0.592
0.481 0.538 0.596
0.482 0.540 0.598
0.483 0.542 0.601
0,484 0.544 0.604
0.485 0.546 0.607
7 8 9
0.644 0.699 0.754
0.648 0.704 0.760
0.652 0.709 0.766
0.656 0.714 0.772
0.660
0.719 0.778
0.664 0.725 0.785
0.669 0.730 0.791
10 11
0.809 0.864 0.919
0.816 0.872 0.929
0.823
12
0.880
0.938
0.830 0.889 0.947
0.838 0.897 0.956
0.845 0.905 0.965
0.852 0.913 0.974
13 14
0.975 1.030
0.985 1.041
0.995 1.052
1.005 1.063
1.015 1.074
1.025 1.085
1.035 1.096
pH
Diff. per pH 0 . 0 5 5 0.056 0.057 0.058 0.059 0.060 0.061 a Based on Clark, “The Determination of Hydrogen Ions,” p. 456, Williams & Wilkins Co., Baltimore, 1922.
There are several convenient methods of transforming values of voltage to pH. One of the easiest is by making use of a chart such as is given in Tables I and 11. These tables give the voltages corresponding to p H units for both the hydrogen and the quinhydrone electrodes for seven different temperatures. Since the differences per p H unit (as found a t the bottom of the several columns) are constant for a given temperature, these values will give a straight line if voltage is plotted against pH. These plots have been found convenient in making the transformation from voltage to pH. The constant differences between p H values makes it easy to interpolate values in between those which are given in Tables I and I1 or to make auxiliary tables which give voltages for each one-tenth p H unit, for example. It will be noticed, from Tables I and 11,that the correction for temperature change varies with the pH. It is evident
677
that, in measurements of acid solutions with the hydrogen electrode this effect is almost negligible, while with the quinhydrone electrode a maximum difference of about 0.15 p H is caused for a temperature difference of 5” C. This latter variation is sufficiently large so that all direct-reading instruments, used with the quinhydrone electrode, should be provided with temperature compensation. Table 11-Voltage of Saturated Calomel Cell and Quinhydrone Electrode for Various pH Valuesa pH 5O C. loo C. 15O C. 20° C. 25’ C. 30° C. 35O C. ’ 0 -0.455 -0.455 -0.454 -0.454 -0.453 -0.453 -0.452 1 2 3
-0.400 -0.345 -0,290
-0,399 -0.342 -0.286
-0.397 -0.340 -0.283
-0.396 -0.338 -0.279
-0.394 -0.335 -0.276
4 5 6
-0.235 -0.179 -0.124
-0,230 -0.174
-0.226 -0.169 -0.111
-0.221 -0.163 -0.105
-0.158
-0,118
7 8 9
-0.069 -0.014 +0.041
-0.062 -0.006 4-0.051
-0.054 +0.003 +0.060
-0.047 +0.011 f0.069
-0.040 f0.020 f0.069
-0.032 $0.028 +0.088
-0,025 +0.036 +0.098
10
f0.096
+0.107
4-0.117
$0.128
$0.138
+0.148
-l-0.159
DiE. per PH
1
0.055
0.056
0.057
0.058
-0.217
-0.099
0.059
-0.393 -0.333 -0.273
-0,391 -0.330 -0,269
-0.212
-0.208 -0,147
-0.152
-0.092
0.060
-0,086
0.061
a Compare Kolthoff and Furman, “Potentiometric Titrations,“ p. 222, John Wiley & Sons, Inc., New York, 1926.
Limitations of Quinhydrone Electrode
It has been shown by Biilmann2 that the quinhydrone electrode gives potentials which are not entirely stable in solutions having p H values above 8. La Mer and Parsons3 made a detailed study of this limitation, being able to divide the effect into four parts: first, a change in the nature of the fundamental electrode reaction as suggested by Clark4which is due to the acidic dissociation of hydroquinone; second, a reaction of the weakly acidic hydroquinone with the solution tending to reduce the hydroxyl-ion concentration; third, the autoxidation of the hydroquinone by air; and fourth, the salt error. La Mer and Parsons have calculated the magnitude of the error involved in the first of these factors. An error of 0.1 pH is introduced at’a p H value of 9.5, and a t a p H of 10 the error has increased to 0.23 pH. Regarding the magnitude of the second effect La Mer and Parsons calculate that the neutralizing action amounts to about 0.006 N when the pH is 9.8. This means that the “alkalinity” of the solution will be reduced 0.006 N if the final pH (after the addition of the quinhydrone) is 9.8. From their calculations the above effect is negligible at pH values below 8, and it is evident that the effect would also be negligible in solutions having a p H as high as 10.0, provided these solutions were well buffered. The autoxidation of quinhydrone by air is a slow reaction. La Mer and Ridea16 found a drift of potential amounting to 0.2 millivolt per minute in a solution having a pH of 8.6. It is evident that this effect will be negligible in buffered solutions below a p H of 10, provided the measurements are taken quickly. This effect occurs in a direct’ion opposite that of the two effects mentioned previously. The salt error of the quinhydrone electrode has been studied by Sorensen, Sorensen and Linderstrom-Lang,‘j who found errors of 0.1, 0.14, and 0.19 p H a t salt concentrations of 2.0, 3.0, and 4.0 normal, respectively. The quinhydrone electrode gave p H values which were too high. From this it is evident that the salt error of this electrode may be considered negligible for most industrial measurements. (In * A n n . chim., 15, 119 (1921). * J. B i d . Chem., 67, 613 (1923). 4 “The Determination of Hydrogen Ions,” 2nd ed., p. 258, Williams & Wilkins Co., Baltimore. 6 J . Am. Chem. SOC.,46, 223 (1924). 6 Ann. chim., 16, 283 (1921).
Figure 3 sllows a iiew "H-Ion Field Kit." This inst.riinient is completely equipped with batt.eries, cliemicals, electrodes, ete., for making field measureinerits With either tho hydrogen or qiiinhydmne electrodes. Measuremerit,s with both electrodes are directly in pH iiriits. A hand teniperature compensator is provided at 9: and a spring beaker rest a t c. Tlie t,wo quinhpdronc electrodes are mounted in the tiibes, E, wliere they may be left, soaking in distilled water (ir a buffer nihtion. The hydrogen eleetriides are mounted at F, wliere they are in position for platinizing. Figure 4 shoms the internal wiring diagram OS the new field kit,. Tlie method for obtaining temperatore compensation is evidrnt Srom tliiti ilirtgnmi. 'This figure also shows IL moss tim of t,he "comhined" cell, which rimy he uiied Sor eithcr ie hydrogen or quinliydn,m! clectrodrs. Procedure for Making a H-Ion Measurement
Figure L-~PorIahlr Acidify Meter
Tliere are in tlie literatiire complete directions for iiirikiiig nccorate 13-ion mcasiirenient,s that, Iiave becii found suitiiblc for iix?in resesrdi iiivestigiltions. Ho\vrrer, many of the iiiit,nii,iiint,i~,nsdescdied are riot npp1ic;rble to the type ( I f electrodes wliidi woiild be siiit,rtble for indiistrinl meit~~iree prccnut,ions mentioned are not. re,iir:ttc to abriiit 0.05 pH are satisincmijnstrnte tlint incliistrial measurem e n t , ~may l i e made \vit,li wrrsiderable rase aiid rimplicit,y, tlie mrtnipiilations rcqiiird Sor this type of meiisuremeiit will bed. For ilIust,ratiiin the procediire will he outinftde with the tiekl kit shown in iiii~a~iiri~i~,ei,ts Figure 3. C a i . o ~ i (:BI~L-T~P ~, v:tlnmel CY# ih made iip cntirdy frnm i:oirrmereially purificd i,lieiiiio:~ls--"s])~ I mercury for ealome1 cells," cloatrolyt,ic ~:alome:. rtiid a siLtirrnteii x h t i u n of c. P. potassitmi clilirridc. Rlcrciiry is aihicd to the i:cII iiiitil the t,ip of tlir platinnm wire, which makes eleclriral coiineetiiin, is well i:overerl. About omive (7 grnms) of calomel is :iildeci am1 the i:dl is filled wit,]>the satrirrrtod soltitimi of
Suinniarizing tlic liniitittions OS t,lie quinhydrone electri,de, it may he said that huffrred S
