surprisingly we have found that the electrophoretic migration of suspended material particles can be produced not only in a d . c . voltage field of force . but by allowing the suspension to flow laminarly and equally in time in an adequate apparatus , and by switching on the proper + or - pole in the direction of flow and applying momentary controlled direct voltage of proper magnitude it may be produced . according to the process of the invention , the v e electrophoretic migration speed can be calculated in another way than described in the prior art . it is observed that local flow velocity ( v ) of the selected particle changes to the flow velocity v v due to the effect of the voltage v applied . in this case the particle does not stop but travels slower or faster depending on the energized direction of poles . in case , the negative pole is switched in the direction of flow and flow velocity of the particle decreases upon the effect of the voltage v , the sign of the zp - value will be negative ; otherwise it is positive . on the basis of the expression electrophoretic migration speed v e can be calculated ( expressed in units cm . s - 1 ) and in such a manner zp can be given by using the formula as described . according to a preferred embodiment of the process , in the constantly flowing liquid the particle intended to be observed will stop , as the magnitude of the electrophoretic velocity arising under the effect of the voltage complies with the velocity of liquid flow . zp - value can be calculated by using the previously described equation , considering that the velocity of flow of the particle corresponds to the value v e . according to a preferred embodiment of the method of the invention , two 0 . 2 mm thick platinum electrodes 10 spaced apart 10 . 0 cm are installed in a cylindrical glasspipe 11 having an inner diameter of 3 to 6 mm , preferably 4 mm . thereafter the measuring pipe 11 is placed under a microscope 12 suitable for 15 - 30 - fold magnification and provided with a calibrated ocular micrometer scale . the suspension is illuminated by fibre - optics or by means of a 100 micron optical slot 13 arranged before the microscope lamp 14 to eliminate the disturbing ( heating ) effect of the light source . one end of the measuring pipe is coupled by a synthetic connection 15 to an injector 16 of 2 - 10 cm 3 , containing the suspension to be tested , while the other end is coupled by a synthetic connection 17 to a vessel 18 for receiving the suspension flowing through the measuring pipe . when entering the suspension into the system , care is to be taken to avoid air bubbles when filling the measuring pipe 11 , the connecting synthetic pipes 15 , 17 and the injector 16 . thereafter the injector 16 is fitted into the equipment and the suspension is allowed to flow at a regulated , steady - state velocity . the electrodes 10 ( made of platinum ) of the measuring pipe are connected to a d . c . voltage supply unit 19 provided with an alternator or pole changer switch 20 and with a stabilized voltmeter 21 which works in the voltage range between 1 and 800 v . steady flow of the particles in the microscope 22 can be controlled by measuring the travelling time of the selected particle , observing the same through the 100 - scale of the ocular micrometer , with an accuracy of 0 . 01 s , separately in the first and second 50 -- 50 pitch of the scale . if the particle takes the first and second 50 pitch within about the same time ( 0 . 01 - 0 . 02 s difference is acceptable ), the flow in the system is considered to be steady - state , laminar . thereafter , another particle is selected and the travelling time thereof is measured at an optional distance ( 10 , 20 , . . . 50 ocular pitch ) with an accuracy of 0 . 01 s . then the d . c . voltage 19 is switched on , the pole changer 20 is positioned , and by regulating voltage controller 22 the voltage applied to the measuring pipe 11 the motion of the chosen particle should stop . this voltage value is registered . the flow velocity measured with the calibrated ocular micrometer with an accuracy of 0 . 01 s is calculated to v e = cm . s - 1 units with an accuracy of four digits . dividing the voltage needed for stopping the particle by the distance of the electrodes ( in the present case , ten cm . ), the value e of voltage density is obtained . in this manner using the known formula ## equ2 ## the zp - value of the desired aqueous suspension can be calculated . it is recommended to perform measurements of a large series of particles migrating with different speeds . thus , the average of the zp - values and a variation coefficient or standard deviation can be calculated by using known mathematic statistical methods . these values characterize the accuracy of measuring . the testing method may be varied , e . g . by scanning the flowing particles , a zp - value of the selected particle can be determined by simultaneously regulating flow velocity and the voltage applied to the measuring pipe . according to a preferred embodiment of the measuring system apparatus suitable for carrying out the method of the invention , includes a measuring pipe 11 with an inner diameter -- within the range 3 to 6 mm --, into which platinum electrodes 10 ( dia . 0 . 2 mm ) at a spacing of 10 . 0 cm are installed . the measuring pipe is fixed with the usual clamping plates to the stage of the microscope 22 . illumination is provided by fibre - optics instead of a condenser or with a 100 micron optical slot 13 arranged in front of the condenser . the ocular micrometer is calibrated and the path length of the particles is calculated in cm . for quickly sedimenting suspensions , it is advantageous to position the microscope at an angle of 90 ° with the stage of the microscope turned from the horizontal to vertical . in this case , the sedimenting particle bends from the line of the ocular micrometer scale , making it possible to examine only those particles which are travelling parallel with the axis of the measuring pipe and with the scale line of the ocular micrometer . the sample to be tested is fed by a syringe 16 ( injector ). practically , for low velocities an injector with a smaller volume is used ; if higher velocities are needed , injectors with a larger volume should be used . for this purpose the so - called tuberculine injectors with a volume of 2 , 5 , perhaps 10 cm 3 may be used . injectors with a far larger volume are recommended only in the case of extremely high zp - values . the connecting pipes 15 , 17 are made of polyethylene or any other structural material . the sample feeder 23 is advantageously the hungarian &# 34 ; infucont &# 34 ; quartz - controlled electronic apparatus , which is well suited for the control of the &# 34 ; infumat or infudriver &# 34 ; drives which drive the piston of injector . the electric energy at the proper frequency , phase , and voltage , required for operating the shifting - motor drive at a continuously controllable rate of feeding is electronically digitally displayed . the digital display ( d ) includes the feeder and flow velocity observed in the measuring pipe ( with an inner diameter of 5 . 0 mm ( expressed in cm . s - 1 ) controlled with a 10 cm 3 synthetic syringe ). within the digital display range 0 - 40 , the apparatus delivered the aqueous suspension with an acceptable linearity . in the described arrangement the linear regression coefficient was found to be 0 . 9999 . the solution of the linear equation will yield different values , when using measuring pipes or injectors with different inner diameters . thus calibration is required for individual arrangements of given dimensions . it is well known that a pure laminar flow will be established in a cylindrical pipe within the range of contemplated velocity , based on the low reynolds - number . the velocity distribution curve has a parabolic shape . along the axis of the pipe , the flow velocity is double the average flow velocity ( v ). the optimal radius r of the cross - section of the radius r o the local flow velocity v can be calculated on basis of the knudsen and katz rule ## equ3 ## ( john h . perry : manual of chemical engineers , technical publishers , budapest , 1953 . volume i , page 564 ). from this it becomes obvious that in the microscope , in dependence of the depth of focus , at a relatively high velocity ( above 100 micron . s - 1 ), parabolic velocity distribution becomes flattened to such an extent that in its visual field particles travelling with different speeds can be observed . at considerably lower velocities particles travelling with nearly identical velocities can be observed . according to an improved mode of realization of the process according to the invention , the measuring pipe 11 is placed on the microscope 12 -- as detailed previously --, the injector 16 is arranged in the feeding device 23 and connection is made with synthetic pipes 15 , 17 . the feeder 23 is calibrated for the suspension to be examined , as specified earlier . the flow velocity of the particles is twice the digital signal of the feeder . by the aid of the voltage regulator of the d . c . voltage current source 19 , the voltage needed for stopping the particles is adjusted . care should be taken that the pole switch 20 is in the proper direction , in compliance with the nature of the material tested . the feeder 23 and the d . c . voltage supply unit 19 deliver an electric signal proportional with the flow velocity and the voltage value needed for stopping the particles , these two signals are fed to a computer and display unit . thereafter , based on a predetermined algorithm , the zp - value of the material tested is obtained automatically . accuracy of the measuring system and the testing method are illustrated by the following examples 2 - 3 mg of 17α - hydroxy - 21 - acetoxy - corticosterone , as a basic material with a grain size below 10 microns , are mixed with a small quantity of water in a frictional mortar . the mixture thus obtained is diluted with 50 cm 3 of distilled water and allowed to stand for three days . measurement is obtained with a measuring pipe having an internal diameter of 4 . 0 nm with electrodes arranged therein with a spacing of 10 cm ; a 5 cm 3 synthetic injector ; and a microscope with 10 x magnification . the flow time on the 1000 micron path in second ( s ), as well as voltage required for stopping the particle are determined . thereafter further measuring is performed for a 318 microns path with 30 x magnification . zp - values calculated from the measured data are as follows : ______________________________________s v zp______________________________________1000 microns path11 . 0 230 - 59 . 312 . 0 222 - 56 . 312 . 0 222 - 56 . 3318 microns path3 . 80 220 - 57 . 13 . 73 220 - 58 . 13 . 73 219 - 58 . 43 . 93 205 - 59 . 23 . 87 208 - 59 . 3______________________________________ these test results demonstrate well that the measuring system and the testing method of the invention can be used for determining the zp - value with an accuracy which corresponds to and often exceeds the experiences reported heretofore in technical literature . in technical literature zp - value has been referred to generally as an &# 34 ; apparent &# 34 ; value , form which conclusions can be drawn on the inaccuracies resulting from the difficulties of determination . taking into consideration that measuring technique based on a flow system eliminates numerous disturbing factors , a zp - value determined in that way may be considered as a virtual value . in order to be able to demonstrate reproducibility of the data of technical literature , comparative tests were also carried out . in the earlier described manner , colloidal iron ( iii ) hydroxide was tested in freshly boiled and cooled distilled water at 22 ° c . after having precipitated ferric - chloride with sodium hydroxide and eliminated chlorine . measured results : ______________________________________318 microns path : s v zp mv______________________________________4 . 58 235 - 44 . 34 . 80 220 - 45 . 14 . 76 222 - 45 . 14 . 90 215 - 45 . 35 . 00 231 - 41 . 35 . 57 203 - 42 . 26 . 43 181 - 41 . 07 . 12 162 - 41 . 3______________________________________ the value given in the technical literature is 45 mv ; the difference is - 3 . 93 % ( martin , a . n . et al . physical pharmacy ( lea and febiger , philadelphia 1969 ) page 458 ). zp - value is considerably influenced by traces of carbonic acid impurities in water , or chloride ion impurities of the sample ; the comparison to the value of technical literature is considered good . the second coparative test performed for determining zp - values in alumina / 10 - 4 m potassium - nitrate solution in dependence of ph was carried out as previously described ( wiese , r . g . et al . : colloid interface sci . 51 , 427 ) ( 1975 )); the results obtained are reflected in fig1 . fig1 illustrates that the zp - values measured in the flowing system show a good comparison with the data of technical literature . difference of larger extent , as observed in high ph - ranges may result from the differences in the structure of the solid material tested in addition to the error sources as mentioned before . it is well known that some elements have allotropic structure . organic compounds can be produce in several crystal modifications , e . g . the pesticidal pharmaceutical basic material mebendazol , the three polymorphous crystal modifications ( a , b and c ) of which are known ( lancos ; krisztina : ph . d . thesis , medical university semmelweis , pharmaceutical institute , budapest , 1985 ). in respect to chemical composition , the single mebendazol polymorphes are quite identical , however , the structure of their solid body is reversibly different . three mebendazol polymorphous crystal modifications we comminuted to a grain size finer than 30 microns and 2 mg thereof were suspended in distilled water . zp - value was measured in a flowing system at 20 ° c . temperature , as specified earlier . the average value of six measurements and variation coefficient of the measuring series were , as follows : ______________________________________sample zp . sub . mv ± ______________________________________mebendazol a 106 . 3 2 . 1mebendazol b 34 . 9 0 . 9mebendazol c 104 . 0 3 . 0______________________________________ according to technical literature stability of a suspension is considered as maximally satisfactory , if the value of zp is approximately - 100 mv . the stability of the suspension does not meet the requirements if the zp - value thereof is lower ; under - 40 agglomeration and under - 10 precipitation will occur . by means of auxiliary materials absorbed onto the surface of the solid body , properties of the so - called electric double - layer and thus the zp - value can be controlled . the possibility of zp - value regulation is illustrated by the example of the aqueous suspension of 17α - hydroxy - 21 - acetoxycorticosterone , as specified hereinabove . if zp - value is measured in distilled water , at a temperature of 20 ° c . in the presence of different auxiliary materials , as described earlier , the following results are obtained : ______________________________________mg · cm . sup .- 3 auxiliary materials zp mv ± ______________________________________ . 0 . ( in pure distilled water ) - 58 . 0 1 . 20 . 5 sodium stearate - 74 . 0 2 . 00 . 5 tween - 80 - 81 . 0 1 . 40 . 5 + 0 . 5 sodium stearate + - 140 . 2 4 . 1tween - 80______________________________________