Δ F( Hz) Δ F( Hz)
Δ F( Hz)
50
0
-50
-100
-150
Blank
-200
Δ F( Hz) Vs. Time( Seconds)
500ppb PEPMA / HPSP
500ppb PBTC
-250 |
-300 |
30 |
35 |
40 |
45 |
50 |
55 |
60 |
|
|
|
Time( Seconds) |
|
|
|
a |
Δ F( Hz) Vs. Time( Seconds) |
50
0
500ppb PEPMA / HPSP
-50
500ppb HEDP
Blank
-100
-150
-200 30 35 40 45 50 55 60 Time( Seconds) b
Δ F( Hz) Vs. Time( Seconds)
50
500ppb PEPMA / HPSP
0
500ppb HEDP
-50
Blank
-100
-150
-200
-250
-300 30 40 50 60 70 80 90 100 110 120 130 Time( Seconds) c
Figure 3- QCM delta frequency: Utilities A, B & C ppm sulfate, 147 ppm chloride and 1 ppm orthophosphate and heated to 71 º C. The resulting pH was 8.8-8.9. The water had a calcite saturation of ~ 43.5x, a Langelier Saturation Index of 2.01 and a momentary excess for calcite of ~ 19.18 mg / l.
The saturation index is a good indication of the driving force for precipitation, while the calculation of momentary excess provides an understanding of how much calcite will precipitate( untreated) before the water returns to equilibrium. In this case, equilibrium can be defined as the condition in which precipitation and dissolution rates are equal resulting in a net zero precipitation or CaCO 3
.
In this and all experiments presented here, the QCM probe was allowed to stabilise in the cation solution until a steady baseline frequency was observed. Once this is achieved, the experiment is started and data recording is initiated. The anion solution is then added to the cation solution after 30 seconds. This becomes Time 0, since the combinations of the cation and anion solutions is the first opportunity for precipitation and potential deposition.
In the untreated blank, an inflection or reduction in frequency of about 10 Hz is recorded within three seconds of the solutions being mixed. This is indicative of the onset of precipitation and actual deposition onto the QCM probe. This is the time in which the induction of
CaCO 3 precipitation is recorded.
While the initial reduction of vibration frequency was observed at just three seconds, the test is typically continued until a full inflection is observed and no further decrease in frequency is measured. In theuntreated blank test, this occurred at about 50-55 seconds. At this point, the overall change in frequency( ΔF Hz) over the duration of the test provides an indication of the relative deposition quantity.
Using this methodology, the QCM instrument was used to compare efficacy of a blend of PEPMA and HPSP to phosphonate and an untreated blank in synthetic solutions replicating specific water and operating
48 SPECIALITY CHEMICALS MAGAZINE ESTABLISHED 1981