Some Applications of the EQCM

In most such applications, the simple relationship between mass and frequency shift given by Eq. (17.1) may not apply, but the sensor could be calibrated to yield useful results. 

If the EQCM acts in this system as a microbalance (i.e., if Eq. (17.1) is applicable), the coordinates of A/ can be recalculated to show Am. The slope of this line is, therefore, the equivalent weight of the metal deposited. Any systematic deviation from the calculated line can be associated with a lower current efficiency, or some other deviation of the system from the conditions under which Eq. (17.1) is applicable. 

The data for silver deposition agree with the calculated line within 0.25%. For deposition of copper the data also follow a straight line, but the slope in this and similar experiments was found to be consistently lower than the calculated slope, by about 6-8%. This deviation from theory was not attributed to the properties of the EQCM. The electroplating of copper is commonly believed to occur in two consecutive charge-transfer steps 

Most of the monovalent copper formed at the solution side of the interface (at the OHP) will be reduced to metallic copper, but some may diffuse away into the solution, without being further reduced. Thus, a fraction of the electrons consumed in Eq. (17.15) may not be used to form atomic copper on the surface, and the effective charge will be less than that measured, leading to the lower slope. The same argument applies to deposition of Au, where the reactant contains Au and the intermediate contains Au .  

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