How to loop through pandas dataframe, and conditionally assign values to a row of a variable?

I'm trying to loop through the 'vol' dataframe, and conditionally check if the sample_date is between certain dates. If it is, assign a value to another column.

Here's the following code I have:

vol = pd.DataFrame(data=pd.date_range(start='11/3/2015', end='1/29/2019'))
vol.columns = ['sample_date']
vol['hydraulic_vol'] = np.nan
for i in vol.iterrows():
    if  pd.Timestamp('2015-11-03') <= vol.loc[i,'sample_date'] <= pd.Timestamp('2018-06-07'):
        vol.loc[i,'hydraulic_vol'] = 319779

Here's the error I received: TypeError: 'Series' objects are mutable, thus they cannot be hashed